Journal of Plankton Research - recent issues

Size-fractionated photosynthesis/irradiance relationships during Phaeocystis antarctica-dominated blooms in the Ross Sea, Antarctica

Shields, A. R., Smith, W. O. — 2009-06-02

In the Ross Sea, there are two major phytoplankton functional groups: diatoms and prymnesiophytes (dominated by Phaeocystis antarctica). Phaeocystis antarctica often occurs in colonial form, but also as solitary cells, and the two forms have distinct ecological roles. A comparison of the growth characteristics of solitary and colonial forms of Phaeocystis sp. is essential to understanding the differential impact each has on biogeochemical and ecological processes, and to allow parameterization of each form in numerical models. We measured the biomass and photosynthetic responses of two size fractions (>20 and <20 µm), representing colonies and solitary cells, at locations dominated by P. antarctica to assess the relative photosynthetic potential of the two forms. While the relative contribution of each form to total P. antarctica biomass differed among years, there were no significant differences between maximum photosynthetic rates of colonial and solitary forms within years. Substantial interannual variations in biomass and maximum photosynthetic rates normalized to chlorophyll a (P_m^B) and initial light-limited rates of photosynthesis () were observed among years for the colonial fractions; however, interannual variations in maximum rates of photosynthesis or of solitary cells were not observed. A laboratory experiment with P. antarctica, together with field data, showed that growth stage of colonies strongly affected the maximum photosynthetic rates. Under nutrient-replete conditions and exponential growth, colonial cells had higher maximum photosynthetic rates than solitary cells, but as growth rate declined and senescence began, the solitary cells' rates became greater. This may be a reason for the high abundance of colonies that is often found in the Ross Sea during austral spring. Our results suggest that photosynthetic rates may influence the composition of the morphotypes of Phaeocystis, but do not appear to be the sole factor in regulating this critical biological variable.

Life cycle traits of two transatlantic populations of Eurytemora affinis (Copepoda: Calanoida): salinity effects

Beyrend-Dur, D., Souissi, S., Devreker, D., Winkler, G., Hwang, J.-S. — 2009-06-02

While the populations of the copepod Eurytemora affinis are often morphologically (i.e. taxonomy) indistinguishable, the species complex is composed of genetically distinct clades, representing divergent evolutionary histories. The most distant clades, genetically and morphologically (i.e. phylogeny), are transatlantic clades: North American and European (Lee, 2000). The study of the life cycle strategies of two populations from St. Lawrence salt-marshes (Canada) and from the Seine estuary (France) at three salinities (5, 15 and 25) revealed differences in their salinity tolerance. Individuals from the Seine exhibited high mortality under the highest salinity suggesting that the St. Lawrence population tolerated a wider salinity range. At the lowest salinity, the development time of St. Lawrence individuals was longer than that of individuals from the Seine suggesting that the Seine population was more adapted to low salinity. The clutch size and the longevity of St. Lawrence adults were on average two times higher compared to Seine adults. Thus, the St. Lawrence population exhibited a higher fitness relative to the Seine population. Such differences could be due to genetic differences resulting from divergent evolutionary history, to phenotypic plasticity and/or to the acclimation to culture conditions. We confirmed that a gamma density function is an appropriate fitting function for copepod development time, based on a large data set on development time. It can therefore be integrated into individual-based models of copepod population dynamics.

Annual assessment of the predation of Mnemiopsis leidyi in a new invaded environment, the Kiel Fjord (Western Baltic Sea): a matter of concern?

Javidpour, J., Molinero, J. C., Lehmann, A., Hansen, T., Sommer, U. — 2009-06-02

The sudden occurrence of the ctenophore Mnemiopsis leidyi has been reported recently from different regions of the Baltic Sea and it has been suggested that the species has invaded the whole basin. Here we provide the first set of quantitative data of seasonal diet composition and life history traits of M. leidyi and its predatory role in the pelagic ecosystem of the Western Baltic Sea. The size structure of the species appeared to be dominated by small size classes and only a few adults were as large as those reported in the native region of the species and in other invaded areas. We show that the species has a high preference for small-sized and slow swimming prey, mainly during the winter low temperature period. Barnacle nauplii appeared to be the main source of carbon for the over-wintering population of M. leidyi. A preference for copepods was only found during August when these prey contributed up to 20% of the gut composition. In summer, planula larvae of the jellyfish Aurelia aurita were the most abundant prey in the gut content (feeding rate of 621 ind. ctenophore^–1day^–1). We further found that at highest densities of the species, in summer, a significant predation on its larvae occurs, this being the major carbon source of adults. Overall, these results are discussed in the context of trade-offs M. leidyi faces in the new environment and adverse environmental conditions, which are likely forcing the species toward reduced sizes and also probably reducing its potential predatory impact in the Baltic Sea.

