Journal of Plankton Research - Advance Access

Effects of fish biomass and planktivore type on plankton communities

Attayde, J. L., Menezes, R. F. — 2008-05-06

We performed a field experiment in 15 fish ponds manipulating the size and biomass of the Nile tilapia, Oreochromis niloticus, to test the hypothesis that the effects of planktivorous fish on plankton dynamics and community structure depends on the planktivore type and fish stocking biomass. Juveniles and adult tilapia were either stocked alone or combined in the same proportion at 20 g/m³ and 40 g/m³. Because tilapias show an ontogenetic niche shift from visual predation on zooplankton to filter (suction) feeding on both zooplankton and phytoplankton, we expected differences in the biomass-related effects of juveniles and adults on plankton communities. Our results show that rotifer abundances were higher in treatments stocked with juvenile tilapia while cladoceran abundances were higher in treatments stocked with adults. However, total zooplankton and copepod abundances, chlorophyll a concentrations and Secchi depth were not affected by tilapia size, suggesting that variation in the feeding mode of tilapias affects the structure of zooplankton communities but that this does not cascade down to affect phytoplankton biomass and water transparency. Competitive interactions among the fish constrained their potential to depress zooplankton abundances and increase algal biomass as fish stocking biomass increased. Competition among size classes of tilapias was highly asymmetric and juveniles were better competitors than adults for plankton resources in our experimental ponds.

Modelling the influence of abiotic and biotic factors on plankton distribution in the Bay of Biscay, during three consecutive years (2004 to 2006)

Zarauz, L., irigoien, X., Fernandes, J. A. — 2008-05-02

Three surveys were carried out in the Bay of Biscay during the springs of 2004, 2005, and 2006. Hydrographic, nano-microplankton (diatoms, ciliates and unidentified particles) and mesozooplankton biomass data were collected at mesoscale spatial resolution. Generalized Additive Models (GAMs) based on a combination of hydrographic, geographic and biological terms were used to understand the factors affecting distribution. The final models accounted for 66% of the variability in the biomass distribution of unidentified particles, 60% for diatoms, 44% for mesozooplankton and 23% for ciliates. The contribution of hydro-geographical terms was greater than the information described by the biological variables. Geographical location (latitude, longitude) was the main explanatory factor for all of the plankton groups identified, revealing that the presence of mesoscale fronts related to geographical structures is more relevant than the hydrographic variables per se, to describe plankton distribution.

Feeding behavior and development of Acartia tonsa nauplii on the brown tide alga Aureococcus anophagefferens

Smith, J. K., Lonsdale, D. J., Gobler, C. J., Caron, D. A. — 2008-04-30

Toxic attributes of the brown tide alga Aureococcus anophagefferens affect the ability of benthic and protistan grazers to control blooms. Yet, little is known regarding the effect of A. anophagefferens on a dominant component of the microzooplankton community, copepod nauplii. This study describes the grazer-prey relationship between nauplii of the calanoid copepod Acartia tonsa and A. anophagefferens. Four laboratory experiments using varying proportions of A. anophagefferens (2-4 µm) and a control alga, Isochrysis galbana (4-6 µm), were conducted to test the effects of A. anophagefferens isolate 1708 during exponential and stationary growth phases and A. anophagefferens isolate 1850 (exponential phase only) on naupliar grazing and development. A fifth experiment compared the effects of A. anophagefferens with an equal-sized control alga, Micromonas pusilla (1-3 µm). Isolate 1708 (exponential or stationary) as a single food item did not suppress naupliar ingestion rates (ng C nauplius^–1 d^–1) when compared to I. galbana. No ingestion was detected on isolate 1850 when offered alone, suggesting that this isolate may be more harmful to nauplii. Overall, nauplii selectively grazed on I. galbana over A. anophagefferens in mixed diets, but size-selection could not be ruled out as selective feeding was not apparent in mixtures with M. pusilla. Both isolates of A. anophagefferens delayed naupliar development. Our results indicate that Acartia tonsa nauplii can graze on A. anophagefferens, and can potentially help suppress brown tides. However, the efficacy of grazing control by copepods will vary with availability of alternate food sources and toxicity of the A. anophagefferens strain(s) comprising the population.

Growth and development of Pseudocalanus spp. in the northern Gulf of Alaska

Liu, H., Hopcroft, R. R. — 2008-04-25

Pseudocalanus are the numerically dominant calanoid species in coastal subarctic Pacific waters. We examined their juvenile growth rates, and explored their relationships to temperature, chlorophyll a and body size for Pseudocalanus spp. from 2002-2004 in the northern Gulf of Alaska. Generally, the monthly mean growth rates increased from 0.049±0.007(SE) day^–1 in March to 0.095±0.016 day^–1 in August, declining slightly to 0.074± 0.009 day^–1 in October. Typically, growth rates at most stations were around 0.05 day^–1, with no consist or significant pattern between stations. After standardization to 5 °C and 10 °C, the mean growth rates were 0.045±0.002 day^–1 and 0.075±0.004 day^–1 respectively, with growth rate decreasing with increasing development stage. Unlike other local calanoid copepod species, Pseudocalanus species tend to be more temperature-dependent than food-dependent, with composite statistical models describing at most 30% of the observed variability in growth rate. Interestingly, development time was comparable to other co-occurring calanoid copepods, however, growth rates of Pseudocalanus spp. were considerably lower. We demonstrate this with a new multi-species model that describes the growth rates of other egg-scattering copepods in this ecosystem, but to which Pseudocalanus does not fit. Thus, the egg-carrying Pseudocalanus species appear to employ a life history strategy optimized for slow growth at low chlorophyll, that keeps individuals relatively small, and may therefore reduce visual predation upon them.

Patchiness and spatial structure of the early developmental stages of clupeiforms in the NW Mediterranean Sea

Maynou, F., Olivar, M. P., Emelianov, M. — 2008-04-18

We analyse the spatial structure of early developmental stages of three ecologically important small pelagic schooling fish species in the northwestern Mediterranean Sea: the anchovy Engraulis encrasicolus, the round sardinella Sardinella aurita, and the sardine Sardina pilchardus. We used data on egg and larval abundance (arranged by development stage), hydrographic data and potential prey biomass collected during 2 cruises in summer and autumn 2005. Our analysis is based on the computation of Lloyd's patchiness index during the ontogeny of early developmental stages, as well as the computation of spatial auto-correlograms and cross-correlograms with environmental variables. The combined analysis allowed determination of the relative role of behavioural traits and the influence of hydrographic conditions in shaping the spatial structure of the three clupeiforms. We show that the spawning and developmental strategies of the two summer-spawning species (E. encrasicolus and S. aurita) are different. E. encrasicolus has relatively low patchiness for all early developmental stages, while the importance of trophic variables in determining spatial structure increases during development. S. aurita spawns in a trophically favourable environment and the patchiness of its early developmental stages increases for older larvae, becoming decoupled from environmental variables. The autumn spawned S. pilchardus eggs and larvae showed a development strategy similar to S. aurita, with limited importance of environmental variables in determining their patchiness.

Geographic distribution of Karlodinium veneficum in the US east coast as detected by ITS- Ferredoxin Real-Time PCR assay

Zhang, H., Litaker, W., Vandersea, M. W., Tester, P., Lin, S. — 2008-04-18

Karlodinium veneficum is an ichthyotoxic dinoflagellate suspected to be responsible for massive fish kills. Because it is morphologically similar to Pfiesteria and related dinoflagellates, accurate abundance data of K. veneficum in the natural environment is difficult to obtain using light microscopy. In this study, species-specific Real-Time PCR assays were developed using the ferredoxin gene (KvFERR) and rDNA locus (partial internal transcribed spacer regions and 5.8S rDNA, KvITS). These assays were used to investigate the geographic and temporal distribution of K. veneficum from various estuaries and coastal regions of the Pacific and West Atlantic Oceans. In general, both genes gave similar quantitative results, detecting down to 0.1 - 1 K. veneficum cells mL^–1. The higher occurrence frequency and abundance of K. veneficum were observed in Neuse River, North Carolina, Narragansett Bay, Rhode Island and Chesapeake Bay, Maryland, USA. Most of the KvITS sequences from North Carolina to Maine were identical to that of K. veneficum strain CCMP1975 (initially isolated from Chesapeake Bay), with 1-2 nucleotide substitutions occurring in sequences from Texas and Narragansett Bay samples. This result suggests the same origin and recent separation of K. veneficum populations living in this wide geographic range. In this study, K. veneficum was more abundant at 5-15 °C and 4-20 PSU.

