Journal of Plankton Research - recent issues

An advanced laser-based fluorescence microstructure profiler (TurboMAP-L) for measuring bio-physical coupling in aquatic systems

Doubell, M. J., Yamazaki, H., Li, H., Kokubu, Y. — Wed, 04 Nov 2009 06:08:08 PST

An advanced model of the Turbulence Microstructure Acquisition Profiler, TurboMAP-L, to investigate the coupling between phytoplankton and turbulence in aquatic systems is described. The profiler provides simultaneous measurement of turbulence, hydrographic and biological parameters at sampling rates between 64 and 512 Hz. Specifically, the addition of a new laser fluorescence-based sensor extends the measurement of in situ chlorophyll fluorescence distributions to millimeter scales. Complementary information on phytoplankton patch and particulate matter size and spatial structure are obtained through the attachment of a separate CMOS mini-camera system. Images of the TurboMAP-L LED fluorescence/turbidity sensor sample volume are obtained with 330 x 330 µm resolution. Results with respect to the performance of the laser sensor and camera system are presented from laboratory tests and field experiments conducted in coastal waters off Tokyo, Japan.

Regulation of filter-feeding house components in response to varying food regimes in the appendicularian, Oikopleura dioica

Wed, 04 Nov 2009 06:08:08 PST

The frequent repetitive secretion of filter-feeding houses of the tunicate, Oikopleura dioica represents investment of a substantial proportion of total body carbon. Despite this investment, the filter-feeding strategy of pelagic tunicates has been proposed as an adaptation to oligotrophic environments. Here, we examined the capacity of O. dioica to modify its house renewal rate (HRR) and expression of component proteins, oikosins, as well as ingestion rates of different sized particles in response to varying food regimes. There were no significant changes in HRR (0.26 ± 0.07 SD house h^–1) with age or food concentration throughout the life cycle. Our data suggest that the complex pattern of endoreduplicating cycles in the oikoplastic epithelium probably limits the capacity to reduce the energetic output of house replacement as a response to a limiting food environment. On the other hand, at the molecular level, there was differential regulation of component house proteins when animals were cultured in standard versus limiting food regimes. Animals pre-conditioned in each of these regimes and subsequently challenged with an identical mixture of large and small particles exhibited different retention efficiencies of larger food particles. Taken together, these results raise the possibility that a limited ability to modulate house architecture may underlie the differential particle retention efficiencies observed.

Evaluation of DNA extraction and handling procedures for PCR-based copepod feeding studies

Wed, 04 Nov 2009 06:08:08 PST

Molecular methods are becoming increasingly common for taxonomic and ecological studies of marine and freshwater plankton. Recently, nucleic acids have been used as target molecules for identification and quantification of prey species in studies of trophic interactions. A critical step in the quantification of mesozooplankton feeding by molecular analysis is the isolation of microalgal DNA from predator guts and in the food environment. It is essential that total genomic DNA extraction provides maximum quantitative yield suitable for downstream analysis. In this study, we compared the efficacy and experimental variability of eight different protocols for total genomic DNA extraction from free-living microalgae and microalgae within the gut of copepods. We also developed and evaluated different sampling procedures for copepods prior to genomic extraction. The optimal protocol was evaluated using real time quantitative polymerase chain reaction (qPCR) and the integrity of the genomic DNA was determined by amplifying PCR targets of increasing size. Considerable variability was observed between purification protocols. Qiagen DNeasy® Blood & Tissue kit was the most efficient of the tested methods for genomic extraction from both free-living microalgae and microalgae inside copepod guts. Furthermore, the appropriate handling of predator copepods prior to genomic extraction was essential for quantitative gut content estimates.

Seasonal succession and composition of the zooplankton community along an eutrophication and salinity gradient exemplified by Danish waters

Zervoudaki, S., Nielsen, T. G., Carstensen, J. — Wed, 04 Nov 2009 06:08:08 PST

The seasonal distribution of the major components of the zooplankton community, protozooplankton, copepods and cladocerans, along a eutrophication gradient were examined in order to establish if eutrophication through increases in phytoplankton biomass and productivity has an impact on biomass and composition of the zooplankton community. Data on salinity, temperature, inorganic nutrients, chlorophyll a, phytoplankton biomass and production, protozooplankton and mesozooplankton biomass were collected during routine sampling at seven stations representing a eutrophication gradient from open to estuarine waters. Mesozooplankton biomass varied seasonally in all areas with a unimodal pattern characterized by a peak between mid-spring and mid-summer. Total mesozooplankton biomass as well as copepod and cladoceran biomass, decreased with DIN concentration. Salinity and the type of ecosystem (well-mixed shallow estuaries versus stratified open waters) were the major factors controlling the biomass proportion of copepod species. Annual production of the total copepod community as well as the total grazing impact of copepods on primary production was higher in open waters than in estuarine waters. In estuarine type ecosystems, phytoplankton production is underexploited by copepod grazing, whereas in the open type ecosystems, the phytoplankton production alone could not satisfy the carbon demand of copepods stressing the potential importance of protozoans in the copepod diet.

Distribution of hyperiid amphipods (Crustacea) of the southern Gulf of Mexico, summer and winter, 1991

Gasca, R., Manzanilla, H., Suarez-Morales, E. — Wed, 04 Nov 2009 06:08:08 PST

The distribution and abundance of the hyperiid amphipods of the southern Gulf of Mexico was analyzed from zooplankton samples collected during summer and winter 1991. We evaluated the effect of the local hydrographic conditions, characterized by mesoscale eddies, on the hyperiid community. We explored the presumed correlation of hyperiids with siphonophores, one of the groups of gelatinous zooplankton to which they are symbionts. It was expected that hyperiids were more abundant in areas related to cyclonic eddies and less abundant in the anticyclonic eddies than in the surrounding waters, but most differences were not significant. Both our data set and previous observations are not conclusive because they are based on low resolution sampling grids. Hyperiids were significantly more abundant at night. Of the 57 species of hyperiids recorded, Lestrigonus bengalensis was widely dominant in both seasons. It was significantly correlated (Spearman r) with the siphonophore Muggiaea kochi during the winter and with Diphyes dispar in the summer, when this siphonophore was most abundant. It is suggested that the association takes place and is detectable when and where both species are highly abundant. There are other significant siphonophore/amphipod correlations; most are not likely to represent symbiotic associations. Variations of the gulf hyperiid community are related to: (i) the dominance of L. bengalensis, (ii) the seasonal abundance of the siphonophore D. dispar, mainly in relation to L. bengalensis, (iii) the anomalous occurrence of deep-living forms and (iv) day/night variations. The presumed influence of mesoscale eddies on the gulf hyperiid community deserves further study.