Zooplankton communities in two contrasting Basque estuaries (1999-2001): reporting changes associated with ecosystem health

Albaina, A., Villate, F., Uriarte, I. — 2009-06-02

This study is a part of the zooplankton monitoring program carried out in the euhaline region of the estuaries of Bilbao and Urdaibai (Basque coast, Bay of Biscay), and analyses between-estuaries differences in zooplankton spatial and temporal patterns in relation to environmental conditions between July 1999 and May 2001. Environmental variables measured were water temperature, dissolved oxygen saturation (DOS), Secchi disk depth (SDD) and chlorophyll a. Relationships between zooplankton community and environmental variables were analysed using canonical correspondence analysis; between-estuaries differences in environmental conditions and distribution of zooplankton taxa in relation to salinity were tested using Mann–Whitney U-test. Spatial differentiation of the zooplankton community was higher in the estuary of Bilbao, with the relative abundance of most of the taxa decreasing more pronouncedly towards the upstream estuary than in the Urdaibai related to significantly lower values of DOS and SDD, reflecting the higher degree of pollution, in the Bilbao estuary. However, the successful establishment of the Acartia discaudata and A. margalefi populations, and the first records of another Acartia species, Calanipeda aquaedulcis and Eurytemora affinis in the Bilbao estuary, along with the increasing similarity between zooplankton assemblages of the Bilbao and Urdaibai estuaries in relation to the period 1997–1999, represent a new step in the recovery of the zooplankton community in the estuary of Bilbao responding to the improvement of water quality.

Near-bottom zooplankton in the continental shelf and upper slope of Heraklion Bay (Crete, Greece, Eastern Mediterranean): observations on vertical distribution patterns

Koulouri, P., Dounas, C., Radin, F., Eleftheriou, A. — 2009-06-02

Near-bottom zooplankton sampled by a newly developed hyperbenthic sledge (TTSS2) on the continental shelf (50–200 m) and upper slope (300 m) of Heraklion Bay (Crete, Eastern Mediterranean) consisted of both meso- and macro-zooplankton. Meso-zooplankton was composed mainly of calanoid copepods and crustacean larvae, while macro-zooplankton consisted almost exclusively of chaetognaths. Results revealed that near-bottom zooplankton is found in very high densities close to the seabed and especially on or just a few centimetres above the sediment surface. Furthermore, near-bottom zooplankton abundance was significantly higher during the night than during the day implying that the performance of "reversed" diel vertical migration by these animals is probably due to the presence of predators that use tactile stimuli, rather than vision, to locate prey.

Small-scale distribution of fish larvae around the Medes Islands marine protected area (NW Mediterranean)

Lopez-Sanz, A., Vert, N., Zabala, M., Sabates, A. — 2009-06-02

During the spring and summer of 2003, the small-scale spatial distribution of fish larvae was analysed in and around the Medes Islands marine protected area (MPA). High specific richness was found during both seasons, which is when most coastal fish species spawn in the NW Mediterranean. Despite the small dimensions of the area, well-defined larval fish patterns were found. Larvae from pelagic eggs were located farther from the adult habitat than those from demersal eggs; however, there were exceptions, which highlight the influence of larval behaviour and pelagic larval duration. Inside the reserve, there was both a high diversity index and a high abundance of shorefish larvae in relation to the high abundance of adults and a possible retention of larvae in the MPA. Although there was a great variability both in species composition and abundance between periods, the sampling station assemblages were similar. Factors such as habitat and spawning location of adults, and the role of the MPA seem to determine the stability of the larval distribution patterns in time. The importance of small-scale approaches is highlighted to understand the fish larval spatial distributions.

