ALGAL CONSTRAINTS ON COPEPOD GRAZING - GROWTH-STATE, TOXICITY, CELL-SIZE, AND SEASON AS REGULATING FACTORS

The copepod Calanus finmarchicus was used to quantify effects on the g razing rate due to prey algae characteristics. Laboratory cultures of the haptophytes Emiliania huxleyi, Prymnesium patelliferum, and Pavlov a lutheri, the diatoms Thalassiosira nordenskioeldii, T. anguste-linea ta, and Chaetoceros calcitrans f. pumiilus, and the cryptophyte Rhodom onas baltica were used as food. There was no significant difference in clearance rate between exponential and stationary cultures. Feeding r ate increased rapidly with cell diameter (ESD) of the algae in the siz e range 7-17 mu m, while there was no apparent relationship for algae between 3 and 7 mu m. E. huxleyi was a suboptimal food with low cleara nce rates (0-1.6 ml cop(-1) h(-1)) and estimated maximum daily food:bo dy carbon ratios (18%). Feeding rate was not affected by Variation in coccolith coverage of the cells. In contrast to the diatoms, there was a pronounced seasonal shift in feeding rate on E. huxleyi. The highes t grazing rates coincided with the maximum abundance of E. huxleyi (Ma y-June), and may be caused by seasonal adaptation of the copepod. The feeding on P. patelliferum was lower (0-1 ml cop(-1) h(-1)) than expec ted from its cell size, and decreased further during incubations >24 h . A hepatocytic cell test demonstrated a high toxicity of the P. patel liferum strain, although the vitality of C.finmarchicus was not influe nced during a two-day exposure in dense suspensions (10(6) cells ml(-1 )). Thus sublethal toxicity may protect P. patelliferum from copepod p redation, being a competitive advantage compared with non-toxic algae. (C) 1995 International Council for the Exploration of the Sea