Seasonal abundance and vertical distribution of siphonophores in western Norwegian fjords

Hosia, A., Bamstedt, U. — 2008-04-16

Seasonal abundance of siphonophores in Korsfjord and Fanafjord was studied by net sampling. Nanomia cara, Lensia conoidea and Dimophyes arctica were dominant species present throughout the year. Abundance of N. cara colonies peaked in late May and early June. Low numbers of relatively large colonies and high numbers of siphonulae were observed during winter. D. arctica and L. conoidea had a generation shift in March, when large over-wintering and small young polygastric colonies co-existed briefly before the former disappeared. Abundances were lowest during winter, and eudoxids were consistently more numerous than polygastric colonies for both species. Maximum abundances of polygastric D. arctica and L. conoidea occurred in early May and late June, respectively. L. conoidea may have more than one annual generation in Korsfjord.

Vertical distributions of siphonophores were studied with an ROV in 9 fjords. The distributions of agalmatid physonects, probably mostly N. cara, differed significantly between the fjords, with the weighted mean depths (WMD) of the distributions ranging from 99 to 412 m. Results from Sognefjord suggested diel vertical migration (DVM). The bulk of the adult colonies in each fjord occurred below sill depth. Siphonophores were not observed at localities known to host persistent populations of Periphylla periphylla.

Phytoplankton blooms in the Huon Estuary, Tasmania: top down or bottom up control?

Thompson, P. A., Bonham, P. I., Swadling, K. M. — 2008-04-11

The roles of ‘top down’ and ‘bottom up’ factors were investigated in terms of their influence on the diatom and dinoflagellate abundances in the microtidal, salt wedge Huon Estuary, Tasmania, Australia. Long term (1996 to 2005) changes in chlorophyll a, the peridinin:chlorophyll a ratio and the abundance of autotrophic dinoflagellates were observed to coincide with the warming of regional surface waters. There were significant seasonal differences in pigment-specific net growth rates for chlorophyll a, peridinin and fucoxanthin. Diatoms dominated the spring bloom when species such as Skeletonema costatum had the highest net growth rates and fucoxanthin-specific gross growth rates were~0.9 d^–1. During late summer the peridinin-specific grazing mortality was significantly less than the fucoxanthin-specific grazing mortality and dinoflagellates increased their dominance of the phytoplankton community. This late summer relaxation of grazing pressure on dinoflagellates was associated with a decline in the overall abundance of microheterotroph grazers and a peak in the abundance, biomass and estimated grazing rates of mesozooplankton. We suggest the composition of the autumn phytoplankton community was dependent upon a trophic cascade where mesozooplankton, such as Noctiluca scintillans, preyed upon microheterotrophs and reduced their grazing upon some species of dinoflagellates.

Summer Planktonic Copepod Communities of Australia's North West Cape (Indian Ocean) during the 1997-99 El Nino/La Nina

McKinnon, A. D., Duggan, S., Carleton, J. H., Bottger-Schnack, R. — 2008-04-10

The community composition of pelagic copepods near Australia's North West Cape (21º 49’ S, 114º 14’ E) was studied during the austral summers of 1997/98 and 1998/99. Most sampling occurred at a shallow (20 m) shelf station and a deeper (90 m) shelf-break station, though on 4 occasions a set of 8 stations were occupied on a 36 km cross-shelf transect. During the El Niño conditions prevalent during the austral summer of 1997/98, episodic upwelling occurred causing intermittent high primary production. During the El Niño conditions of 1997/98, there was little difference between stations in the spring (Oct- Nov), but communities differentiated later in the sampling season (Dec-Feb) with a more characteristic inshore community developing at the shelf station. In the La Niña conditions of 1998/99, the community at the shelf break was invariant, but the shelf community was mainly offshore copepods as a result of seasonal downwelling during the spring that was later replaced by an inshore community of more widely distributed species. Over 120 species of copepods were identified, of which the most speciose families the Corycaeidae (22 spp.), Oncaeidae (>20 spp.), Paracalanidae (15 spp.) and the Oithonidae (11 spp.). Cross-shelf transects confirmed the existence of a distinct inshore community of copepods, dominated by small species of Paracalanidae and Oithonidae, and which was at least twice as abundant as those at the shelf break. In both summers there was an onshore-offshore gradient in community composition, with the inshore stations characterised by small paracalanid and oithonid species.

Benthic-pelagic coupling in sediment associated populations of Karenia brevis

Sinclair, G. A., Kamykowski, D. — 2008-04-08

Nutrient delivery to populations of Karenia brevis in oligotrophic water columns in the Gulf of Mexico remains uncertain. Aggregations of Karenia brevis near the sediment-water interface suggest that cells derive nutrients from the sediment. Video of cells near the sediment suggest that cells either access nutrients that flux out of the sediment or migrate into the sediment pores where higher nutrient concentrations exist. Experiments tested K. brevis’ ability to migrate into the sediment using chambers divided by a 100 µm mesh overlain with a thin layer of sediment. Since the diel vertical migration of K. brevis typically displays a nocturnal descent, experiments tested migration response at night in response to sub-sediment nutrient sources. The experiments suggest that while the sediment affects the progress of descending cells, migration occurs through thin layers of sediment and increases in response to elevated nutrient concentrations below the sediment. Since all cells found below the sediment had significantly higher C/N ratios than those remaining above the sediment, migration appears related to a cell's internal biochemical state. The vertical migration behavior of K. brevis may help alleviate bottom-up controls and permit populations to persist as vegetative cells near the sediment-water interface.

Dual control of the levels of photoprotective compounds by ultraviolet radiation and temperature in the freshwater copepod Boeckella antiqua

Garcia, P. E., Perez, A. P., Dieguez, M. d. C., Ferraro, M. A., Zagarese, H. E. — 2008-03-27

Photoprotective compounds (PPCs), such as carotenoids and mycosporine-like aminoacids (MAAs), confer photoprotection to aquatic organisms against harmful ultraviolet-B (UV-B) radiation. The natural variability of these compounds in zooplankton has been related to temperature, radiation and diet, but the ultimate mechanisms regulating the observed patterns in the field are still unclear. In this study we analyzed the variability of carotenoids and MAAs in a population of the calanoid copepod Boeckella antiqua in a shallow pond located in Northern Patagonia (Argentina). During our field survey, carotenoids and MAAs in B. antiqua varied without a clear seasonal pattern. Nevertheless, both groups of PPCs reached their maxima during spring and minima during summer. Inverse relationships were found between carotenoid concentrations vs. temperature and irradiance. For MAAs the same relationships were not significant.

Tolerance experiments showed that mortality of B. antiqua was significantly influenced both by temperature and UV-B dose, being more vulnerable at high temperature. We further investigated the effect of radiation regime and temperature on the bioaccumulation of PPCs in controlled laboratory experiments. We found that the concentrations of PPCs could be experimentally modified by manipulating radiation exposure and temperature.

In addition, by breaking down the bioaccumulation processes into uptake and elimination, we were able to show that (i) the uptake rate was stimulated by PAR+UVA exposure, and (ii) both uptake and elimination rates increased with temperature. Thus, the net accumulation (i.e, the balance between uptake and elimination), which ultimately dictates the concentration observed in an animal, could be either positive or negative depending on the specific combination of radiation exposure and temperature. The dual regulation of PPCs by radiation exposure and temperature should be considered in future efforts to reconstruct or predict the photoprotective responses of aquatic organisms to the past or future climate scenarios.

