ALBERT

Description: In a mesocosm study, the appendicularian Oikopleura dioica bloomed after the reduction of copepod abundance, and in a second treatment showed a significantly negative correlation with copepod densities. Calculations, together with field data from the Baltic Sea, suggest that common calanoid copepods may control appendicularian population dynamics.

Description: Selectivity-size spectra, clearance and ingestion rates and assimilation efficiencies of Acartia clausi (Copepoda), Penilia avirostris (Cladocera) and Doliolum denticulatum (Doliolida) from Blanes Bay (Catalan Sea, NW Mediterranean) were evaluated in grazing experiments over a wide range of food concentrations (0.02–8.8 mm3 L−1 plankton assemblages from Blanes Bay, grown in mesocosms at different nutrient levels). Acartia clausi reached the highest grazing coefficients for large algae 〉70 μm (longest linear extension), P. avirostris for intermediate food sizes between 15 and 70 μm, and D. denticulatum for small sizes from 2.5 to 15 μm. Penilia avirostris and D. denticulatum acted as passive filter-feeders. Acartia clausi gave some evidence for a supplementary raptorial feeding mode. Effective food concentration (EFC) decreased linearly with increasing nutrient enrichment for D. denticulatum and followed domed curves for A. clausi and for P. avirostris with maximum values at intermediate and high enrichment levels, respectively. Clearance rates of crustacean species showed curvilinear responses with narrow modal ranges to increasing food concentration. Clearance rates of D. denticulatum increased abruptly and levelled into a plateau at low food concentrations. Mean clearance rates were 13.9, 25.5 and 64.1 mL ind.−1 day−1, respectively. No clearance could be detected for A. clausi at food concentrations 〈0.1 mm3 L−1 and for P. avirostris at food concentrations ≤0.02 mm3 L−1. Ingestion rates indicate a rectilinear functional response for A. clausi and for P. avirostris and showed a sigmoidal curve for D. denticulatum. Mean ingestion rates were 1.3, 2.8 and 7.7 μg C μg Cind.−1 day−1, respectively. Conversion of ingested carbon to tissue was 30–80% for the investigated crustaceans and 20–50% for doliolids. Food niche calculations suggest that food niche separation may explain the coexistence of the three species in summer in Blanes Bay.

Description: The δ15N of marine mesozooplankton species was measured on four occasions. Significant differences were found between copepods and meroplanktonic larvae, yet not between holoplanktonic species. On average, mesozooplankton was enriched by 3.4 ± 0.9‰ relative to selected seston size fractions. Despite suggesting small differences (∼0.5 to 1‰) in the δ15N of different phytoplankton taxa on one occasion, the size fractionation procedure generally proved inadequate in separating major taxonomic groups composing seston. This circumstance, and phase-shifts in the transmission of rapid changes (〉2‰) in seston δ15N to mesozooplankton complicate the calculation of mesozooplankton trophic levels.