Epibiotic suctorians and enigmatic ecto- and endoparasitoid dinoflagellates of euphausiid eggs (Euphausiacea) off Oregon, USA

Gomez-Gutierrez, J., Kawaguchi, S., Nicol, S. — 2009-06-02

We discovered euphausiid eggs infested with epibiotic suctorian ciliates (Ephelotidae), with ectoparasitoid blastodinian dinoflagellates (Chytriodinium sp.), and infected with unidentified endoparasitoid syndinian-like dinoflagellates. The discovery of two parasitoid protists suggests an unquantified source of embryonic mortality and a potential control on euphausiid larval recruitment rates.

Novel virus dynamics in an Emiliania huxleyi bloom

Sorensen, G., Baker, A. C., Hall, M. J., Munn, C. B., Schroeder, D. C. — 2009-06-02

Diel studies of an Emiliania huxleyi bloom within a mesocosm revealed a highly dynamic associated viral community, changing on small times scales of hours.

Responses of coastal osmotrophic planktonic communities to simulated events of turbulence and nutrient load throughout a year

Guadayol, O�s., Marrase, C., Peters, F., Berdalet, E., Roldan, C., Sabata, A. — 2009-04-30

A year-long series of monthly experiments with laboratory enclosures were conducted with water from Blanes Bay (NW Mediterranean) to analyse the change in the short-time response of the osmotrophic planktonic community to simulated turbulence and nutrient input events. Both experimental factors triggered a relative increase of biomass in the enclosures, in terms of chlorophyll a, bacteria and particulate organic matter. Ratios of particulate organic nitrogen to phosphorus became lower in the water than in the sediment, although turbulence partially smoothed out this difference. Initial physico-chemical conditions significantly influenced the short-time responses to experimental forcing. The response to turbulence, in terms of chlorophyll a, was maximum in spring. The response to nutrient enrichment was found to be seasonal, and was correlated with photoperiod and temperature, and also in situ nitrate and silicate concentrations and Secchi depth, which are proxies of recent inputs of nutrients resulting from episodes of resuspension and river discharge. This study shows robust qualitative regularities in the response of the osmotrophic planktonic community to episodes of turbulence and nutrient enrichment, with quantitative variability throughout the year, depending mostly on the recent record of hydrodynamic forcing.

Short-term dynamics of late-winter phytoplankton blooms in a temperate ecosystem (Central Cantabrian Sea, Southern Bay of Biscay)

Alvarez, E., Nogueira, E., Acuna, J. L., Lopez-Alvarez, M., Sostres, J. A. — 2009-04-30

The Spring Phytoplankton Bloom takes place in the Central Cantabrian Sea (Southern Bay of Biscay) from late-winter to spring as a series of blooms with variable biomass accumulation. In late-winter of 2004 and 2005, phytoplankton blooms occurred in this area following a change in the weather. In order to describe the dynamics of these late-winter blooms, two oceanographic cruises which involved high-frequency sampling (every 2–3 days) were carried out. Meteorological conditions during the cruises showed similar changes in variables relevant to phytoplankton physiology and population dynamics. Before the bloom, phytoplankton started to grow actively when underwater photosynthetic active radiation (PAR) increased. However, biomass accumulation did not occur until wind, and hence turbulence levels in the water column, decreased. The observations presented here suggest that before the onset of a late-winter bloom a preliminary physiological activation phase is necessary driven by increased availability of underwater PAR. Afterwards, biomass accumulation can take place provided wind-derived water column turbulence decays. The development of the bloom is reinforced by the shoaling of the surface mixing layer depth. The timing of this sequence of events can be altered by meteorological disturbances, such as an increase of wind speed. The composition of the bloom differed across-shelf: phytoplankton larger than 5 µm in equivalent spherical diameter (ESD) dominated on the coast and inner shelf, whereas smaller phytoplankton (<5 µm ESD) were more important in the oceanic area, markedly when a frontal structure separating both domains developed at the mid-shelf.