Comparison between automated analysis of zooplankton using ZooImage and traditional methodology

Gislason, A., Silva, T. — Wed, 04 Nov 2009 06:08:08 PST

The objective of this study was to evaluate the accuracy of the ZooImage image analysis system for taxonomic classification of zooplankton samples. For this purpose, automated analysis with the ZooImage software was compared with traditional analysis, using zooplankton samples collected in the Iceland Sea in July 2006. When compared with the traditional methodology, ZooImage was able to classify zooplankton into main taxonomic entities (size classes and families or genera in some cases), while being less successful in identifying the zooplankton into species. Other important information, that is difficult and time consuming to obtain by traditional methods such as biomass and size distributions are, however, easily obtained with ZooImage. The automated analysis takes much less time than the traditional methods. While the study confirms that ZooImage is a promising tool for rapidly analysing zooplankton samples, it is also clear that the traditional approach will be needed in future investigations, particularly studies addressing zooplankton community structure, development and life history.

The effects of light availability in shallow, turbid waters: a mesocosm study

Wed, 04 Nov 2009 06:08:08 PST

Seasonal patterns of water transparency in turbid Pampean lakes suggest that the amount of seston is strongly influenced by incident solar radiation. However, the unequivocal identification of light as the major forcing variable is precluded due its co-variation with several potentially confounding variables. In order to specifically investigate the role of light, we performed a mesocosm (3000 L) experiment aimed at investigating the effects of different degrees of light availability on planktonic community structure and metabolism. The experimental setup successfully mimicked the light limiting conditions prevailing in Pampean turbid lakes. We used neutral density filters to create four light treatments, approximately covering the natural annual range of incident irradiance. Our results are consistent with field patterns and light-limitation theory: (i) light attenuation and (ii) primary production increased with light availability. In contrast, the abundance of most grazers displayed negative relationships with light availability. The decreases in the density of grazers could have resulted from changes in the nutritional value of the food. This possibility is also supported by (iii) decreases in seston P-content and (iv) increases in community respiration with increasing light availability.

Photosynthetic picoplankton in Lake Tanganyika: biomass distribution patterns with depth, season and basin

Wed, 04 Nov 2009 06:08:08 PST

Photosynthetic picoplankton (PPP), particularly picocyanobacteria (PCya), are known to be a major component of phytoplankton in oligotrophic waters. We investigated the dynamics of PPP and heterotrophic bacteria (HBact) in Lake Tanganyika during the dry and rainy seasons of 2004 to 2007, in the two basins of this large lake. Flow cytometry analyses showed that PPP was mainly composed by PCya of the phycoerythrin-rich Synechococcus type, with maximal abundances (2.3 x 10⁴–8.2 x 10⁵ cells mL^–1) found in the mixolimnion (10–20 m depth). PPP biomass integrated over the water column depth ranged between 0.41 and 3.09 g C m^–2, with maximal values in the south basin during the dry season (2.28 ± 0.62 g C m^–2 on average). The contribution of PPP to total phytoplankton biomass ranged from 41 to 99%, with highest values in the south basin in the dry season. Cellular measurements by image analysis of epifluorescence microscopy images showed a significant increase of the cell volume of the PCya during this period. Flow cytometry also allowed enumeration of photosynthetic picoeukaryotes. Assuming a conversion factor of 530 fg C cell^–1, they contributed on average to 6% to PPP biomass, except during the dry season in the south basin, where their contribution increased to up to 20% of PPP biomass. Integrated over a 100-m water column depth, PCya biomass was on average 1.4 times higher than HBact biomass. This study establishes reference values for the biomass contribution of this part of the microbial food web, covering for the first time, spatial (different sites), vertical (profiles at different depths) and seasonal variations on a multi-year basis. The results strengthen the view of a major role of PPP in the pelagic food web of large tropical Lake Tanganyika.

Short-term variability of abundance, diversity and activity of estuarine bacterioneuston and bacterioplankton

Wed, 04 Nov 2009 06:08:08 PST

Short-term variability in abundance, heterotrophic activity and structural diversity of bacterioneuston and bacterioplankton were assessed over 24-h cycles at a fixed point in the estuarine system of Ria de Aveiro (Portugal). Lipase activity was the only descriptor of bacterial activity significantly different in samples collected during the day and the night (up to 40% higher during the night) at equivalent tidal conditions. The lack of diel patterns in other microbiological parameters indicates uncoupling of estuarine bacteria from primary production, pointing to the relevance of allochthonous nutrient sources for heterotrophic bacteria in this mesotrophic system. Occurrence of strong positive correlations between the values of microbiological parameters (total bacterial abundance, chlorophyll a concentration, leucine-aminopeptidase, lipase, phosphatase, sulphatase and acetate and glucose incorporation) in bacterioneuston and bacterioplankton suggests that the surface community may originate from the underlying waters (UWs) concentrating at the surface through upward passive transport, and that bacterioneuston and bacterioplankton are subjected to similar environmental regulation. The results also indicate that the microbial enrichment of the surface microlayer relative to the UW is highly variable during the day, pointing to the importance of the sampling time for the outcome of comparative analyses of bacterioneuston and bacterioplankton dynamics.