Diversity and distribution of the mesozooplankton in the tropical Southwestern Atlantic

2008-03-19

Mesozooplankton species distribution and abundance were studied in the tropical Southwestern Atlantic, in neritic and oceanic regions off the state of Rio Grande do Norte, northeastern Brazil (3^o to 6^o S and 32^o30' to 35^o30' W). Samples were collected with a 120 µm mesh net towed horizontally at 40 stations along 5 transects in June 1986. A total of 151 taxa were identified. Copepoda (93 species) numerically dominated (78%) the community. The highest zooplankton abundance was recorded at a coastal station (1,098 ind m^–3), and the lowest at the station farthest offshore (82 ind m^–3). The low abundance of zooplankton corresponded to oligotrophic water masses, and differences in abundance were locally affected by topographic upwelling. Three groups were distinguished: 1) Oceanic with very low total abundance (< 15 ind m^–3), 2) a mix of neritic and oceanic species; and 3) Coastal neritic group. Species diversity was high (mean 3.14 ± 0.42 bits ind^–1) and increased with the ecological stability of the area. The local oceanographic conditions, with a strong connection between the oligotrophic oceanic water masses and the enriched coastal water, is of paramount importance to the local food chain and patterns of zooplankton diversity.

DIET OF ROUND SARDINELLA, Sardinella aurita, LARVAE IN RELATION TO PLANKTON AVAILABILITY IN THE NW MEDITERRANEAN

Morote, E., Olivar, M. P., Villate, F., Uriarte, I. — 2008-03-19

The present study analyses for the first time the diet of larvae of the clupeid Sardinella aurita in the NW Mediterranean. Gut contents of larvae from first feeding (3.9 mm) to early postflexion stages (14.7 mm) were analysed. The type and abundance of ingested prey were related to the plankton composition in the environment in order to determine prey-type selectivity. The feeding incidence observed was among the highest reported for clupeid larvae, with higher values during the day (68.6%) than at night (7.7%), indicating that Sardinella aurita larvae are diurnal predators. The mean number of ingested prey was 3.3 (± 2.51). The width of the prey increased from <30 to 410 µm with the development of the larvae, but the niche breadth did not vary with the size of the larvae. The diet was based on different stages of copepods (nauplii and postnauplii) and cladocerans (mainly Evadne spp.), though the preference for each type of prey varied, with higher preference for copepod nauplii in larvae <8 mm and a higher preference for Evadne sp. in larvae =8 mm. The copepod based diet of S. aurita larvae is very similar to that reported for E. encrasicolus larvae in the Mediterranean, indicating a possible competition for food between the first-feeding larvae in situations of low prey abundance.

Hydromedusa Blooms and Upwelling Events in the Bay of Panama, Tropical East Pacific

Miglietta, M. P., Rossi, M., Collin, R. — 2008-03-19

Seasonal upwelling events dominate coastal areas of some regions of the Tropical East Pacific. The effects of upwelling on gelatinous zooplankton are largely unknown and undocumented for this or any region, although upwelling is known to shape phytoplankton dynamics. Small hydromedusae, the most widespread and diverse representatives of gelatinous plankton, are often neglected in plankton ecology as they are inconspicuous and escape direct observation. Their occurrence is seasonal and standard plankton sampling techniques easily overlook their blooms. In order to investigate whether upwelling affects hydromedusae dynamics we monitored their abundance and diversity in the Bay of Panama, a region on the Pacific Coast of Panama characterized by seasonal upwelling events. Our results show that, although the number of species is relatively constant throughout the year, hydromedusa abundance in the Bay of Panama can be up to two orders of magnitude higher during upwelling than non-upwelling conditions. The difference in the numbers of hydromedusae between the two seasons is mostly due to temporally short medusa blooms that occur only during the upwelling season. Our results point to a link between hydromedusa blooms and upwelling events, and the increased productivity associated with them. The results are consistent with a scenario in which upwelling events act on the benthic colonies thus inducing medusa production.

Light availability for Chlorobium phaeobacteroides development in Lake Kinneret

Rimmer, A., Ostrovsky, I., Yacobi, Y. Z. — 2008-03-11

The green sulfur bacterium Chlorobium phaeobacteroides (Chl. phaeobacteroides) forms a dense population in the oxygen devoid layers of Lake Kinneret, Israel, from July throughout November. The typical vertical distribution of the species is characterized by a prominent peak at the center of the metalimnion, but, the light available at that depth in Lake Kinneret is far below the values that enable maximal photosynthetic activity of Chl. phaeobacteroides. In this paper we report on our findings regarding the characteristics of Chl. phaeobacteroides development in Lake Kinneret, both in space and time. We farther quantified the available light in the metalinmion, and suggest a model for the development of Chl. phaeobacteroides in the lake. Spatial analysis of Chl. phaeobacteroides suggest that a combination of diurnal north-west wind, diurnal oscillations of the metalimnion, and the hours of sunlight, result in a situation in which the metalimnion in some parts of the lake is exposed to slightly higher light intensities than the other parts of the lake, and therefore allows higher concentrations of Chl. phaeobacteroides to develop. The analysis of Chl. phaeobacteroides with time suggests that it can be described fairly well by the logistic growth model.

Successful establishment of the Ponto-Caspian alien cladoceran Evadne anonyx G.O. Sars 1897 in low-salinity environment in the Baltic Sea

Pollupuu, M., Simm, M., Pollumae, A., Ojaveer, H. — 2008-03-11

Our new results confirm that the Ponto-Caspian cladoceran Evadne anonyx was first found in the Gulf of Finland in 1999. In the Gulf of Riga, the first specimens were detected in 2000. The observed distribution pattern, together with the recorded increase in the population density of the species of ca. 10 times during the years 2000-2006 indicate recent successful establishment of E. anonyx in the low-salinity conditions (< 5 psu) of the NE Baltic Sea. E. anonyx and the native E. nordmanni are present in mesozooplankton community from May to October. In the seasonal cyle, the maximum abundance of E. anonyx may occur later in the season than that of E. nordmanni. As the fecundity of the alien E. anonyx significantly exceeds that of the native E. nordmanni we suggest that population abundance of E. anonyx will very likely increase in future and the species may colonize new areas in the recently invaded ecosystem.

Heterogeneity of LSU rDNA sequences and morphology of Gymnodinium catenatum dinoflagellate strains in Bahia Concepcion, Gulf of California, Mexico

2008-03-08

We report the LSU rDNA sequences and morphology of several Gymnodinium catenatum dinoflagellate strains isolated from Bahía Concepción, Mexico. PCR amplifications of the D1-D2 fragment of the nuclear large subunit rDNA gene resulted in a single product of approximately 889 bp. A phylogenetic tree was constructed from the partial LSU rDNA (350 bp) sequences from six strains of G. catenatum isolated from the bay and 12 isolates from around the world. Strains from the bay grouped within the G. catenatum clade, however a constant characteristic of G. catenatum strains from the bay is that, at position 453, a single nucleotide polymorphism was observed, presenting cytosine instead of guanine. This single base polymorphism could indicate a mutation or genetic isolation from other G. catenum populations. It is suggested that the Western Pacific could be the ancestral population of this species. Significant differences were found between cell length (CL), cell width (CW), and cell width/cell length ratios in laboratory cultures in the late exponential growth phase. Average cell length varied between 41 and 53.69 µm, with a range between 26.94 and 69.99 µm; average cell width varied between 33.75 and 40.68 µm, with a range between 25.04 and 54.73 µm; the average CW/CL varied between 0.76 and 0.97 µm, with a range between 0.49 and 1.27 µm. Cingulum thickness was between 3 and 4.11 µm, with a range between 1.97 and 6.74 µm. Cell size varied with the length of the chain, and position in the chain.

