Feeding and reproductive responses of marine copepods in South China Sea to toxic and nontoxic phytoplankton

The influences of the toxic and nontoxic dinoflagellate Alexandrium tamarense and the diatom Thalassiosira weissflogii on the feeding and reproductive responses of the calanoid copepods Calanus sinicus and Paracalanus crassirostris were examined. Experiments were carried out to investigate the functional feeding response of copepods at different food concentrations for toxic and nontoxic A. tamarense and diatoms. Egg production of the copepods was compared at the same cell concentration (0.21 mg C 1^-1) for the three algal diets. The results demonstrated that the two copepods were able to feed on toxic dinoflagellates at rates similar to or higher than the rates with nontoxic dinoflagellates. Maximum ingestion rates were recorded at a food carbon concentration of 0.5 mg C 1^-1. The toxic and non-toxic dinoflagellates were radiolabeled with ¹⁰⁹Cd and ⁷⁵Se, respectively, and fed to the copepods as a mixture. The ratio of the two radiotracers in the copepods after feeding was comparable to the ratio measured in the cell mixture, indicating that the copepods were not able to select the particles based on the paralytic shellfish poisoning toxin content of the cells within a short exposure period. Copepods fed with the toxic dinoflagellates produced on average 17-46% fewer eggs than the animals fed with the nontoxic dinoflagellates and diatoms, although there was no significant difference among the three algal treatments due to the great variation among different individuals. The hatching success of eggs produced by copepods fed on the three diets and the larval development were, however, comparable. A field study was conducted to determine the grazing pressure of copepods on phytoplankton during a massive dinoflagellate bloom dominated by Prorocentrum dentatum in summer. Our calculations indicated that copepod grazing was able to remove < 10% of the phytoplankton standing stocks under these conditions.