Interactive effects of phosphorus loads and ambient ultraviolet radiation on the algal community in a high-mountain lake

2009-04-30

Interactions among multiple stressors generated by global change exert cumulative effects on ecosystems. In order to identify the long-term interactive effects of ultraviolet radiation (UVR) and increasing atmospheric nutrient inputs (P-inputs) on aggregate (growth rate, chlorophyll a and abundance) and emergent (evenness and diversity) algal community variables, a 2 x 5 factorial experiment [two levels of light quality (+UVR, –UVR) and five levels of nutrient enrichment] was conducted using in situ mesocosms in a high-mountain lake in the Sierra Nevada (Spain) over a 70-day period. Addition of P suppressed and inverted the stimulatory UVR effect on the non-flagellate growth rate (mid-term scale). This interactive effect was propagated as increased harmful UVR effect on chlorophyll a and abundance across the P-gradient. Interestingly, P-pulse reversed the positive effect of UVR on phytoplankton evenness and diversity on the long-term scale. These findings support our hypothesis that the UVR x P interactive effect would favor a few UVR-tolerant rapid-growing species. We show that the algal community is acclimated to UVR-flux but not to high P-pulse. The loss of diversity and disappearance of mixotrophic flagellates may have important implications for the functioning of clear-water ecosystems.

Essential fatty acid concentrations of different seston sizes and zooplankton: a field study of monomictic coastal lakes

Kainz, M. J., Perga, M.-E., Arts, M. T., Mazumder, A. — 2009-04-30

Plankton particle size is a central issue for aquatic ecologists. In this field study, we examined essential fatty acids (EFAs) in different plankton particle sizes and obtained initial estimates of how EFA concentrations, and thus high dietary lipid quality for consumers, vary with seston size. We measured EFA concentrations in three seston size classes (i.e. micro = 0.7–35 µm; meso = 35–64 µm and macroseston = 64–100 µm) and in cladocerans and calanoid copepods of monomictic, coastal lakes. Algal pigment analysis identified a mixture of Chlorophyta, Cryptophyta, diatoms and Cyanophyta in all seston size classes. Total EFA concentrations did not vary with increasing seston size. There was no difference in linoleic (LIN), eicosapentaenoic (EPA) and docosahexaenoic (DHA) acid concentrations among the three seston size classes; however, -linolenic (ALA) and arachidonic (ARA) acid concentrations were significantly higher in microseston. For herbivorous zooplankton, concentrations of LIN, ALA and EPA did not differ significantly between cladocerans and calanoid copepods; however, ARA concentrations were significantly higher in cladocerans and DHA concentrations were significantly higher in calanoid copepods. Our results indicate that microseston represents the most nutritious dietary component, per unit biomass, with respect to ALA and ARA and that seston size alone does not predict EFA concentrations, i.e. dietary access to larger seston particle sizes is not necessarily associated with access to particles with higher EFA concentrations.

Contribution of auto- and heterotrophic protozoa to the diet of copepods in the Ulleung Basin, East Sea/Japan Sea

Yang, E. J., Kang, H.-K., Yoo, S., Hyun, J.-H. — 2009-04-30

We estimated the ingestion rates and prey selectivity of two calanoid copepods, Calanus sinicus and Neocalanus plumchrus, on auto- and heterotrophic protozoa in the Ulleung Basin in the East Sea/Japan Sea. Both copepod species preferentially ingested auto- and heterotrophic protozoa. The heterotrophic protozoa comprised between 42.9 and 86.7% of the total carbon ration ingested by the copepods, although they comprised on average 21.5% of the total available prey carbon. In particular, ciliates comprised the major dietary component for the copepods in the study area. The copepod consumption of ciliates was on average 38.2% of the total daily carbon ration of auto- and heterotrophic protozoa. Auto- and heterotrophic protozoa in the 20–50 µm size were the fraction that was most actively consumed by copepods and they comprised on average 45.1% of the total carbon ration ingested. In all experiments, the copepods selected the ciliate and heterotrophic dinoflagellate (HDF) groups over the other protozoa. Our results indicate that the copepod feeding regime was influenced by the composition and size of the potential food and that, in this study area, feeding behavior of copepods may control the populations of ciliates and HDFs that are larger than 10 µm in size. Thus, the selective feeding patterns and higher grazing pressure of copepods on heterotrophic protozoa, compared with autotrophic protozoa, indicate a trophic coupling between copepods and the microbial food web in the Ulleung Basin.