Bacterial chemotaxis towards the extracellular products of the toxic phytoplankton Heterosigma akashiwo

Seymour, J. R., Ahmed, T., Stocker, R. — Wed, 04 Nov 2009 06:08:08 PST

Marine bacteria exhibit positive chemotactic responses to the extracellular exudates of the toxic phytoplankton Heterosigma akashiwo. In the environment, this will support bacteria–algae associations with potential implications for harmful algal bloom dynamics.

Uptake of dissolved organic matter by ephyrae of two species of scyphomedusae

Skikne, S. A., Sherlock, R. E., Robison, B. H. — Wed, 04 Nov 2009 06:08:09 PST

Ephyrae of Aurelia labiata given dissolved organic matter (DOM) in seawater had higher carbon content than starved ephyrae. DOM-fed ephyrae of Chrysaora colorata did not have significantly higher carbon content than starved ephyrae, although the mean trend was higher. DOM-fed ephyrae of A. labiata developed aberrant morphologies similar to starved ephyrae, and neither starved nor DOM-fed ephyrae of either species developed into medusae. In addition to higher carbon content, there was evidence for DOM utilization by A. labiata shown through uptake of and increased ammonium production after exposure to a fluorescently labeled lysine polymer.

Predictability of patches of neritic salps and doliolids (Tunicata, Thaliacea)

Deibel, D., Paffenhofer, G.-A. — Wed, 04 Nov 2009 06:08:09 PST

The occurrence of large patches of gelatinous zooplankton has for decades been considered to be unpredictable. An evaluation of our own data and published results on abundances of doliolids and salps from ocean margins reveals that a considerable degree of prediction is possible, based upon meteorological and boundary current intrusion dynamics.

Lipids in Aquatic Ecosystems

Pond, D.W. — Wed, 04 Nov 2009 06:08:09 PST

Planktonic ecosystem models: perplexing parameterizations and a failure to fail

Franks, P. J. S. — Mon, 05 Oct 2009 08:56:57 PDT

Planktonic ecosystem models have been used for many decades; modern models are only subtle variations on model structures established in the 1970s or earlier. Here I explore two problems that I see with these models: (i) their formulation and parameterization and (ii) their use. Using nutrient uptake by the phytoplankton as an example, I trace the history of why we use Michaelis–Menten kinetics in our models, and show that this functional form may not be the best representation of nutrient uptake by a diverse and changing phytoplankton community. I then discuss how models are used—not as the hypotheses they are, but more like toasters. I make the point that treating models as hypotheses could lead to much stronger and more robust insights into planktonic dynamics. However, this requires better statistical comparisons of models and data, including the "right" kinds of data. Finally I suggest some ways to move forward, to make planktonic ecosystem models much more powerful tools in our investigations of ocean dynamics.

Photosynthetic picoplankton dynamics in Lake Tahoe: temporal and spatial niche partitioning among prokaryotic and eukaryotic cells

Winder, M. — Mon, 05 Oct 2009 08:56:57 PDT

Dynamics of prokaryotic and eukaryotic picophytoplankton were investigated over a 2-year time period using flow cytometry and combined with an in situ experiment in Lake Tahoe, USA to better characterize to which extent environmental factors control these communities. Pronounced seasonal patterns and clear temporal and spatial partitioning were observed between picocyanobacteria and picoeukaryotes. Picocyanobacteria dominated in the nutrient deficient upper water column during the stratified season, while picoeukaryotes reached maximum abundance during isothermal conditions and maintained high numbers in deep-water layers during the stratified season. Picocyanobacteria were more sensitive to high solar and UV radiation compared with picoeukaryotes, which were not affected by high solar radiation and nutrient enrichment stimulated their growth. The opposing response of these two populations is consistent with their vertical distribution: picocyanobacteria dominate below the 30% isolume and above the nitrocline depth, whereas picoeukaryotes increase in the vicinity of the nitrocline and thus increased nutrient concentration. This spatial separation of picophytoplankton groups along environmental gradients in Lake Tahoe is consistent with other deep-oligotrophic lakes and the marine environment, suggesting that these marine and freshwater organisms have similar ecophysiological requirements. These results highlight that the smallest photosynthetic communities show taxon-specific responses to mixing and resource availability, which affect the structure and dynamics of picophytoplankton.

Cyanobacterial cytoskeleton immunostaining: the detection of cyanobacterial cell lysis induced by planktopeptin BL1125

Mon, 05 Oct 2009 08:56:57 PDT

The aim of this study was to detect cyanobacterial cytoskeletal elements and the cytoskeletal framework using immunostaining with anti-bovine -tubulin mouse monoclonal antibodies. After strong permeabilization of axenic cyanobacterial cell lines, the cytoskeleton elements become visible in all cells. A mild cell permeabilization procedure allows the discrimination between healthy and senescent or otherwise stressed cyanobacteria whose cell integrity has been jeopardized. This technique can be useful to investigate cyanobacterial bloom lysis provoked by various natural or artificial factors. Viruses, among others, are important mortality agents of cyanobacteria. The presence of non-hepatotoxic cyclic cyanopeptides can provoke lysis of non-axenic Microcystis aeruginosa cell lines. This is presumably due to lytic cycle induction in lysogen cyanobacteria. A susceptible cyanobacterial cell line exposed to the depsipeptide planktopeptin BL1125 has been analysed with transmission electron microscopy to corroborate the involvement of virus like particles (VLP) in the process of lysis. VLP that correspond in shape and size to tailed cyanophages have been observed only in samples where the process of lysis has been triggered. The immunostaining of cytoskeletal elements by using epifluorescence and confocal microscopy has confirmed that the lysis expands from single infected cells or cell groups, the focal points, to their immediate environment. Our in vitro experiments demonstrate that lysogen focal point formation, which follows induction by endogenous cyanobacterial cyclic peptides, could constitute, also in the natural environment, the basis for an extremely rapid and extensive cyanobacterial bloom collapse.