Feeding and egg production of the planktonic copepod Calanus sinicus in spring and autumn in the Yellow Sea, China

Huo, Y.-Z., Wang, S.-W., Sun, S., Li, C.-L., Liu, M.-T. — 2008-03-04

Shipboard incubations were conducted in spring (April) and autumn (October/November) 2006 to measure the feeding and egg production rates (EPR) of Calanus sinicus in the Yellow Sea, China. The ingestion rate (2.08-11.46 and 0.26-3.70 µgC female^-1 d^-1 in spring and autumn, respectively) was positively correlated with microplankton carbon concentrations. In the northern part of the Yellow Sea, feeding on microplankton easily covers the respiratory and production requirements, whereas in the southern part in spring and in the frontal zone in autumn, C. sinicus must ingest alternative food sources. Low ingestion rates, no egg production and the dominance of the fifth copepodite (CV) stage indicated that C. sinicus was in quiescence inside the Yellow Sea Cold Bottom Water (YSCBW) area in autumn. C. sinicus ingested ciliates preferentially over other components of the microplankton. The EPR (0.16-12.6 eggs female^-1 d^-1 in spring and 11.4 eggs female^-1 d^-1 at only one station in autumn) increased with ciliate standing stock. Gross growth efficiency (GGE) was 13.4% (3-39%) in spring, which was correlated with the proportion of ciliates in the diet. These results indicate that ciliates have higher nutrient quality than other food items, but the low GGE indicates that the diet of C. sinicus is nutritionally incomplete.

Niche segregation factors in an assemblage of pelagic rotifers of a deep high-mountain lake (Redon, Pyrenees)

Bartumeus, F., Catalan, J. — 2008-03-03

The niche space of a species has been defined as the region in N-dimensional environmental hyperspace in which the fitness of the individuals is positive. In zooplankton assemblages, there has been little consideration of how different density-dependent (i.e. resources) and density-independent (i.e. conditions) environmental factors relate to fitness components in the field, namely survival and reproduction. We studied the abundance and reproduction (egg ratio) variability of seven species of pelagic rotifers living in a high-mountain lake and their relation to a number of environmental factors by means of partial canonical correspondence analyses. The generally higher explanatory capacity of conditions over resources suggests that habitat partitioning among species largely relies on historical processes, which are internalised in life history traits of spatial and temporal habitat use. However, the increase in the relevance of resources when considering reproduction, as compared to abundance, indicates that there is still an on-going interactive niche segregation process among rotifer species, which affects present and future rotifer assemblages in the lake. Our study shows the value of considering measurements close to fitness components (e.g. egg ratio) for detecting on-going niche segregation processes in zooplankton, which may help to resolve paradoxes in relation to species coexistence in natural assemblages.

The response of zooplankton assemblages to variations in the water quality of four man-made lakes in semi-arid northeastern Brazil

Sousa, W., Attayde, J. L., Rocha, E. D. S., Eskinazi-Sant'Anna, E. M. — 2008-02-28

The response of zooplankton assemblages to variations in the water quality of four man-made lakes, caused by eutrophication and siltation, was investigated by means of canonical correspondence analysis. Monte Carlo simulations using the CCA eigenvalues as test statistics revealed that changes in zooplankton species composition along the environmental gradients of trophic state and abiogenic turbidity were highly significant. Brachionus calyciflorus, Thermocyclops sp. and Argyrodiaptomus sp. were good indicators of eutrophic conditions, whereas Brachionus dolabratus, Keratella tropica and Hexarthra mira were good indicators of high turbidity due to suspended sediments. Overall, our results showed that changes in the water quality of man-made lakes in a tropical semi-arid region have significant effects on the structure of zooplankton assemblages that can potentially affect the functioning of these ecosystems.

WANTED: ALIVE AND NOT DEAD - FUNCTIONING DIATOM STATUS IS A QUALITY CUE FOR THE SUSPENSION-FEEDER CRASSOSTREA GIGAS

Beninger, P. G., Valdizan, A., Cognie, B., Guiheneuf, F., Decottignies, P. — 2008-02-28

Understanding the quality cues which govern the grazing of phytoplankton by suspension-feeders is essential to understanding seston dynamics and benthic-pelagic coupling. We studied the effect of functioning cellular status on selection of two common coastal diatoms, Coscinodiscus perforatus var. pavillardii and Actinoptychus senarius, grazed upon by the commercially-farmed oyster, Crassostrea gigas Thunberg. Two contexts were investigated, corresponding to those naturally encountered by oysters and their prey: (1) diatoms (C. perforatus var. pavillardii) larger than the size allowing selection on the gill (hence subject to selection on the labial palps only), and (2) diatoms (A. senarius) within the size range allowing selection on the gill (hence subject to selection on both the gills and labial palps). Both intact and naturally-dead cells possessed a perifrustular envelope, but the naturally-dead cells lacked intracellular organic content. Oysters were able to discriminate between intact and naturally-dead (empty) cells, both in the absence of prior selection on the gill, and subsequent to prior selection on the gill. In contrast to previous findings in scallops, functioning cellular status is thus an important determinant of diatom selection and ingestion by oysters. This fine degree of discrimination may have evolved as an adaptation to the high-turbidity, low food - quality estuarine environment typically colonized by oysters. The reduced organic content of rejected diatom cells in oyster pseudofaeces may affect the organic content of biodeposits, and subsequently the dynamics of nutrient release to the water column during mixing and resuspension.

Effects of intra- and inter-annual variability in prey field on the feeding selectivity of larval Atlantic mackerel (Scomber scombrus)

Robert, D., Castonguay, M., Fortier, L. — 2008-02-19

We identified to the lowest taxonomical level possible the preferred prey of Atlantic mackerel larvae from the southern Gulf of St. Lawrence, and assessed the extent to which prey selectivity varied within and among years. Mackerel larvae and their zooplankton prey were sampled in the summer of four consecutive years (1997-2000). The nauplii of the calanoid copepod Pseudocalanus sp. strongly dominated the diet of larvae <7 mm both in terms of numbers and carbon content, while larvae ≥7 mm mainly fed on fish larvae (including conspecifics) and cladocerans. Chesson's alpha index revealed strong selectivity in all years for Pseudocalanus sp. nauplii in first-feeding larvae. Selectivity shifted to cladocerans and fish larvae around a body length of 7 mm. Intra- and inter-annual prey selectivity changes were mainly observed for alternative prey, during the period surrounding the shift in diet from small to large prey. Our results underscore the importance of considering the availability of the main prey Pseudocalanus sp. nauplii (early larval stage) as well as cladocerans and fish larvae (late larval stage), rather than the entire prey field in the future assessment of the role played by prey availability on larval mackerel vital rates.

Population dynamics and life strategies of Rhincalanus nasutus (Copepoda) at the onset of the spring bloom in the Gulf of Aqaba (Red Sea)

2008-02-18

Abundance, distribution, population structure, lipid content, lipid composition and reproductive and feeding activity of Rhincalanus nasutus were studied in the Gulf of Aqaba and in the northern Red Sea during RV "Meteor"-cruise M 44-2 in February/March 1999. R. nasutus occurred in higher numbers in the Gulf of Aqaba (585 ind. m^–2) than in the northern Red Sea (254 ind. m^–2). Young developmental stages (nauplii, copepodite stages CI and CII) were absent. In the southern Gulf of Aqaba, the bulk of the population developed from stage CV to adult in the course of the 3-week study period. In contrast, immature CV stages dominated at the adjacent stations in the northern Gulf of Aqaba and in the northern Red Sea. Development was associated with the seasonal vertical migration from wintering mid-water layers and initiation of feeding starting as early as beginning of March in the southern Gulf of Aqaba. No upward migration was observed in the northern parts of the Gulf and in the northern Red Sea, where more than 90% of the females remained immature during our study. Lipids were dominated by wax esters in females and CV. The fatty acid and fatty alcohol compositions of females were very similar throughout the study region and period. Major fatty acids were 18:1(n-9) 16:1(n-7), 16:2(n-4) and 20:5(n-3). Our results support previous reports of a seasonal dormancy of R. nasutus in the Gulf of Aqaba and suggest that the timing of vertical migration, feeding and maturation is closely coupled to the development of the spring bloom in oligotrophic subtropical waters.

