Nutrient and fish effects on the morphology of the dinoflagellate

The effects of nutrient loading and fish density on Ceratium hirundinella morphology were investigated during an experimental mesocosm study in Lake Creteil during the summer of 1990. In the presence of zooplanktivorous fish, more than 80 % of C. hirundinella cells had an hypotheca with two horns. In the enclosures without fish, we observed a highly significant increase in the proportion of C. hirundinella with three antapical horns. The proportion of C. hirundinella cells with three antapical horns tended to increase with nutrient loading, but this effect was not significant and no significant interaction effect between nutrients and fish was observed. Temperature profiles were similar in fish and fishless enclosures and could not explain the morphological variability of C. hirundinella. Our results show that biotic factors should be taken into account in polymorphism studies on Ceratium. Frequently invoked hypotheses based only on water temperature, nutrient availability or turbulence level, may not be sufficient to explain this phenomenon. ((C) Academie des sciences/Elsevier, Paris.).