Cyanobacterial equilibrium phases in a small tropical impoundment

Soares, M. C. S., Vidal, L. O., Roland, F., Huszar, V. L. M. — Mon, 05 Oct 2009 08:56:57 PDT

The cyanobacterium Limnothrix has been reported especially in shallow, nutrient-rich lakes and lowland rivers of temperate regions. However, the first bloom of a member of this genus in Brazil was reported in Manacás Lake. Phytoplankton seasonal succession was studied weekly for 2 years, with the aim of identifying steady-state periods of Limnothrix dominance. During a 2-year period, L. bicudoi comprised 87% of the total phytoplankton biomass. Two (13- and 7-week period) steady-state phases of L. bicudoi dominance were observed during two cold dry seasons, although cyanobacterial blooms are more expected during summer months, even in the tropics. Compared with temperate systems, L. bicudoi may represent an example of steady-state formation at higher temperatures. Low disturbance during cold dry seasons, especially related to temperatures below 20°C, long mixing periods, nutrient availability and high turbidity have favored these long steady-state phases. Although competitive exclusion is the primary mechanism that selects species towards equilibrium, high adaptability to these conditions seems to be the factor related to the occurrence of these steady-state phases, with the unusual dominance of Limnothrix in a tropical impoundment.

Spatial distribution of autotrophic picoplankton in relation to physical forcings: the Gulf of Cadiz, Strait of Gibraltar and Alboran Sea case study

Echevarria, F., Zabala, L., Corzo, A., Navarro, G., Prieto, L., Macias, D. — Mon, 05 Oct 2009 08:56:57 PDT

In this paper, we analyze the pattern of distribution of three populations of picoplanktonic autotrophs measured using flow cytometry. Prochlorococcus sp., Synechococcus sp. and eukaryotic picoplankton were investigated at 58 stations to the south of the Iberian Peninsula including the northeastern Gulf of Cádiz, Strait of Gibraltar and northwestern Alborán Sea, during July 1995. There were important differences in the oceanographic conditions in these regions, with warmer, low-nutrient and low-chlorophyll waters in the Atlantic sector and colder, high-nutrient and high-chlorophyll waters in the Alborán sector in which upwelling occurred. The biomass concentration of picoplanktonic cells was two times higher in the Atlantic area than in the richer Alborán Sea waters. The average integrated abundance of picophytoplankton was negatively correlated with chlorophyll a concentration as well as with inorganic nitrogen availability. Prochlorococcus and Synechococcus were the most abundant groups, and they showed a complementarity in their spatial distribution, with a shallower and more coastal distribution of Synechococcus and the presence of Prochlorococcus mainly at offshore stations and at deeper depths. The ratio of autotrophic picoplankton/total autotrophic biomass showed significant differences on both sides of the Strait, with much higher values in the Gulf of Cádiz sector.

Species- and stratification-dependent diel vertical migration behaviour of three dinoflagellate species in a laboratory study

Jephson, T., Carlsson, P. — Mon, 05 Oct 2009 08:56:57 PDT

This study shows that different species of the same genus can behave differently in relation to a salinity gradient and suggests that a halocline can act as a barrier to the diel vertical migration (DVM) of dinoflagellate species. In a laboratory study, we found that the dinoflagellates Ceratium furca, C. tripos and Dinophysis acuta did not cross a salinity gradient of 5 psu, but instead exhibited DVM exclusively below and above the halocline. Furthermore, C. tripos aggregated in the halocline during the night and migrated to the surface at midday, whereas C. furca aggregated in the halocline at midday and migrated downwards to the bottom during the dark period. Dinophysis acuta was found exclusively above the halocline, with the highest density in the surface water at midday. The present results suggest that different dinoflagellate species display unique DVM behaviours in stratified waters. The species-specific DVM strategies revealed in this study are of central importance for understanding the ecology of these taxa and to further elucidate both the ecological significance and the mechanisms of DVM.

Effect of marine autotrophic dissolved organic matter (DOM) on Alexandrium catenella in semi-continuous cultures

Loureiro, S., Garces, E., Collos, Y., Vaque, D., Camp, J. — Mon, 05 Oct 2009 08:56:57 PDT

Alexandrium catenella was grown in semi-continuous cultures in L1 growth-medium enriched with concentrated dissolved organic matter (DOM) from a diatom bloom. In medium with full nitrate (880 µM), the average growth rate was 0.32 ± 0.01 day^–1 (L1, control without added DOM). Adding natural marine dissolved organic nitrogen (DON) at levels of 20–30 µM above background (10 µM) led to a higher growth rate (L1+DOM, 0.40 ± 0.00 day^–1). In medium with lower nitrate level (175 µM) and higher DON (L1/5+DOM treatment), both dissolved inorganic nitrogen and DON were used, leading to the highest growth rates (0.43 ± 0.03 day^–1). In medium without nitrate [(L1-N)+DOM treatment], the low ammonium concentrations observed throughout the experiment (<1 µM) as well as the uptake kinetics of A. catenella could not have supported the observed growth rates, leading us to conclude that DON was directly used by this organism, rather than using N remineralized by bacteria (from DON). The decrease of bacteria in DOM enriched bioassays could point to a nutrient limitation and competition with A. catenella for organic matter. Viruses likely contributed as an additional factor to keep the bacterial population from becoming dominant.

