AUTOTROPHIC AND HETEROTROPHIC NANOPLANKTON IN THE DIET OF THE ESTUARINE COPEPODS EURYTEMORA-AFFINIS AND ACARTIA-BIFILOSA

The ingestion of autotrophic and heterotrophic nanoplankton by two est uarine copepods, Eurytemora affinis and Acartia bifilosa, was measured in various environmental conditions using the incubation method and e pifluorescence microscopy. Egg production of the species was also dete rmined in order to estimate their carbon requirements. Assuming a gros s efficiency of egg production of 0.3, nanoplanktonic carbon ingested always met the carbon requirements suggesting that, most of the time, other carbon sources could be unnecessary. Nanoplankton ingestion by A . bifilosa (from 128 to 1693 cells ind.(-1) h(-1)) was dominated by au totrophic forms (60-97%) and was seriously affected by high (>100 mg l (-1)) suspended particulate matter (SPM) concentrations. Nanoplankton ingestion by E. affinis (from 300 to 1049 cells ind.(-1) h(-1)) was re latively stable in comparison, but this latter species seemed to switc h its grazing pressure from autotrophic to heterotrophic forms when SP M concentrations increased. Thus, two copepod species, living in the s ame estuary, presented two different feeding behaviours, probably to m aximize energy input per unit of energy expenditure. Such differences could contribute to the spatial and seasonal segregation of these spec ies which is usually observed.