Decreased astaxanthin at high feeding rates in the calanoid copepod Acartia bifilosa

Holeton, C., Lindell, K., Holmborn, T., Hogfors, H., Gorokhova, E. — 2009-04-30

In marine food webs, copepods are the major producers of a carotenoid pigment astaxanthin, which is an important antioxidant. The availability of astaxanthin for higher trophic levels can be affected by changes in phytoplankton stocks and copepod feeding; however, the functional relationship between food availability and astaxanthin production is poorly understood. We hypothesized that with a given food type and quality, astaxanthin content in copepods is positively related to feeding and egg production rates. The hypothesis was tested by measuring astaxanthin accumulation in concert with ingestion and egg production rates in the copepod Acartia bifilosa exposed to different algal concentrations (Tetraselmis suecica; 0 to 1200 µg C L^–1). Egg production and ingestion rates increased with increasing food availability and reached a plateau at ≥400–600 µg C L^–1. In contrast, increasing accumulation of astaxanthin with increasing food availability was observed only at concentrations ≤150 µg C L^–1. Contrary to our hypothesis, at 600–1200 µg C L^–1 copepods had maximal ingestion and egg production rates, but low astaxanthin contents. It is suggested that this low accumulation of astaxanthin at high food concentrations results from a food-dependant decrease in assimilation efficiency. These findings are important for the understanding of astaxanthin dynamics within marine food webs, where increases in phytoplankton biomass may translate to a trade-off between zooplankton quantity and its nutritional quality for zooplanktivores.

Trophic relationships of zooplankton in the eastern Mediterranean based on stable isotope measurements

Koppelmann, R., Bottger-Schnack, R., Mobius, J., Weikert, H. — 2009-04-30

Abundance and stable isotope composition of large and small mesozooplankton were analyzed in samples taken with 333 and 100 µm nets, respectively, at four sites in the eastern Mediterranean down to 4200 m depth in October 2001. Large mesozooplankton (333 µm nets) was sieved into five size fractions, and the ¹³C and ¹⁵N values of the fractions were measured as well as the ¹⁵N values of total small mesozooplankton (100 µm nets) and specific mesozooplankton taxa. These measurements allow insights into the source of the diet and the trophic level relative to sinking and suspended particulate organic matter. Overall, biomass and abundance of zooplankton was low, reflecting the oligotrophic character of the eastern Mediterranean. Stable nitrogen isotope values of mesozooplankton were low (1–4) and close to zero in suspended particles at the surface. This indicates that the fixation of atmospheric nitrogen probably contributes to the N-pool in the eastern Mediterranean. Such low values were also found in sinking particles in deep waters and in most zooplankton size classes. However, suspended particles and mesozooplankton in the size class 0.5–1 mm, which was primarily composed of the deep-sea species Lucicutia longiserrata, showed higher values at depths below 1000 m. There is some indication that L. longiserrata was able to utilize the suspended particle pool in the deep eastern Mediterranean.

A hierarchical analysis of the spatial distribution of larval fish prey

Young, K. V., Dower, J. F., Pepin, P. — 2009-04-30

We quantified the small-scale distribution of larval fish prey (copepod nauplii and copepodites) to (i) investigate the variability in prey abundance across a range of spatial scales and (ii) determine the statistical distribution (normal, negative binomial or Poisson) that best describes the distribution of prey at scales relevant to larval fish foraging. We used a hierarchal sampling program to collect zooplankton at scales from metres to kilometres using a rosette sampler that collected replicate 2.5 L samples (~ volume searched by larval fish) at 10 m. A generalized linear model framework was used to investigate the underlying distribution of the zooplankton at different spatial scales. The majority of variance (51%) in zooplankton abundance was found at the metre-scale indicating a high degree of small-scale patchiness was present. The distribution of zooplankton was significantly different from a Poisson distribution but not a normal or negative binomial distribution. Thus, larval fish prey were not randomly distributed within the upper mixed layer, but were more aggregated than predicted by the standard Poisson distribution used in most foraging models. Our results suggest that efforts should be made to estimate the variability of prey abundance at the scale of larval fish foraging rather than using large-scale average abundance estimates, as small-scale prey patchiness likely plays a role in larval fish feeding dynamics.