Phytoplankton diel and vertical variability in photobiological responses at a coastal station in the Mediterranean Sea

Brunet, C., Casotti, R., Vantrepotte, V. — 2008-02-13

Photophysiological parameters provide useful insights into the effects of environmental forcings on phytoplankton physiology. We present data on the short-term photoacclimative responses of phytoplankton throughout the water column during a diel sampling (every 1.5-3 h for 33 h) at a coastal station in the Gulf of Naples (Italy) in November 1996. Liposoluble pigments (HPLC), variable fluorescence (Prim-Prod probe) and picoplankton cell counts and autofluorescence (flow cytometry) were investigated every 1.5 to 3 h over a period of 33 hours. The phytoplankton was phased to the alternation of light and dark and also showed acclimation to the different light intensities. Photoprotective pigments were synthesized during the day at the surface (0 to 20 m), and were significantly correlated with light intensity changes, as well as with the effective quantum yield of fluorescence. At night, recovery of photosystems from excess light was observed as was redistribution of nutrients and algae due to vertical convective motions caused by thermal dissipation. Equations linking photobiological parameters and time or light evolution were inferred to obtain kinetic coefficients. These were then used as biological tracers of vertical mixing whose velocity in the surface layer was estimated to be <0.05 cm sec^–1.

Multifactorial nature of rotifer water layer preferences in an oligotrophic lake

Obertegger, U., Flaim, G., Sommaruga, R. — 2008-02-13

Rotifer preference for the upper (0 - 2 m) or deeper layer (5 - 35 m) of the water column was assessed at midday and midnight in an oligotrophic mountain lake during summer, and related to temperature, food availability, presence of predators, and exposure to solar ultraviolet radiation (UVR). Whereas Keratella cochlearis and Synchaeta pectinata showed a population maximum in the deeper layer during midday and in the upper layer during midnight, Asplanchna priodonta, Synchaeta kitina, and Filinia terminalis always remained in the deeper layer. In contrast, Polyarthra dolichoptera and Synchaeta grandis were the only rotifer species that remained in the upper layer. Possession of mycosporine-like amino acids (MAAs), a family of photoprotective compounds seemed to be an important strategy for occupying the upper layer. For other species, midday positioning in the deeper layer seemed to be related to UVR avoidance rather than to predation pressure, while the upward shift at night was species dependent. Migrating species seemed favoured by higher temperatures in the upper layer, while non-migrating species seemed restricted by factors such as food supply. Our study indicates that rotifers exhibit different species-specific strategies for dealing with factors such as UVR exposure, temperature, and food availability.

Reproduction, growth and secondary production of Pseudocalanus elongatus Boeck (Copepoda, Calanoida) in the southern North Sea

Renz, J., Mengedoht, D., Hirche, H.-J. — 2008-02-11

The seasonal cycle of population dynamics and production of the dominant copepod Pseudocalanus elongatus in the southern North Sea was described using for the first time concurrent shipboard measurements of abundance, egg production and stage durations. Biological and physical parameters were measured from February 2004 to May 2005. Highest population abundance (592*10³ n m^–2) was recorded in June. Maximum egg production rate (EPR) was 9.1 eggs f^–1 day^–1 in April 2004. High EPR occurred at several stations where chl a concentration was low, suggesting P. elongatus to be able to use alternative food resources like microzooplankton and detritus or to maintain high production rates by using lipid reserves. Stage durations derived from molting experiments ranged from 1 day (CII) in February up to 9.2 days (CV) in April, with fastest development for most stages occurring in April. The increasing temperature in summer did not further shorten developmental times. Therefore food might be an important determinant of development of P. elongatus in the North Sea. Estimated generation times in August based on shipboard measurements of stage durations were 2.3 times longer than predicted from laboratory investigations and point towards the importance of field measurements when calculating secondary production.

Evaluation of a new technique for characterising resting stage zooplankton assemblages in riverine slackwater habitats and floodplain wetlands

Ning, N. S. P., Nielsen, D. L., Hillman, T. J., Suter, P. J. — 2008-02-11

We evaluated a new technique to characterise resting stage zooplankton assemblages in both riverine slackwater habitats and floodplain wetlands, using coconut fibre mats (CFMs). Results obtained using the CFMs were compared with those obtained using the conventional sediment coring technique, over three different mat-setting durations. Presently, there is a paucity of suitable sampling techniques available for characterising resting stage zooplankton assemblages, particularly for assessing resting stage production and/or use in intervention experiments. Results from this study indicate that, for slackwaters and wetlands, the structure and composition of resting stage communities obtained using mats did not differ from that obtained using the sediment coring technique, irrespective of the mat-setting duration. This suggests that the CFM technique can be used as an effective surrogate for the sediment coring technique to characterise resting stage assemblages in slackwaters and floodplain wetlands. In addition, the CFM technique offers the capacity to measure resting stage production over a known time period, and can be used in intervention experiments. Hence, the mats should prove to be useful in a number of settings, ranging from investigative research to management applications.

Feeding of tropical cladocerans (Moina micrura, Diaphanosoma excisum) and rotifer (Brachionus calyciflorus) on natural phytoplankton: effect of phytoplankton size-structure

Pagano, M. — 2008-02-11

The proliferation of large phytoplankton in tropical shallow freshwater ecosystems may be attributable to inefficient feeding by the dominant zooplankton (small cladocerans and rotifers) on large particles, but more information on the feeding behavior of tropical organisms is required to explore this hypothesis. In this study, food size selectivity and functional feeding responses of three major tropical freshwater zooplankton species (Moina micrura, Diaphanosoma excisum and Brachionus calyciflorus) were studied to test their ability to control phytoplankton. Eleven grazing experiments were performed, using natural phytoplankton assemblages as a food source. M. micrura fed efficiently on a wide range of sizes of phytoplankton particles, from unicellular picoplankton Chlorella sp. (2-4 µm equivalent spherical diameter, ESD) to large Coelastrum reticulatum coenobia (20-40 µm ESD) but the selectivity depended on the nature and size distribution of the phytoplankton. D. excisum ingested only very small particles (Monoraphidium, Chlorella). B. calyciflorus fed on a wide size range but showed a clear preference for the largest algae (Cyclotella sp, Scenedesmus opoliensis). These three species increased their ingestion rate linearly with the food concentration and the saturation point was reached for M. micrura within the range of experimental conditions. The results suggest a strong food partitioning between these three species and showed that B. calyciflorus and M. micrura were better able to exploit and control algal blooms than D. excisum, which was a more selective feeder controlled by the availability of small food particles.

Distribution and multi-annual abundance trends of the copepod Temora longicornis in the US Northeast Shelf Ecosystem

Kane, J., Prezioso, J. — 2008-02-09

The average spatial distribution and annual abundance cycle are described for the copepod Temora longicornis from samples collected on broadscale surveys (1977 - 2006) and along Continuous Plankton Recorder (CPR) transects (1961 -2006) of US Northeast continental shelf waters. After its annual low in winter, T. longicornis abundance begins to increase in coastal waters with the northern progression of spring conditions. Annual maximum shelf concentrations were found in the more southern inshore waters of the region during the summer months. Abundance throughout most of the ecosystem increased sharply in the early 1990s and remained high through 2001. During this period the copepod became more numerous and widespread in offshore shelf waters. Abundance declined to approximately average levels in 2002 for the remainder of the time series, but its extended offshore range remained intact. Correlation analysis found that the copepods interannual abundance variability had a significant negative relationship with surface salinity anomalies throughout the ecosystem, with higher correlations found in the northernmost subareas. T. longicornis abundance in the ecosystem's southernmost subarea (Middle Atlantic Bight) did not increase in the 1990s and was found to be negatively correlated to surface temperature, indicating that continued global warming could adversely impact the copepods annual abundance cycle in this region.