Changes in marine prokaryotic community induced by varying types of dissolved organic matter and subsequent grazing pressure

Mon, 05 Oct 2009 08:56:57 PDT

We analysed changes in the abundance, biomass, activity and composition of coastal marine prokaryotic communities after the addition of organic substrates, such as glucose, leucine and yeast extract, and the effect of grazing pressure exerted by nanoflagellates. The addition of a carbon source (i.e. glucose) promoted the growth of Gammaproteobacteria, while a combined source of C and N (i.e. leucine) favoured the development of Alphaproteobacteria. The addition of yeast extract, a complex substrate rich in N and growth factors, promoted the proliferation of Alphaproteobacteria and Gammaproteobacteria. Grazing pressure exerted by nanoflagellates produced marked differences on the size structure of the prokaryotic biomass. A pronounced tendency to filamentation and aggregation was observed in the glucose treatment, while in the case of yeast extract, small and mainly freely dispersed prokaryotes were maintained throughout the incubations. Thus, the final community in the yeast extract treatment showed a high percentage of edible biomass, while an important fraction of potentially grazing-resistant prokaryotes (more than 50% of total prokaryotic biomass) was detected in the microcosms enriched with glucose. These results suggest a marked effect of DOM sources on the development of grazing-resistant prokaryotes.

Thin layer formation during runaway stratification in the tidally dynamic San Francisco Estuary

Bochdansky, A. B., Bollens, S. M. — Mon, 05 Oct 2009 08:56:57 PDT

During the course of a year, we repeatedly collected high-resolution vertical fluorometer data timed to coincide with a specific state during the tidal cycle. The time (end of the ebb during neap tide) and the location (a deep channel half-way between the Golden Gate and the point of tidally averaged bottom salinity of 2 psu) were chosen with the goal to observe runaway stratification. We consistently found at least one pronounced chlorophyll peak in the water column; however, the vertical location of these peaks varied within three types including surface, bottom and subsurface maxima. Our results showed that heterogeneity of chlorophyll in the water column and thin layer formation do occur in systems that are characterized by high tidal flow speeds thus resulting in patchy prey fields for zooplankton that migrate in tidal cycles through the water column. Using these chlorophyll profiles, approximate calculations suggested that only during the spring phytoplankton bloom did all layers of the water column provide sufficient food for maximum egg production of the San Francisco Estuary copepod Acartia sp.

Variation in diatom biochemical composition during a simulated bloom and its effect on copepod production

Diekmann, A. B. S., Peck, M. A., Holste, L., St John, M. A., Campbell, R. W. — Mon, 05 Oct 2009 08:56:57 PDT

The biochemical quality of phytoplankton depends in part upon nutrient availability, which has implications for the population dynamics of grazers. Here, we examined how nutrient availability influenced the growth dynamics and biochemical content of the marine diatom Thalassiosira weissflogii and how these biochemical changes impacted the vital rates of a calanoid copepod (Acartia tonsa). Changes in biochemistry (protein, carbohydrate and fatty acids) were compared in diatom cultures that simulated bloom conditions (B-algae) and those maintained in near exponential growth (E-algae) over the course of a 16-day experiment. Egg production rates (EPRs, eggs female^–1 day^–1) and the developmental success of copepodite stages of A. tonsa fed these different diets were quantified. Copepod EPR was significantly lower (reduced by half) when B-algae entered the senescent phase due to silicate limitation. In a crossover (diet switch) experiment, EPR increased when copepods fed B-algae were switched to E-algae and vice versa. Copepodites of A. tonsa developed normally and reached the adult (C6) stage when fed E-algae, but ceased development (approximately at stage C2) when reared on senescent phase B-algae. Given the importance of copepods as prey for higher trophic levels, our results highlight how nutritional changes that naturally occur during a phytoplankton bloom may influence the productivity of copepods and higher trophic levels.

Gulf Stream position and winter NAO as drivers of long-term variations in the bloom phenology of the diatom Skeletonema costatum "species-complex" in Narragansett Bay, RI, USA

Borkman, D. G., Smayda, T. J. — Mon, 05 Oct 2009 08:56:57 PDT

Bloom behavior of the Skeletonema costatum "species-complex", a keystone diatom cluster, in Narragansett Bay (NBay), USA, during a 39-year (1959–1997) time series was statistically related to variations in the path and latitudinal position of the north wall of the Gulf Stream (GSNW). Winter–spring abundance of the Skeletonema complex was ca. 10-fold greater in years when the GSNW was displaced to the south; summer abundance was 3-fold higher in years of southerly displaced GSNW flow. The relationships with the North Atlantic Oscillation (NAO), the influence of temperature on growth and grazing of Skeletonema and the advective connectivity of NBay with contiguous regional waters potentially impacted by GSNW dynamics are evaluated. The two major, regionally coupled ocean–atmosphere systems, the NAO and the GSNW, differ in their seasonal and interannual impacts and do not directly explain the specific causes of the associated variations in bloom behavior of the Skeletonema complex. Summer bloom behavior was correctly forecast in 14 of 17 years based on advanced predictions of GSNW position at its extreme northern (GSNWI > +0.5) and southern (GSNW < –0.5) latitudinal limits.

Vertical migration, feeding and colouration in the mesopelagic shrimp Sergestes arcticus

Vestheim, H., Kaartvedt, S. — Mon, 05 Oct 2009 08:56:57 PDT

Intraspecific variation in vertical distribution, timing of vertical migration, and colouration of the mesopelagic shrimp Sergestes arcticus were studied in the >400 m deep part of Masfjorden, Norway. Very few individuals were caught in the upper strata during daytime, and larger individuals occurred deeper during the day than smaller ones. Vertical migration was prominent and no overall trend of increasing length with depth was found at night. Small individuals arrived in the upper layers earlier than larger ones. Animal colouration assessed by digital photography revealed significant variance in individual redness. Depth of capture was the most important factor explaining colouration, with increasing degree of redness with depth. Assessing the gut fullness of the transparent shrimps provided a rapid way of estimating feeding activity and showed that feeding took place mainly at night.

Zebra mussels negate or mask the increasing effects of nutrient enrichment on algal biomass: a preliminary mesocosm study

Dzialowski, A. R., Jessie, W. — Mon, 05 Oct 2009 08:56:57 PDT

Phosphorus enrichment increased algal biomass in mesocosms without zebra mussels. However, with zebra mussels, there were no differences in algal biomass between controls and mesocosms that received 150 µg P/L.

Molecular and morphological methods for identifying plankton: what makes a successful marriage?

