Production rates and characteristics of fecal pellets of the copepod Acartia tonsa under simulated phytoplankton bloom conditions: implications for vertical fluxes

Over the course of a bloom, the quantity and quality of algae available for copepods to eat may vary dramahcally The effects of varying phytoplankton concentration and quality on fecal pellet produchon rates and fecal pellet charactenstics of the calanoid copepod Acartia tonsa were examined in the laboratory Bloom conditions were siinulated by feeding copepods cells of the diatom Thalassioslra weissflogii grown in batch cultures Experiments were run when algae were rapidly growing (early bloom) and when algae were in stationary growth phase (late bloom) at concentrations ranging from 0 to 3200 cells ml-' Daily pellet production rates and volume, density, and carbon and nitrogen content of pellets were measured Results show that for a given algal growth stage pellet production rate and pellet volume increase linearly w ~t h cell concentration up to a crit~cal point beyond whlch no further increases in these parameters occur In general, (1) maximum pellet volumes were higher when copepods ate late-bloom cells, (2) pellet densities were higher when copepods ate early-bloom cells and (3) volume-specific pellet carbon was higher when copepods ate early-bloom cells Sinking rates of pellets were estlrnated from an empincal model based on pellet volume and density Sinking rates of pellets were lowest at low concentralons (200 cells ml-l) concentrahons of early-bloom cells Sinking rates were highest at high concentrations (2000 cells ml-l) of early-bloom cells S i n l n g rates of fecal pellets were relatively high regardless of food concentration when copepods ate late-bloom cells Daily volume-speclfic fecal matter produced per copepod was also calculated Maxlmum values of total fecal matter index were h ~g h e r when copepods ate early-bloom cells Our results indicate that copepod fecal pellets change in a predictable manner over the course of a bloom We conclude that copepod fecal pellets may play different roles in recychng and vertically transporting carbon over the course of a phytoplankton bloom