Louise K. Poulsen

Marine microalgae attack and feed on metazoans

Free-living microalgae from the dinoflagellate genus Karlodinium are known to form massive blooms in eutrophic coastal waters worldwide and are often associated with fish kills. Natural bloom populations, recently shown to consist of the two mixotrophic and toxic species Karlodinium armiger and Karlodinium veneficum have caused fast paralysis and mortality of finfish and copepods in the laboratory, and have been associated with reduced metazooplankton biomass in-situ. Here we show that a strain of K. armiger (K-0688) immobilises the common marine copepod Acartia tonsa in a density-dependent manner and collectively ingests the grazer to promote its own growth rate. In contrast, four strains of K. veneficum did not attack or affect the motility and survival of the copepods. Copepod immobilisation by the K. armiger strain was fast (within 15 min) and caused by attacks of swarming cells, likely through the transfer and action of a highly potent but uncharacterised neurotoxin. The copepods grazed and reproduced on a diet of K. armiger at densities below 1000, cells ml−1, but above 3500 cells ml−1 the mixotrophic dinoflagellates immobilised, fed on and killed the copepods. Switching the trophic role of the microalgae from prey to predator of copepods couples population growth to reduced grazing pressure, promoting the persistence of blooms at high densities. K. armiger also fed on three other metazoan organisms offered, suggesting that active predation by mixotrophic dinoflagellates may be directly involved in causing mortalities at several trophic levels in the marine food web.

The plankton community on Sukkertop and Fylla Banks off West Greenland during a spring bloom and post-bloom period: Hydrography, phytoplankton and protozooplankton

Abstract The plankton community structure was investigated on Sukkertop and Fylla Banks off West Greenland during the spring bloom in May 2000 and the post-bloom period in June 1999. In May a small change in density, clearly illustrated by the profile of potential energy, was sufficient to support a spring bloom in the upper part of the water column. The spring-bloom phytoplankton community displayed high biomass (92 ± 45 mg C m3) dominated by species of the genera Thalassiosira and Chaetoceros. The phytoplankton species composition, ongoing sedimentation and nutrient depletion indicated late spring-bloom conditions. Spring-bloom heterotrophic biomass (17 ± 7 mg C m-3) was dominated by large heterotrophic dinoflagellates (> 20 μm). In June, low autotrophic biomass was present at post-bloom stations (2 ± 1 mg C m-3) and small autotrophic flagellates (< 10 μm), mostly haptophytes, dominated the phytoplankton community. Heterotrophic biomass was low (5 ± 1 mg C m-3) and an important part was comprised by heterotrophic nanoflagellates (24 ±1%). Protozooplankters (heterotrophic dinoflagellates and ciliates) were important grazers of the phytoplankton community in the post-bloom period (estimated grazing impact 112 ± 15% d-1) and in the spring bloom (9 ± 2% d-1).

Growth and respiration in blue mussels (Mytilus spp.) from different salinity regimes

ABSTRACT Shell growth, weight-specific growth of the soft tissue, and oxygen consumption were measured in native blue mussels, Mytilus spp., fromdifferent locations inDenmark, covering a salinity range from ∼10 to ∼30. The greatest growth rates were observed in mussels growing at average salinities of 25.7 and 29.5, the lowest rates occurred at the location exhibiting the most fluctuating salinity regime over time, with an average 20.5. Individuals in waters with a salinity of 25.7 also displayed the greatest condition index of all locations (12.1mg/cm3, P < 0.05).Mussels from five of six locations displayed similar oxygen consumption rates (P ≤ 0.83) when standardized to weight (range, 0.78–0.88 mg O2/g/h. Of the salinities noted in the experiment, 25.7 appears to be the optimal salinity in terms of growth and condition, whereas strongly fluctuating salinity obviously involves reduced growth. At first glance, this study may appear to be just one among numerous attempts to describe the effect of salinity on growth and respiration in Mytilus edulis and its Baltic hybrids. However, themajority of studies focus on field transplants and responses to salinity alterations in the laboratory, whereas only sparse information exists on locally adapted blue mussels in relation to their ambient, native salinity.