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 dramatically. The effects of varying phytoplankton concentration and quality on fecal pellet production rates and fecal pellet characteristics of the calanoid copepod Acartia tonsa were examined in the laboratory. Bloom conditions were simulated by feeding copepods cells of the diatom Thalassiosira weissflogii grown in batch cultures. Experiments were run when algae were rapidly growing (early bloom) and when algae were iu stationary growth phase (late bloom) at concentrations ranging from 0 to 3200 cells ml(-1). 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 with cell concentration up to a critical point, beyond which 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 estimated from an empirical model based on pellet volume and density. Sinking rates of pellets were lowest at low concentrations (200 cells ml(-1)) concentrations of early-bloom cells. Sinking rates were highest at high concentrations (2000 cells ml(-1)) of early-bloom cells. Sinking rates of fecal pellets were relatively high regardless of food concentration when copepods ate late-bloom cells. Daily volume-specific fecal matter produced per copepod was also calculated. Maximum values of total fecal matter index were higher 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 recycling and vertically transporting carbon over the course oi a phytoplankton bloom.