Genetic and morphologic characterization of four putative cylindrospermopsin producing species of the cyanobacterial genera Anabaena and Aphanizomenon

2009-04-09

Cylindrospermopsin (CYN) is a potent hepatotoxic alkaloid that has been detected in freshwater samples worldwide and is produced by a number of cyanobacterial species, mainly of the genera Cylindrospermopsis, Aphanizomenon and Anabaena. Cylindrospermopsis raciborskii is a morphologically distinctive species which forms a genetically well-defined cluster. In contrast, some species within Aphanizomenon and Anabaena are morphologically not clearly assignable to either genera and both genera are polyphyletic. In the Cylindrospermopsis cluster CYN producing and non-producing strains co-occur, but it is not known if CYN producing and non-producing strains are closely related in Anabaena and Aphanizomenon. Here we attempt to disentangle the phylogenetic relationships of four taxa of the genera Anabaena and Aphanizomenon, some of which are known as CYN producers. We have sequenced and phylogenetically analysed partial sequences of the 16S rRNA gene, cpcBA-IGS and rpoC1 from 31 cyanobacteria isolates of the genera Aphanizomenon and Anabaena and have documented morphotypic characteristics of new and recently isolated strains. Our results do not corroborate the separation of Aph. gracile and Aph. flos-aquae into separate species. In contrast, they support the distinction of Ana. bergii and Aph. ovalisporum into distinct taxa. Further, Ana. bergii is most likely not a CYN producing species.

Effects of light intensity and light quality on growth and circadian rhythm of saxitoxins production in Cylindrospermopsis raciborskii (Cyanobacteria)

2009-04-09

Effects of light intensity and light quality on saxitoxin and neosaxitoxin production by Cylindrospermopsis raciborskii (Woloszynska) Seenaya and Subba Raju (strain T3) were investigated in batch culture conditions. Three light intensities (50, 100 and 150 µmol photons m^–2 s^–1) and three light qualities (white and blue 470 nm and red 630 nm) were used. Growth rates were higher under 100 and 150 when compared to 50 µmol photons m^–2 s^–1 (control condition). Saxitoxin (STX) and neosaxitoxin (NSTX) production showed an increase during the early stationary phase under 50 µmol photons m^–2 s^–1 (12 days). Higher concentrations of STX and NSTX were observed in cells under 100 µmol photons m^–2 s^–1. Production of these toxins showed a circadian rhythm synchronized with light:dark (L:D) cycle and also with a period of 24 h under the three light intensities tested. The circadian rhythm of STX and NSTX production under white light conditions was confirmed. Red light induced the loss of circadian rhythm in NSTX production. These data demonstrate a circadian rhythm in saxitoxins production modulated by light.

Warm spring and summer water temperatures in small eutrophic lakes of the Canadian prairies: potential implications for phytoplankton and zooplankton

Dupuis, A. P., Hann, B. J. — 2009-04-09

Shallow, polymictic lakes with low heat storage capacity are especially susceptible to warmer spring conditions, predicted for a changing climate. In these lakes, atmosphere to water mass heat transfer is efficient as a result of high wind exposures and large surface areas relative to volumes. We examined effects of warmer water temperatures on phytoplankton and zooplankton abundance and species composition in three small eutrophic lakes (5.2–14.9 ha) of the Canadian prairies in Winnipeg, Manitoba, Canada, over two open-water seasons with contrasting spring weather conditions, i.e. 2005-a "normal" spring and 2006-a warm spring. Warmer spring and summer water temperatures were associated with decreased water transparency, increased phytoplankton biomass, increased relative filamentous cyanobacteria biomass and shifts in dominant genera from Aphanizomenon to Anabaena and Planktothrix. Zooplankton responded strongly; abundance of Daphnia (D. pulicaria, D. ambigua and D. parvula) decreased while rotifers, Skistodiaptomus oregonensis and Bosmina longirostris increased in abundance. Of several factors influencing phytoplankton dynamics, total dissolved nutrients [nitrogen (N), phosphorus (P) and N:P] and water column stability did not show important changes between years. In contrast, water temperature [described as the metric degree-days (°C day)] was related to changes in phytoplankton and % cyanobacteria biovolume. Daphniid abundance showed a significant negative relationship with an increase in filamentous cyanobacteria biomass and, thus we suggest, was indirectly associated with increased water temperatures.

