Journal of Plankton Research - current issue

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.