MOLECULAR SYSTEMATICS OF THE PLEUROCHRYSIDACEAE, A FAMILY OF COASTAL COCCOLITHOPHORES (HAPTOPHYTA)

Saez, A. G., Zaldivar-Riveron, A., Medlin, L. K. — 2008-02-09

Within the marine phytoplankton community, coccolithophores are important players in the carbon cycle, because of their great abundance and unusual ability to form an external cover of calcium carbonate. Most described species are oceanic, and so little attention has been devoted to coastal species, the majority of which belong to two families: Pleurochrysidaceae and Hymenomonadaceae. Here, we study the biodiversity and evolutionary relationships within the Pleurochrysidaceae, for which we have compiled DNA sequences from a total of 18 strains plus three members of the Hymenomonadaceae. Most of the analysed sequences have been obtained for this study and belong to three genetic fragments: 18S rDNA, elongation factor Tu (tufA), and internal transcribed spacers 1 and 2, which include the 5.8S rDNA. We find some discordance between the current taxonomy of the group and our molecular analyses. First, we observe that several different recognised species are in fact highly similar taxa, probably belonging to the same evolutionary lineages. Secondly, we show some unexpected phylogenetic relationships. In the light of these results, we suggest that, when possible, DNA sequences be considered in future taxonomic classifications of the Pleurochrysidaceae and other planktonic organisms.

Scale patterns of diel distribution of the copepod Cyclops abyssorum in a regulated lake: the relative importance of physical and biological factors

Ludovisi, A., Todini, C., Pandolfi, P., Taticchi, M. I. — 2008-02-09

The relative importance of hydrological and biological factors in driving the diel distribution of the copepod Cyclops abyssorum from small to large scale is here evaluated in a lake where a complex hydrodynamics is at work and a clear environmental gradient is present. A set of statistical tools (which include the proposed "partial variance") are used for characterising the scale patterns of distribution and their daily trend. A day-night aggregation/dispersion process is observed at any of the spatial scales and directions considered. The value of the fractal dimension of the copepod distribution (D-horizontal =1.89) suggests that short-range effects prevail over long-range ones in affecting the overall complexity of distribution. The geostatistical analysis shows that individuals form isotropic swarm-like assemblages of a few meter diameter during daytime which relax during night. The cohesion among population members of different age and sex also shows a daily fluctuation, with separation distance increasing vertically during daytime and horizontally during night. The present study shows how visual predation affects the whole structure of patchiness and explains the diel spacing among population fractions, whereas food availability prevails over water transport in driving the copepod distribution at a large scale.

On the effect of abiotic environmental factors on production of bioactive oligopeptides in field populations of Planktothrix spp. (Cyanobacteria)

Halstvedt, C. B., Rohrlack, T., Ptacnik, R., Edvardsen, B. — 2008-02-08

The variability in abundance of bioactive oligopeptides in Planktothrix spp. biomass in Lake Steinsfjorden, Norway, was examined for the period May 2003 - August 2004, both temporally (over the year) and spatially (by depth). Of the 33 oligopeptides recorded, desmethyl-microcystin RR and LR, anabaenopeptin B, aeruginosin 583, oscillamide Y, oscillaginin B, and oscillapeptin G were studied in detail. Of these desmethyl-microcystin RR had the highest average concentration (2.5 ± 3.0 µg L^–1) and showed similar distributional patterns as the Planktothrix spp. biomass. Multivariate regression models, called generalized additive models (GAMs), were used to investigate the effect of temperature, irradiance, macronutrients, depth, and date on the cellular oligopeptide content of Planktothrix spp. GAMs explained between 50 and 94 % of the total variance in the abundance of oligopeptides per unit Planktothrix spp. biomass, and also ranked parameters according to their contribution to this variance. Date was the most important contributor, linked to the waxing and waning of the four Planktothrix chemotypes found in Lake Steinsfjorden. There was an overall trend of increase in oligopeptide abundances per unit Planktothrix spp. biomass by depth. The factors affecting growth, temperature, irradiance and macronutrients had only a minor effect, explaining on average less than 10 % of the variance included in the statistical models. The various classes of oligopeptides appeared to be influenced by environmental factors in a similar manner, suggesting that these oligopeptides may have a similar function in cyanobacteria.

Use of fluorescence information for automated phytoplankton investigation by image analysis

Hense, B. A., Gais, P., Jutting, U., Scherb, H., Rodenacker, K. — 2008-02-07

Automated identification and quantification of algae in microscopic images is a tool that allows high taxonomic resolution with reasonable technical efforts. However, in samples containing various non-algal objects this is still not a satisfactorily solved problem. We show that autofluorescence information improves discrimination of algae from non-algal objects as well as phycoerythrin containing algae from others. We analyze the stability of the autofluorescence to estimate its constraints. Cold and dark storage of glutaraldehyde fixed samples maintains autofluorescence sufficiently for three weeks. Under repeated excitations, chlorophyll a or phycoerythrin autofluorescence show an exponential decrease followed by an intermediate maximum. A peak also occurs in emission wavelength ranges without chlorophyll and phycoerythrin fluorescence. The unspecific autofluorescence causing the peaks is at least partly identical with the blue-green fluorescence (BGF) in plant cells. BGF interferes with identification of algae, thus correction of pigment autofluorescence with such unspecific fluorescence allows a more reliable algal discrimination procedure. A classification scheme for discrimination of chlorophyll a and phycoerythrin containing algae shows a high performance in a test with natural samples. Integration of fluorescence and bright-field image information provides a powerful tool for phytoplankton analysis in complex samples.

A global estimation of mesozooplankton ammonium excretion in the open ocean

Hernandez-Leon, S., Fraga, C., Ikeda, T. — 2008-02-05

Mesozooplankton ammonium excretion rates in the ocean, based on published data, were studied across all latitudes in order to assess the amount of ammonium excretion by this community. Specific ammonium excretion rates were highest in equatorial waters and decreased rapidly pole-ward. Global community excretion in the upper 200 m of the oceans, integrated over all latitudes, accounted in general for 1.78 ±0.60 Gt N yr^–1. Excretion rates showed a decrease from tropical (0.65 ±0.14 GtN yr^–1) to polar waters (0.05 ±0.02 GtN yr^–1). The substrate metabolized by organisms as indicated by the O/N ratio showed a protein-based metabolism of mesozooplankton with the exception of the temperate and subpolar regions, which showed a more lipid-based metabolism. The largest contribution of ammonium to autotrophs was observed in the tropical and subtropical areas, decreasing to polar areas. On a global basis, nutrient regeneration by mesozooplankton in the oceanic realm was estimated to be in the range of 12-23% of the requirements for phytoplankton and bacterial production.

"Impact of large storm events with different meteorological characteristics on estuarine ciliate biomass"

Wetz, M. S., Paerl, H. W. — 2008-02-05

Microzooplankton are important consumers of phytoplankton production in estuaries and are links to higher trophic levels. We examined the impact of several large storms varying in their meteorological characteristics on the distribution of ciliates, a key component of the microzooplankton community in North Carolina's Neuse R. Estuary (NRE). Ciliate biomass was largely unaffected by Tropical Storm ("TS") Helene, except for dissipation of a patch of elevated ciliate biomass at a frontal zone in the upper estuary. Following passage of Hurricane ("H") Isabel, mean ciliate biomass increased 2-fold, with the most dramatic increases occurring in the upstream region that was influenced by freshwater runoff. H Alex had minimal impact on ciliate biomass, but shortly after its passage, Tropical Depression Bonnie and TS Charley passed over the region with significant rainfall. Ten days after passage of those storms, ciliate biomass increased throughout the NRE, with the most dramatic increases again seen in the upper freshwater-influenced region of the estuary. Overall, these findings suggest that the response of estuarine ciliates to storm events is complex, although some coherent patterns were detected in terms of the overall biomass response. Given that storms represent important drivers of ecological processes in estuaries and that the U.S. East and Gulf Coasts are in a period of elevated storm activity, further work is warranted to fully understand the impact of storms on trophic transfer in estuaries.