Differential tolerance of UV radiation between Chaoborus species and role of photoprotective compounds

Nagiller, K., Sommaruga, R. — 2009-04-09

Previous studies with larvae of the American species Chaoborus punctipennis suggest that this genus is highly sensitive to solar ultraviolet radiation (UVR). Here, we first tested the UV sensitivity of late-instar larvae of two European Chaoborus species collected from a fishless shallow lake and second assessed their strategies to minimize UV damage. Exposure of late-instar larvae of C. pallidus to artificial UVR caused 50% mortality at a dose of 63.9 J cm^–2. In contrast, no mortality was observed in C. obscuripes at the same or even at a higher dose (85.1 J cm^–2). Several UV-absorbing compounds identified as mycosporine-like amino acids (MAAs) were found by high-performance liquid chromatography analysis in C. obscuripes [mean total MAAs: 0.142 µg mg^–1 dry weight (DW)], but not in C. pallidus. Larvae of C. flavicans also had the same suite of MAAs (shinorine, mycosporine-glycine, palythine, usujirene and palythene) and similar MAAs concentration (0.186 µg mg^–1 DW) as C. obscuripes, whereas C. crystallinus had very low concentrations (0.008 µg mg^–1 DW) and only two MAAs. The same suite of MAAs found in larvae of C. obscuripes was also detected in pupae, adult midges and eggs of this species. Whereas C. obscuripes was able to accumulate MAAs through the diet, the origin of these secondary metabolites in the lake remains unclear. Our results indicate that sensitivity to UVR in Chaoborus is species-specific and suggest that resistance in C. obscuripes is associated with the presence of photoprotective compounds. Populations of UV-sensitive Chaoborus species may be at disadvantage in water bodies becoming more transparent to UVR as consequence of climate change.

Daphnia diel vertical migration: implications beyond zooplankton

Haupt, F., Stockenreiter, M., Baumgartner, M., Boersma, M., Stibor, H. — 2009-04-09

Diel vertical migration (DVM) is a common behaviour of many pelagic zooplankton species. While the causes (mostly predator avoidance) and ecophysiological consequences of DVM for zooplankton have been well studied, little is known about the consequences of DVM for the pelagic food web. DVM creates a temporal and spatial grazer-free niche for edible phytoplankton, and theoretical models predict that parts of the phytoplankton community should use this niche. Conceivably, DVM could also cause nutrient transport between separated water layers. We experimentally investigated the influence of DVM of the zooplankton species, Daphnia hyalina, on the nutrient and phytoplankton dynamics of an oligotrophic lake. We used 10-m deep field enclosures with a 4-m deep, well-mixed surface layer. The enclosures contained either migrating or non-migrating Daphnia populations; temperature was kept nearly constant across the entire enclosure depth. Our results show that DVM had significant quantitative and qualitative effects on the phytoplankton community. There was no measurable net nutrient transport between hypolimnion and epilimnion. The gelatinous green alga Planktosphaeria gelatinosa, was the dominant algal species in our experiment. Its abundance decreased in DVM treatments, and thus also influenced the total biomass and diversity of phytoplankton communities.

Metazooplankton distribution across the Southern Indian Ocean with emphasis on the role of Larvaceans

Jaspers, C., Nielsen, T. G., Carstensen, J., Hopcroft, R. R., Moller, E. F. — 2009-04-09

The abundance and depth distribution of metazoans >20 µm were investigated at seven stations across the Southern Indian Ocean (SIO), October–November 2006. Copepod nauplii, copepodites and larvaceans dominated the metazooplankton community. Copepodites were most abundant within Agulhas Current and Southern Ocean waters, decreasing toward subtropical/tropical areas, whereas larvaceans showed the inverse pattern. The fraction <200 µm contained the majority of the zooplankton enumerated, including 81, 23 and 93% of the larvacean, copepodite and nauplii abundances, respectively. The relative abundance of larvaceans compared with copepodites increased from 7 to 44% from South Africa towards Australia. Peak copepodite biomass was observed off South Africa, while larvacean biomass was <1% of the copepodite biomass there, increasing to 6% in tropical waters. Both copepodite and nauplii biomass were positively correlated to total Chl a (P < 0.0001), larvacean biomass was only significantly related to temperature (P = 0.0213). Despite their low biomass, larvacean production was estimated to exceed the copepod production up to five times. It appears that the abundance and role of larvaceans in the SIO has been severely underestimated in previous studies; thus future investigations into the fate of organic matter will remain incomplete if this group is not adequately considered.

