FEEDING AND REPRODUCTION BY CALANUS-FINMARCHICUS, AND MICROZOOPLANKTON GRAZING DURING MESOCOSM BLOOMS OF DIATOMS AND THE COCCOLITHOPHORE EMILIANIA-HUXLEYI

The aim of this study was to quantify zooplankton feeding interactions and copepod reproduction during blooms of diatoms and flagellates (in cluding Emiliania huxleyi) in fertilised mesocosms. A number of microz ooplankton grazing (dilution series), copepod feeding (bottle incubati on) and egg production experiments were performed during a 4 wk summer period. Microzooplankton (mainly ciliates) peaked during an initial b loom dominated by the diatom Skeletonema costatum and flagellates grea ter than or equal to 10 mu m, which apparently became grazer-controlle d. Maximum grazing rates were 1.5 to 1.8 d(-1) for diatoms, the calcif ying haptophyte Emiliania huxleyi and flagellates 2 to 10 mu m, and 65 to 80% of the average standing stock of these algae were removed dail y. During a subsequent bloom of E. huxleyi the microzooplankton compos ition changed and its biomass decreased to <25%, and the daily turnove r of diatoms and E. huxleyi fell to ca 50%. In contrast to other algae , E. huxleyi specific growth was never surpassed by microzooplankton g razing. The copepod C. finmarchicus (CV and CVI females) preferred cil iates greater than or equal to 30 mu m, but ciliates <30 mu m, diatoms and rotifers were also occasionally preyed upon at high rates. E. hux leyi was barely ingested at low concentrations (0.4 to 6 x 10(5) cells l(-1), but was cleared at 106 ml ind.(-1) d(-1) at peak concentration s (1.2 x 10(7) cells l(-1)). It then made up 74% of total carbon inges tion. Although copepod ingestion rates were similar during blooms of d iatoms and E. huxleyi, egg production rates were significantly higher during blooms of the latter, and mesozooplankton biomass increased 3 t imes more in mesocosms dominated by E. huxleyi compared to mesocosms w ith diatom blooms at similar algal biomass. Impact by copepods on the phytoplankton development was mainly induced indirectly by selective p redation on the microzooplankton. A method to correct copepod feeding rate measurements for errors due to loss of microzooplankton grazing i n the incubation bottles is presented.