MEPS 257:139-158 (2003)  -  doi:10.3354/meps257139

ABSTRACT: We measured diet composition, prey preferences and feeding rates of Acartia spp., an abundant copepod group in San Francisco Bay. Monthly incubations with Acartia feeding upon the natural planktonic assemblage were conducted during spring 2000 at 2 locations: South Bay (SB, lagoonal estuary) and San Pablo Bay (SPB, partially mixed estuary). Prey assemblages in SB and SPB were always dominated by nanoplankton, however Acartia never consumed cells <10 µm in either location. Overall abundance >15 µm was always higher in SB, comprised primarily of autotrophic cells (diatoms and pigment-containing flagellates). The assemblage in SPB was typically dominated by heterotrophic prey (ciliates and small non-pigmented flagellates). Acartia consumed a diverse diet but were highly selective for motile prey, especially ciliates and nanoflagellates. Acartia selectivity for individual prey taxa was strongest during periods of high food abundance, consistent with optimal foraging theory. In SB at least 50% of Acartia diet consisted of autotrophic biomass (diatoms, flagellates and the autotrophic ciliate Mesodinium). Ingestion rates were low and accounted for only 6.3% of body carbon per day, except during the March bloom, when Acartia ingested diatom biomass at 217 ngC copepod^-1 h^-1, or 188% of body carbon per day. In SPB Acartia diets were dominated by heterotrophic prey >10 µm, with ciliates and non-pigmented flagellates always >60% of total biomass consumed. Ingestion rates were lower than in SB (typically equivalent to 2.2% of body carbon per day), but in the April bloom Acartia increased consumption of heterotrophic flagellates to 121 ngC copepod^-1 h^-1, or 101% of body carbon per day. These results indicate that protozoans provide an essential nutritional supplement for San Francisco Bay copepods, especially in SPB, and that bloom periods may be important for copepod production and, in turn, higher trophic levels.

KEY WORDS: Copepod feeding · Acartia · Zooplankton · Ciliates · Clearance rates · Prey preference · Selectivity · San Francisco Bay