McManus, G. B., Katz, L. A. — Mon, 07 Sep 2009 08:35:54 PDT

Precise identification of species is critical for the study of biogeography of plankton and for applying laboratory culture results to the same organism in situ. Traditionally, identification has been based on knowledge of morphological traits transmitted from generation to generation of planktologists in monographs or at the bench. Despite recent rapid growth of molecular methods, taxonomists have been slow to incorporate molecular information in a formal way into species descriptions. Likewise, molecular biologists have often been less than thorough about making precise identifications of the species they sequence, as the large number of sequences in the public databases that are linked to mis- or unidentified species will attest. Although some have advocated for a new taxonomy built solely on a scaffold of DNA, for the present it seems wise to use a "total evidence" approach in identifying plankton, relying on both molecular and morphological information whenever possible. There is a large body of information on morphology, phenotypic variation, distribution and ecology of many species that is recorded in their formal descriptions, and this would be lost in a DNA-only approach. Without a successful marriage of molecular and morphological methods, it will be more difficult to solve the mystery of cryptic species. For now, we recommend that molecular approaches to identification be developed and extended where possible, that serious effort be committed to ensuring correct identification of species when DNA sequences are published and that new species of plankton should not be named based on morphology alone without supporting molecular information, especially for protists.

Subtropical ocean ecosystem structure changes forced by North Pacific climate variations

Mon, 07 Sep 2009 08:35:54 PDT

Biological responses to basin-scale climate forcing in the subtropical North Pacific Ocean are assessed based on temporal variations in plankton community structure observed at Station ALOHA and results of a coupled physical–biogeochemical model. Observational data and model simulations for the period 1990–2004 reveal distinct temporal patterns, with significant increases in net primary productivity, modeled nitrate flux into the euphotic zone and the measured downward flux of particulate nitrogen during 1999–2004. Concurrent increases in microalgae, cyanobacteria and modeled and measured zooplankton biomass were also observed during this period. We provide evidence that these responses were a consequence of climate forcing that destratified the upper ocean, making it more susceptible to mixing events and nutrient entrainment. These findings underscore the importance of nitrate flux and plankton community structure, as modulated by climate forcing, in regulating particle export over interannual and decadal time scales.

Genetic heterogeneity of the marine cyanobacterium Leptolyngbya valderiana (Pseudanabaenaceae) evidenced by RAPD molecular markers and 16S rDNA sequence data

Premanandh, J., Priya, B., Prabaharan, D., Uma, L. — Mon, 07 Sep 2009 08:35:54 PDT

The distribution and diversity patterns of the cosmopolitan marine cyanobacterium Leptolyngbya valderiana (Pseudanabaenaceae, Cyanobacteria) were studied. To asses the level of genetic diversity, morphotypes from different geographical locations (Coast of south India and Andaman) were subjected to randomly amplified polymorphic DNA (RAPD) analysis and partial 16S rRNA gene sequence studies. Morphologically and ecologically, all strains fit the currently circumscribed L. valderiana. However, RAPD primers yielded 100% polymorphism among the studied strains, indicating a considerable degree of intra-specific genomic heterogeneity, with percent similarity between 13 and 82%. Partial 16S rDNA sequence similarity values ranged from 91 to 99%. In conclusion, the genetic variability observed within the marine strains of L. valderiana indicates the presence of cryptic species. Thus, future investigations combining well-characterized ultra-structural morphology with additional molecular markers are essential to characterize and clarify the patterns in the evolution and biogeography.

Impact of the inoculum size on the population of Nostocales cyanobacteria in a temperate lake

Rucker, J., Tingwey, E. I., Wiedner, C., Anu, C. M., Nixdorf, B. — Mon, 07 Sep 2009 08:35:54 PDT

Akinete formation enables cyanobacteria of the order Nostocales to survive adverse conditions (e.g. winters in temperate zones) and is assumed to be a prerequisite for the invasion of tropical species like Cylindrospermopsis raciborskii into the temperate zones. We chose C. raciborskii for a 4-year study of the seasonal patterns of all life cycle stages in the pelagic and benthic zones of a shallow eutrophic lake in NE Germany. Akinete production started when C. raciborskii populations reached maximum cell abundance and water temperatures started to decrease, resulting in an increase in the number of akinetes at the sediment surface. The number of akinetes on the sediment (inoculum for the next year) was population size dependent. It decreased slightly in winter but dropped sharply from end of April to June/July, reflective of germination. Thus, germination started at temperatures of about 13°C. Typically, first filaments appeared in June and populations peaked in August. However, the size of the population in summer was not determined by the size of the inoculum. Consequently, it is assumed that population development is determined by growth conditions and loss processes after germination.

Seasonal variations of microbial abundances and virus- versus flagellate-induced mortality of picoplankton in three peri-alpine lakes

Personnic, S., Domaizon, I., Sime-Ngando, T., Jacquet, S. — Mon, 07 Sep 2009 08:35:54 PDT

Viruses and small heterotrophic flagellates are known to exert an important control on bacterial populations. In parallel with the study of picoplankton dynamics (abundance and distribution) in surface waters (0–50 m) of Lakes Geneva, Bourget and Annecy, we used a dilution technique during different seasonal periods in order to assess flagellate- versus virus-induced mortality of heterotrophic bacteria, picocyanobacteria and small eukaryotic phytoplankton. Although it was not always possible to detect a significant viral effect (typically in winter), viral lysis and protozoan grazing could be responsible for up to 71% of the bacterial mortality (in summer). Viral impact, considered alone, never equalled or exceeded predation for heterotrophic bacteria, but could for picocyanobacteria, typically in autumn. In addition, during summer, complex interactions between grazing- and virus-induced mortality of bacteria (e.g. synergism versus antagonism) could be highlighted (for instance with bacterial lysis susceptible to enhance picocyanobacterial growth). The temporal variations observed for experimental viral parasitism and flagellate predation were consistent with the in situ dynamics and statistical relationships found between the targeted communities. This study thus provides new evidence on the critical role played by viruses and small flagellates in the functioning of freshwater microbial food webs and also that these mortality processes vary strongly throughout the seasons.