Monthly occurrence of parasites of the chaetognath Sagitta friderici off Mar del Plata, Argentina

Daponte, M.C., de Pertierra, A. A. G., Palmieri, M.A., de Nunez, M. O. — 2008-02-05

During a one-year study period between March 2000 and 2001, 195 (6%) out of 3236 Sagitta friderici were found to be parasitised by metacercariae of digenean species and metacestodes. Parasites belonged to Derogenes sp., Ectenurus virgulus, Monascus filiformis, Lepocreadiidae, Parahemiurus merus and tetraphyllidean metacestodes. Among the chaetognaths parasitised, 178 (91%) harboured one parasite, and the remaining 17 (9%) showed 2 or 3 parasites of the same or two different species. The highest prevalences for most of the species occurred in autumn-winter (April-August), except for E. virgulus, which had maximum values in summer (December-January). Species richness varied during the year, reaching a peak in May and June 2000 with 6 species. The metacercariae of Lepocreadiidae were present in autumn, winter and summer at all maturity stages of S. friderici, with higher prevalence values for stages 0 and I, whereas the rest of the digeneans were absent in stage 0, with higher prevalences for the more advanced maturity stages II and III. The different types of metacercariae did not seem to induce castration in the host. Copepods (Corycaeus amazonicus) were observed attached to the body of chaetognaths or partially digested in their gut. No nematodes, internal parasitic copepods or polychaetes were found.

Microcopepod community structure in the Gulf of Aqaba and northern Red Sea, with special reference to Oncaeidae.

Bottger-Schnack, R., Schnack, D., Hagen, W. — 2008-02-05

The abundance and vertical distribution of microcopepods sampled by nets with 55 µm mesh size was compared for two neighbouring but hydrographically different areas, the Gulf of Aqaba and the northernmost Red Sea, during spring 1999. The vertical structure of the total microcopepod communities differed considerably between the two regimes: In the stratified waters of the Red Sea, calanoids outnumbered oncaeids as well as oithonids at 0-100 m, whereas oncaeids dominated in all meso- and bathypelagic layers below 100 m depth. In the unusually deep vertically mixed waters of the Gulf of Aqaba, calanoids outnumbered each of the non-calanoid taxa as deep as 250 m or 350 m, while the oncaeid dominated deep water community was restricted to depth ranges below 400 m. Dominant non-calanoid species in both areas were Oncaea bispinosa, Paroithona pacifica, Oithona simplex, Spinoncaea ivlevi, Oncaea tregoubovi and Oncaea cristata. Oncaea scottodicarloi occurred in exceptionally high numbers in the northern Gulf. Pronounced differences between the two areas were found in the vertical distribution of poecilostomatoid species. By comparing the present results with published data from the central and southern Red Sea and other tropical and warm-temperate oceanic areas, intra- and interoceanic differences in the structure of microcopepod communities in oligotrophic areas are discussed. The high abundance and vertically extended range of calanoid copepods during spring appears to be a specific feature of the Gulf of Aqaba, indicating an unusual vertical succession in the trophodynamic structure of the copepod fauna in this area.

Limnoecology - The Ecology of Lakes and Streams

Weisse, T. — 2008-02-01

Life history and population dynamics of the marine cladoceran Penilia avirostris (Branchiopoda: Cladocera) in the Catalan Sea (NW Mediterranean)

Atienza, D., Saiz, E., Skovgaard, A., Trepat, I., Calbet, A. — 2008-02-01

Penilia avirostris is a cosmopolitan marine cladoceran that inhabits coastal and shelf waters of most low and mid latitude ecosystems. In this study we describe the life history and population dynamics of P. avirostris at a fixed coastal station in the NW Mediterranean Sea. This marine cladoceran was very seasonal, having population maxima in summer (2500-3000 ind m^–3), and being practically absent from the water column for the rest of the year. The population typically collapsed in late summer – early autumn, and this decline was accompanied by a shift to gametogenetic reproduction, allowing the production of resting eggs to ensure the continuity of the species in the following season. Estimated birth rates of P. avirostris in the Catalan Sea ranged between 0.097 and 0.46 d^–1 and seemed to be decoupled from changes in P. avirostris standing stocks. Birth rates were mainly dependent on the changes in abundance of embryo-carrying females and their brood size, because during the period of population plateau the temperature-dependent embryonic developmental time was rather similar (2.3 to 2.7 days). Neonates are like miniature adults and need a very short period of growth and moulting to reach the reproductive stage. This life-history trait can explain the fast blooming and dominance of plankton communities by P. avirostris under suitable conditions. Finally, the causes for the seasonal decline of P. avirostris populations are also discussed.

Morphology of the final stage phyllosoma larva of Scyllarus pygmaeus (Crustacea: Decapoda: Scyllaridae), identified by DNA analysis

Palero, F., Guerao, G., Abello, P. — 2008-01-31

The morphology of the final phyllosoma larval stage of the slipper lobster Scyllarus pygmaeus is described and illustrated based on larvae captured from Mediterranean waters and compared with those described in other Scyllaridae. These larvae were always found in deep waters (>200 m). Nucleotide sequence analysis of a region of the nuclear 28S rDNA gene identified these larvae as S. pygmaeus, the morphology of which had been previously un-described. An intensive review revealed a misidentification of Scyllarus arctus larvae in the literature during the last 180 years, since the identification of S. pygmaeus larvae as Chrysoma mediterraneum(Risso, 1827). Detailed examination indicated that the final-stage larvae examined belonged to a clearly defined phyllosoma larval group within the genus Scyllarus, morphologically very similar to some phyllosoma larvae collected from Japan, Hawaiian Islands, New Zealand or Juan Fernandez Islands. This constitutes the first complete description of a phyllosoma stage of a member of the family Scyllaridae, with the specific identity of the larva being validated with molecular techniques.

Distribution, growth and survival of anchovy larvae (Engraulis encrasicolus L.) in relation to hydrodynamic and trophic environment in the Bay of Biscay

2008-01-31

European anchovy larvae were sampled in June 2004 in the south-eastern part of the Bay of Biscay (limits: 4°W, 46°N). Larval distribution was analysed and compared with spawning distribution in previous weeks. Patches of old larvae were found around the French and Cantabrian shelf edge and even in oceanic waters, far from the main spawning areas, while the youngest larvae appeared in the middle French shelf, associated with adult spawning locations. The analysis of environmental conditions showed significant differences from coastal to oceanic stations as well as in areas under the influence of river discharges. A principal component analysis carried out on temperature, stratification, chlorophyll-a and plankton concentration distinguished 5 areas: Gironde, Northern French Shelf, Southern French Shelf, Cantabrian Shelf and Oceanic area. Growth and mortality rates were estimated for anchovy larvae in these contrasting areas. Advective processes due to dominant north-easterly winds during summer influence larval distribution in the Bay of Biscay after spawning. However higher food concentration on the shelf did not result in significant differences in the growth rate. Mortality rates seem to be negatively related to larval size, although this effect may be partially due to advection. This is the first attempt to study the effect of environmental factors on anchovy larval distribution and mortality in the Bay of Biscay.

PHYTOPLANKTON-LINKED VIABLE NON-CULTURABLE VIBRIO CHOLERAE O1 [VNC] FROM RIVERS IN TUCUMAN, ARGENTINA

2008-01-31

Plankton-linked Vibrio cholerae has been detected in aquatic environments since the first decades of the 20th century. Several mechanisms have been proposed to explain the survival of V. cholerae during inter-epidemic periods, one of which is through existence in its viable non-culturable [VNC] form in biological reservoirs. This latent form of V. cholerae [VNC] would explain the seasonal outbreaks and the way the pathogen survives in the environment. The objectives of this study were to assay V. cholerae O1 in its viable non-culturable form in two rivers in Tucumán, Argentina, as well as its possible association to phytoplankton and its relation to four environmental variables (pH, temperature, conductivity and dissolved oxygen), using direct immunofluorescence.

Analysis of phytoplankton revealed that diatoms were predominant with percentages between 85 and 100%. Nitzschia palea was the only species found at all three sample sites with percentages between 0 and 38%. Of the 54 samples obtained during the 18 sampling periods V. cholerae VNC was detected through direct immunofluorescence in 39% of the cases and at all three sampling sites. Positive samples were analysed for association of VNC with phytoplankton and between 1 and 10 bacteria were found adhered to a single algal cell. This confirms for the first time in northwestern Argentina adherence of this microorganism to the genera Stigeoclonium and Nitzschia as environmental reservoirs. No correlation could be found between the latent form of Vibrio and the environmental variables assayed.

