Feeding and reproduction by Calanus finmarchicus, and microzooplankton grazing during mesocosm blooms of diatoms and the coccolithophore Emiliania huxleyi
Nejstgaard JC, Gismervik I, Solberg PT
The aim of this study was to quantify zooplankton feeding interactions and copepod reproduction during blooms of diatoms and flagellates (including Emiliania huxleyi) in fertilised mesocosms. A number of microzooplankton grazing (dilution series),
copepod feeding (bottle incubation) and egg production experiments were performed during a 4 wk summer period. Microzooplankton (mainly ciliates) peaked during an initial bloom dominated by the diatom Skeletonema costatum and flagellates >=10 µm,
which apparently became grazer-controlled. Maximum grazing rates were 1.5 to 1.8 d-1 for diatoms, the calcifying haptophyte Emiliania huxleyi and flagellates 2 to 10 µm, and 65 to 80% of the average standing stock of these algae were
removed daily. During a subsequent bloom of E. huxleyi the microzooplankton composition changed and its biomass decreased to <25%, and the daily turnover of diatoms and E. huxleyi fell to ca 50%. In contrast to other algae, E. huxleyi
specific growth was never surpassed by microzooplankton grazing. The copepod C. finmarchicus (CV and CVI females) preferred ciliates >=30 µm, but ciliates <30 µm, diatoms and rotifers were also occasionally preyed upon at high rates. E.
huxleyi was barely ingested at low concentrations (0.4 to 6 x 105 cells l-1), but was cleared at 106 ml ind.-1 d-1 at peak concentrations (1.2 x 107 cells l-1). It then made up 74% of
total carbon ingestion. Although copepod ingestion rates were similar during blooms of diatoms and E. huxleyi, egg production rates were significantly higher during blooms of the latter, and mesozooplankton biomass increased 3 times more in
mesocosms dominated by E. huxleyi compared to mesocosms with diatom blooms at similar algal biomass. Impact by copepods on the phytoplankton development was mainly induced indirectly by selective predation on the microzooplankton. A method to
correct copepod feeding rate measurements for errors due to loss of microzooplankton grazing in the incubation bottles is presented.
Calanus · Microzooplankton · Emiliania · Skeletonema · Grazing · Food selection · Top-down · Reproduction · Mesocosm