Phytoplankton productivity increased in Lake Geneva despite phosphorus loading reduction

Tadonleke, R. D., Lazzarotto, J., Anneville, O., Druart, J.-C. — Mon, 07 Sep 2009 08:35:54 PDT

Long-term (1972–2005) data from Lake Geneva were analyzed to assess whether primary productivity (PP) has declined over years in response to phosphorus (P) reduction measures implemented in the 1970s, and to determine which factors were responsible for the response observed. The results revealed increases in PP and chlorophyll a, and a concomitant decrease in water transparency, which contrasts sharply with other large lakes undergoing nutrient loading controls. Our data revealed long-term increases in incident light, water temperature, water column stability (WCS), N:P ratios and Baccilariophyceae. Nitrogen, Daphnia and WCS, rather than P, were the major factors driving PP during the first part of the study (1972–1988). When P became the main nutrient driving PP (after 1988), the latter still increased, partly as a result of long-term increases in light and WCS, two factors not often considered when exploring climate influence on phytoplankton. Daphnia abundance decreased over time, and its coupling with phytoplankton changed from positive during the period of P abundance (1972–1988) to negative during the period of P scarcity (1989–2005), suggesting a change in the influence of Daphnia on phytoplankton and in food web structure. These data support the view that increases in climate-related factors may render the restoration of lakes more difficult, and showed that neglecting nitrogen, light and WCS, may hamper our understanding of lake responses to re-oligotrophication or lead to inappropriate management decisions, particularly in the context of global warming.

Environmental factors controlling the vertical distribution of phytoplankton in lakes

Longhi, M. L., Beisner, B. E. — Mon, 07 Sep 2009 08:35:54 PDT

Observations from single lake and experimental studies predict that vertical habitat heterogeneity in lakes can influence phytoplankton community structure. We examined the nature of water column physical habitat structure (light penetration, thermocline depth and shape and relative thermal resistance to mixing), and in turn, how these structures influenced the distribution of bulk chlorophyll a and the biomass of several major phytoplankton groups across 45 lakes in eastern Canada, within two lake districts which varied in watershed geology and water chemistry. Across all lakes, more pronounced temperature gradients favoured the distribution of bulk phytoplankton into more defined layers. The depth at which peak chlorophyll a was observed was affected by temperature heterogeneity and environmental factors related to light penetration. Peak depths and vertical heterogeneity of the major phytoplankton groups were differentially related to epilimnetic water colour and total phosphorus concentration across all lakes. Further insight was gained by comparing the physical structure and phytoplankton responses in the two regions. Lakes from the Laurentians Region had less wind exposure, shallower thermoclines, but greater vertical temperature variability than in the Eastern Townships Region. As a result, total and major phytoplankton group biomass showed more heterogeneous distributions in the Laurentians. The depth of peaks in total biomass and for the major phytoplankton groups was similar in both regions; the exception being a deeper chlorophyte maximum in the ETR, suggesting that there may be important differences between regions in the taxonomic composition of this group.

Dormancy and germination features in resting cysts of Alexandrium tamarense species complex (Dinophyceae) can facilitate bloom formation in a shallow lagoon (Thau, southern France)

Genovesi, B., Laabir, M., Masseret, E., Collos, Y., Vaquer, A., Grzebyk, D. — Mon, 07 Sep 2009 08:35:55 PDT

This study investigated the dormancy length, germination patterns and cyst progeny fate of Alexandrium catenella and A. tamarense found in Thau lagoon, France. In laboratory-produced cysts, the dormancy period was estimated to be less than 2 weeks. A vernalization period was not required prior to germination. However, after 1 month of dark-storage at 6°C, excystment was obtained 2–3 days from re-exposure to favourable conditions thereby suggesting that germination was synchronized. Germination patterns and germling cell viability in laboratory-produced cysts were similar to those exhibited in natural cysts. The following conditions optimized germination: temperature between 14 and 26°C, salinity between 30 and 38 psu and light exposure for >1 h at 100 µmoles photons m^–2 s^–1. Similar conditions prevailed at the commencement of and during Alexandrium blooms in Thau lagoon, suggesting that cysts may germinate shortly after being recruited from sediment re-suspensions. Accordingly, cyst bank germinations contribute to the initial seeding of blooms, and subsequently germinations of newly formed cysts can help to maintain blooms. In cyst banks, the high germination capabilities seem, however, to be counter-balanced by the low viability of cyst progeny, as estimated from the realized seeding ratio. This ratio is proposed as a means of quantifying the potential for cyst banks to regenerate planktonic populations.

The influence of cyanobacterium exudates on copper uptake and toxicity to a tropical freshwater cladoceran

Mon, 07 Sep 2009 08:35:55 PDT

Many studies have shown the reduction of metal toxicity in the presence of dissolved organic matter in natural waters; however, possible roles of metal-edible dissolved organic matter complexes as a vehicle of metal entrance into trophic chains have been poorly investigated. In this study copper uptake and toxicity to Ceriodaphnia cornuta Sars in the presence and absence of exudates produced by Anabaena spiroides Klebahn were investigated. Groups of 30 adult C. cornuta individuals were exposed to a range of copper concentration (10^–8–10^–6 mol L^–1) during a 24 h period. The "Trimmed Spearmen Karber" method was used to calculate EC₅₀. Copper toxicity was almost 4-fold lower in the presence of 30 mg L^–1 of cyanobacterial exudates; without exudates, EC₅₀ was 8.1 x 10^–8 mol L^–1, while with exudates it was 3.2 x 10^–7 mol L^–1. Total copper concentration in the animals was determined using differential pulse anodic stripping voltammetry and the results showed that, for both treatments, metal accumulated by the organisms was not significantly different from the negative control, except for immobile individuals. The present results suggest that the animals were able to regulate their body copper content.