Inter-annual variability in abundance and community structure of zooplankton south and north of Iceland in relation to environmental conditions in spring 1990-2007

2009-04-09

An 18 year zooplankton time series from two standard sections differing in hydrographic conditions (Subarctic Water north of Iceland and Atlantic water south of Iceland) was examined in relation to hydrography and phytoplankton dynamics, and large-scale climatic changes in the North Atlantic Ocean, particularly the North Atlantic Oscillation (NAO). The long-term mean zooplankton abundance was higher in south (~160 000 individuals m^–2) than that in north of Iceland (~120 000 individuals m^–2). Abundance fluctuated markedly between years. Copepods (mainly Calanus finmarchicus and Oithona spp.) dominated the zooplankton, comprising >60–70% of the plankton in most years. Among the copepods, C. finmarchicus was more abundant in south of Iceland (~20–70%) than to the north (~10–60%). Abundance and community structure were related to environmental variables using regression and multivariate techniques (principal component analysis and redundancy analysis). Temperature and salinity were the most important environmental variables in explaining the differences in species composition in north and south of Iceland, with species and groups like Podon leuckarti and cirripede larvae being relatively abundant to the south, and C. hyperboreus to the north of Iceland. A significant year-to-year variability in community structure was observed both to the south and to the north, with salinity and used nitrate dictating the variability to the south and temperature to the north (Monte Carlo permutation tests, P < 0.05). In neither region was a unidirectional temporal trend in species composition.

Active prey selection in two pelagic copepods feeding on potentially toxic and non-toxic dinoflagellates

Schultz, M., Kiorboe, T. — 2009-04-09

Grazing on two red tide dinoflagellates, the potentially toxic Karenia mikimotoi and the non-toxic Gyrodinium instriatum, was examined in two species of marine copepods, Pseudocalanus elongatus and Temora longicornis. Both copepods cleared K. mikimotoi at rates that were a little lower but comparable to those at which they cleared the slightly larger G. instriatum when the two dinoflagellates were offered separately. However, when feeding on mixtures of the two prey species, the clearance rates on K. mikimotoi were substantially reduced in both copepods while their clearances of G. instiatum remained unaltered, suggesting active prey selection. Video observations of individual prey capture and feeding events showed prey rejection frequencies (caught and then released cells) that did not differ between mixed and mono-specific diets. This suggests that the selection between prey cells occurs prior to capture and that it is based on remote characterization of the cells.

Molecular systematics of medusae in the genus Craspedacusta (Cnidaria: Hydrozoa: Limnomedusae) in China with the reference to the identity of species

Zhang, L. Q., Wang, G. T., Yao, W. J., Li, W. X., Gao, Q. — 2009-04-09

The objective of this study was to illustrate the phylogenetic relationship of the species in the genus Craspedacusta in China. The medusae samples were collected at 28 localities in China representing seven described species with their entire ITS region (the contiguous sequences of ITS-1, 5.8S and ITS-2 rDNA) rDNA sequences cloned. Among the 28 samples, the range of sequence variation in the complete ITS and 5.8S region was between 0 and 36.2%. Three main clades were revealed by both maximum likelihood and neighbour-joining trees, with sequence difference of 0–0.9, 0–3.7 and 0.1–1.5% in the three clades. The nesting of C. xinyangensis representatives within C. sowerbii, C. brevinema within C. sinensis and C. sichuanensis within C. kiatingi is strongly supported, with interspecific sequence divergence of 0–0.9, 0.1–1.4 and 0.0–0.4%, respectively. Thus, it is suggested that C. xinyangensis should be the synonym of C. sowerbii, C. sichuanensis the synonym of C. kiatingi and C. brevinema the synonym of C. sinensis. However, the taxonomic status of C. ziguiensis is still uncertain. According to the tree topology, C. kiatingi was closer to C. sowerbii than to C. sinensis. Craspedacusta sinensis was the most genetically distinct from distance matrix values, and located at the base of the phylogenetic trees, so it can be speculated that the C. sinensis may be the ancestral form in the genus Craspedacusta.

Does Eurytemora affinis (Copepoda) control the population growth of Keratella cochlearis (Rotifera) in the brackish water Darss-Zingst Lagoon (southern Baltic Sea)?

Feike, M., Heerkloss, R. — 2009-04-09

Laboratory experiments and long-term zooplankton monitoring data suggest a possible suppression of the population growth of the rotifer Keratella cochlearis f. tecta by predation by the copepod Eurytemora affinis.

Nondestructive quantification of phytoplankton gut content of brachyuran crab megalopae using in vivo chlorophyll a fluorescence

2009-04-09

This work describes a new method to quantify phytoplankton gut content of the crab Carcinus maenas megalopae, based on the non-destructive measurement of in vivo chlorophyll fluorescence emitted from ingested microalgae.