Bacteria associated with a marine planktonic copepod in culture .2. Degradation of fecal pellets produced on a diatom, a nanoflagellate or a dinoflagellate diet

Copepods (Acartia tonsa Dana) were allowed to feed on cultured algal species simulating the yearly boreal phytoplankton succession, where the spring is characterized by diatoms, the summer by nanoflagellates and the autumn by dinoflagellates. Degradation of freshly produced fecal pellets in suspension was followed over time by volume recording and estimation of carbon loss. The fecal pellets produced on a diatom diet were very dense and showed a significantly slower degradation than those produced on nanoflagellate or dinoflagellate diets. This indicates that the algal texture, i.e. the mechanical robustness given by inert structures, determines the degradation rate of the pellets. The degradation rate was also very dependent on the algal concentration for the grazer (565 mu g C l(-1) T-1/2 = 75 h; excess food T-1/2 = 12 h). Bacteria associated with the fecal pellets were monitored just after egestion and through 160-400 h of incubation. The bacteria associated directly with diatom-based pellets decreased rapidly in number over time and reached zero after 400 h. In contrast, the bacteria associated with the nanoflagellate- and dinoflagellate-based pellets increased in number, describing a saturation after 150 and 50 h, respectively. The bacterial number in the incubation water wherein the pellets were suspended increased and reached a plateau after similar to 100, 25 and 20 h for the diatom-, nanoflagellate- and dinofiagellate-based pellets, respectively. The carbon density of diatom-based pellets was lower than that of flagellate-based pellets. The difference in bacterial growth on carbon from the pellets indicates a difference in substrate quality, where diatom-based pellets are suggested to be a poor substrate compared to flagellate-based pellets. This leads to the conclusion, taking the dominating algal taxa and the pellet sedimentation rates into consideration, that flagellate-based fecal pellets are degraded in the surface waters in summer and autumn, while diatom-based pellets are likely to be degraded on the bottom of the sea in spring.