Spatial and temporal distributions of zooplankton in Tampa Bay, Florida, including observations during a HAB event.

Badylak, S., Phlips, E. J. — 2008-01-29

This study describes spatial and temporal distributions of the zooplankton community in Tampa Bay, including their distributions during a bloom of the toxic dinoflagellate Pyrodinium bahamense. Three sites in the bay were sampled thirty times from April 2002 to April 2003. Abundances of holoplankton (copepod nauplii included), meroplankton and microzooplankton were determined using light microscopy. This is the first observation of microzooplankton in Tampa Bay. Phytoplankton composition, temperature, salinity, Secchi depth, total nitrogen, total phosphorus and chlorophyll a were also determined. Generally, the inner bay had higher zooplankton abundances than the mid- and outer regions of the bay. The dominant holoplankton species were copepod nauplii, Oithona colcarva, Acartia tonsa and the larveacean Oikopleura dioica. Bivalve larvae were the most abundant meroplanktonic taxa and tintinnids the most abundant microzooplankton. A major bloom of Pyrodinium bahamense was observed in the inner bay during the summer. During the bloom, numerical abundances of O. colcarva and A. tonsa declined, while abundances of O. dioica mimicked the pattern of P. bahamense abundance. Picoplankton and diatom concentrations also decreased during the bloom. Spatial patterns and temporal trends in zooplankton distribution suggest that regional differences in water residence times, salinity regimes and grazing pressures are important driving factors in the composition and abundance of zooplankton in Tampa Bay. The results of the study are discussed within the context of historical observation of the zooplankton community.

Molecular probe sets for the detection of toxic algae for use in sandwich hybridisation formats

Diercks, S., Metfies, K., Medlin, L. K. — 2008-01-29

Molecular probes can be used for early and rapid detection of toxic algal species. The sandwich hybridisation requires two probes for each species, a capture probe and a nearly adjacent signal probe. Probe sets for the species-specific identification of the toxic algal species Gymnodinium catenatum, Protoceratium reticulatum, Lingulodinium polyedrum, Prymnesium parvum, Chrysochromulina polylepis, Pseudo-nitzschia multiseries, P. australis, P. seriata, and P. pungens were designed. A genus probe set for Pseudo-nitzschia species was adapted, and all probe sets were tested for specificity. The target molecules for the probe sets are the large and the small subunit ribosomal RNAs. The specificity of the different probe sets was tested using a sandwich hybridisation assay (SHA) in a microtiter plate with ribosomal RNA isolated from laboratory strains of the target species and closely related species. The assay showed that eight probe sets were highly specific. For Gymnodinium catenatum and Prymnesium parvum, the probe sets detected one non-target species outside the target species. These ten probe sets are valuable tools for identifying and monitoring different toxic algae. The microtiter plate assay is a cheap and effective means for testing probe specificity.

Spatial differences in mortality of Calanus pacificus within the California Current System

Ohman, M. D., Hsieh, C.-h. — 2008-01-28

A Vertical Life Table method was used to estimate patterns of mortality of late developmental stages of Calanus pacificus in the southern sector of the California Current System from 7 CalCOFI cruises (3 spring, 4 late autumn-early winter). Copepodid stage durations were described as a continuous function of both food concentration and temperature from reanalysis of Vidal (1980). This function was combined with vertical profiles of temperature and chlorophyll a to estimate stage durations in the field and solve for instantaneous mortality rates of two stage pairs: C5/Adult Females and C5/Adult Males. Male mortality rates averaged 2-3 times higher than corresponding female mortality rates. There was no significant difference between seasons (spring vs. winter) in either male or female mortality rates. There was a consistent spatial pattern of higher mortality rates of both males and females in the inshore region relative to the offshore region. This inshore increase in mortality rates is the opposite that expected from the effects of food limitation, but is correlated with an index of abundance of zooplanktivorous fishes obtained from the CalCOFI ichthyoplankton series (a fishery-independent data source). Elevated predation risk may mitigate the inferred benefits to zooplankton of increased food supply.

The importance of the form of the quota curve and control of non-limiting nutrient transport in phytoplankton models

Flynn, K. J. — 2008-01-27

The description of phytoplankton growth and nutrient acquisition plays a pivotal role in affecting the performance of plankton ecosystem models. A comparison was made between the behaviour of multi-nutrient phytoplankton models with alternative descriptions of the relationships between the nutrient:C quota and growth rate (µ) and for the control of the transport of the non-limiting nutrient. Using Droop kinetics (i.e. employing a hyperbolic curve form fixed by the ratio of maximum:minimum quotas), or conversely a linear quota-µ relationship, gave models that could display behaviour in strong disagreement with that of the optimal model when operated in a dynamic scenario. The appropriate description of the control of the transport of the non-limiting nutrient is also important; in particular, a fixed algal N:P should not be assumed. Behaviour was most likely to be aberrant when P was limiting; behaviour under nutrient limitation when the N:P nutrient supply ratio accorded with Redfield expectations (16:1 mole) was least aberrant. Deviations of model output from expected behaviour would affect the simulation of the dynamics of phytoplankton growth, nutrient cycling, competition, and have serious repercussions in models in which the stoichiometric quality of the phytoplankton prey affects zooplankton activity. It is important that the behaviour of phytoplankton behaviour accords with that of the groups they purport to represent. However, we currently lack the data upon which to parameterise such models.

Influence of abiotic and biotic factors on microcystin content in Microcystis aeruginosa cells in a eutrophic temperate reservoir

2008-01-27

The Sulejow Reservoir (Poland) was sampled on a weekly basis between May and September in 2003 and 2004 to examine changes in weight-specific microcystin content of Microcystis aeruginosa and to examine which abiotic or biotic factors may be the key factors governing microcystin content. The variables examined in this study included: temperature; total and dissolved inorganic phosphorus and nitrogen; and Daphnia biomass. Comparing summertime from both years, despite the similar levels of cyanobacterial biomass, a significant difference between microcystin concentrations was observed which resulted from a difference in microcystin content. Differences in weight-specific microcystin content were discussed in relation to different Daphnia biomass (R = 0.34, n = 40, p < 0.05). It is possible that exposure to Daphnia and/or chemical signals released by predators may have resulted in increased microcystin content of M. aeruginosa. Nevertheless, the influence of weight-specific microcystin content on microcystin concentrations (R = 0.56, n = 40, p < 0.05) was less than that of cyanobacterial biomass (R = 0.82, n = 40, p < 0.05), which was strongly correlated with temperature and phosphorus ability. This study indicated that not only abiotic factors, but also the presence of herbivorous zooplankton, may determine microcystin content of M. aeruginosa.

The grazing impact of microzooplankton off south west Western Australia: As measured by the dilution technique

Paterson, H. L., Knott, B., Koslow, A. J., Waite, A. M. — 2008-01-27

Grazing rates of microzooplankton feeding on picophytoplankton (Flow-cytometry) and total phytoplankton (Chlorophyll a) were measured in the eastern Indian Ocean off south west Western Australia from February 2003 - December 2004. Three sites representing different oceanographic habitats, the coastal lagoon, the outer shelf and the continental slope (1000 m) were sampled. The dilution method of Landry and Hassett (Landry and Hassett, 1982) was used and analysed by chlorophyll a analysis and flow-cytometry. During summer the apparent growth rate of total phytoplankton exceeded loss due to microzooplankton grazing in the lagoon and at the outer shelf. On the slope, the phytoplankton assemblage was always dominated by small cells (<5 µm). Although their apparent growth rates were also higher in summer, these were matched by increasing microzooplankton grazing rates. Saturated feeding responses at the outer shelf and slope stations during summer were detected. In this low prey, low productivity environment, this response is either a new type of threshold feeding or an artefact of the dilution method which would result in an over-estimate of both phytoplankton growth and microzooplankton grazing.

WITHDRAWN: Morphological variation in Alexandrium tamiyavanichii and Alexandrium minutum (Dinophyceae)

Lim, P.-T., Ogata, T. — 2005-05-26

This paper has been withdrawn at the request of the authors.