Toxin concentration in Nodularia spumigena is modulated by mesozooplankton grazers

Gorokhova, E., Engstrom-Ost, J. — Mon, 07 Sep 2009 08:35:55 PDT

The ecological role of nodularin in cyanobacterium Nodularia spumigena is still largely unknown, as are the conditions that promote toxin production. We report a modulating effect of mesozooplankton grazing on cellular nodularin content in N. spumigena expressed as a decrease in cell-bound toxin concentration in the presence of copepods compared with the cyanobacterium in similar assemblages without copepods. In our experiments, N. spumigena was incubated in an ambient plankton (<90 µm) assemblage (Expt I) or in 0.2-µm filtered seawater (Expt II), with and without the copepod Eurytemora affinis. Following ~28-h incubation, we measured the changes in N. spumigena abundance and nodularin concentration, frequency of Nodularia DNA occurrence in copepods as a proxy for grazing pressure on the cyanobacterium and individual RNA content in E. affinis as a proxy for copepod growth response. In all copepod-free treatments, intracellular nodularin concentrations were up to four times higher than in the treatments containing copepods. In Experiment I, the copepods also had a positive effect on the cyanobacterium growth, presumably due to a selective removal of more edible algal species and thus decreased competition for nutrients. Nodularia DNA was detected with high frequencies, 18–80%, increasing in treatments with no alternative food or high copepod densities. Simultaneously, no noxious effects of N. spumigena on the copepods were detected as indicated by higher RNA content in copepods exposed to N. spumigena with or without ambient plankton organisms compared with starved controls. These findings stress the need to understand the importance of intra-specific interactions for nodularin production in relation to population dynamics of N. spumigena.

Evaluation of estuarine mesozooplankton dynamics at a fine temporal scale: the role of seasonal, lunar and diel cycles

Marques, S. C., Azeiteiro, U. M., Martinho, F., Viegas, I., Pardal, M. A. — Mon, 07 Sep 2009 08:35:55 PDT

In order to study the influence of physical forcing at different temporal scales, zooplankton was sampled at a fixed station located at the mouth of Mondego estuary (southern Europe). Samples were collected during diel cycles, over neap and spring tides. Zooplankton abundance and diversity were estimated for each sampling period. Holoplankton were dominated by three taxa: Copepoda, with 48% of total zooplankton abundance, Cladocera (16%) and Medusae (12%). Meroplankton occurred mainly as barnacle and decapod larvae. Copepoda were the most diverse group, represented by 26 species followed by Decapoda larvae (21) and Medusae (16). In order to assess significant differences between seasons, a univariate analysis was carried out. Higher abundance and diversity were found in warm months, particularly at neap tides, when water temperature and salinity were higher. Multivariate analysis revealed significant seasonal differences in species composition. The estuarine community was strongly dependent on allochthonous events, such as tidal exchange and river inflow. The results of our study show that the period of higher river flow, coincident with winter, resulted in changes in the zooplankton community. Short-term temporal variations in the species composition and abundance were also attributed to tidal and diel cycles. Zooplankton reached significantly higher densities at night (P < 0.05), suggesting the occurrence of vertical migrations. By emphasizing the importance of different timescale changes in the zooplankton community of the Mondego estuary, this study will be useful for the design of more efficient sampling programs, aiming at documenting changes in the zooplankton at a broad but also at a fine temporal scale.

Diel vertical migration of Antarctic krill (Euphausia superba) is flexible during advection across the Scotia Sea

Mon, 07 Sep 2009 08:35:55 PDT

We model a summer snapshot of the behavior of Antarctic krill (Euphausia superba) during advection across the Scotia Sea. Individual krill respond to a changing landscape of predation risk and food availability by migrating vertically in the water column and choosing an average distance to their nearest neighbor (swarm density). We determine the optimal behavior of 30, 40 and 50 mm krill using a state-dependent life history model where individuals move along 30-day segments of hypothetical journey tracks in three different regions of the Scotia Sea, with the tracks extracted from a combination of circulation models and surface drifter data. Food availability is based on satellite data for surface Chl a with additional heterotrophic and detritus food components, and mortality is parameterized with respect to distance from shore, daylight and krill swarming-behavior. We predict that proximity to predator colonies has a distinct effect on behavior, particularly on depth choice when food-availability is low. Observations made during an acoustic survey of the region found swarms to be deeper at the Antarctic Peninsula compared with South Georgia, in line with model predictions. Our predictions are also consistent with observations that swarm density changes little on a logarithmic scale across the region. We show that being able to change behavior on short time scales has distinct advantages to krill.

Multivariate characterization of spawning and larval environments of small pelagic fishes in the Gulf of California

Mon, 07 Sep 2009 08:35:55 PDT

Spawning and nursery areas of Sardinops sagax (Pacific sardine) and Engraulis mordax (northern anchovy) were characterized during early winter in the Gulf of California, using near-surface horizontal and oblique Bongo tows. The main spawning area for anchovy was located near the islands of Tiburón and Ángel de la Guarda and for sardine near both coasts on either side of the central region of the gulf. A hydroacoustic survey showed a close spatial overlap between the distribution of small pelagic fish schools, their spawning areas and areas with the highest zooplankton biomass. Distribution and abundance of eggs and distinct larval stages showed that the nursery area is considerably larger than the spawning area. Sardines and anchovies had distinct inter-specific larval drift patterns, mostly caused by differences in the spawning locations that were influenced by regionally distinct advection caused by coastal upwelling, filaments and eddies. Abundance of eggs and early larvae of both species were closely associated with higher abundance of fucoxanthin and chlorophyll a, zooplankton biomass and the small copepod Acartia clausi. Older larvae were mostly associated with abiotic environmental variables and large-sized copepods, such as Centropages furcatus and Calanus pacificus. These results suggest the importance of sequential spatial overlap of larvae with food availability as they drift in the Gulf of California.