Subantarctic copepods in an oceanic, low chlorophyll environment:ciliate predation, food selectivity and impact on prey populations
Copepod feeding rates and food selectivity were measured at an oceanic subantarctic site over 5 d in February 1994. A series of gut fluorescence and gut evacuation experiments was carried out for species ranging in size from Oithona similis to Pleuromamma robusta. The copepods were also incubated in ambient seawater to compare their clearance rates on ciliates, diatoms and total chlorophyll a (chl a), using microscope counts and chlorophyll budgets. Mesozooplankton were concentrated into the top 50 m layer where biomass was high (5.5 g dry mass m-2) and comprised mainly (65%) of copepods. Copepod biomass was dominated by copepodite CV of Calanus simillimus. Numerically, however, Oithona spp. (mainly O. similis) constituted 85% of the copepods. These 2 species dominated copepod grazing and contributed to community grazing in about equal proportions. Chl a concentrations were low (~0.7 mg m-3) and were comprised of mostly nano- and picoplankton. Low daily phytoplankton carbon rations were derived from the 2 methods which measured total chl a consumption. These ranged from 1.5% (female CVI C. simillimus) to 34% of body carbon (late stage copepodites of Oithona spp.). These values were generally well below their estimated respiratory requirements, and probably reflected phytoplankton shortage because clearance rates were fairly high. Clearance rates for total chl a in the incubations tended to be less than those for the counted taxa >10 um, suggesting that some of the chl a was in cells too small to be eaten. Feeding selectivity followed a similar pattern across the 4 incubations. Highest clearance rates were on ciliates (~15 um), dinoflagellates (~20 um) or the diatom Corethron sp. (~200 um). Ciliates and dinoflagellates were cleared faster than centric diatoms of similar overall dimensions. It is suggested that the copepods were preying on protozoans preferentially, in response to phytoplankton shortage. Allometric relationships were used to derive the grazing and predation impact of the whole copepod community on primary production and estimated ciliate production. Less than 4% of primary production was removed per day. Copepod predation on ciliates was higher: 57% of daily production, assuming a net growth rate for ciliates of 0.1 d-1.