Natural ingestion rates and the vertical distribution of copepods in the Gulf of Mexico, and an investigation of the gut fluorescence method

The feeding and distribution of three species of copepods (Eucalanus attenuatus Dana, Rhincalanus cornutus Dana and Neocal anus gracilis Dana) were investigated in continental slope waters of the Gulf of Mexico in August of 1982, June of 1983 and February of 1984. A gut fluorescence method was used to estimate ingestion rates in the field. One of the working assumptions of this method is that the rate of gastric evacuation (R,/min) remains constant with different ingestion rates. We found R to increase slowly but significantly with ingestion rate, however. This method was compared to the conventional (incubation) method for the determination of ingestion rate, with the copepod Temora longicornis. Different results were obtained with each method, but we found the gut fluorescence technique satisfactory for field use. Eucalanus, Rhincalanus and Neocalanus performed diel vertical migrations, but their migratory behaviors were highly variable. Nonetheless, the patterns of movements in the water column are similar for all species at a particular sampling episode. In February of 1984 E. attenuatus and R. cornutus performed reversed vertical migrations. The copepods were not associated with the depth of highest primary productivity or the depth of the chlorophyll maximum. Significant differences in ingestion rates of phytoplankton were found for copepods collected at different depths and different times of day. The amount of photosynthetic pigments in the guts of these copepods was generally correlated to the ambient pigment concentration, but ranged widely and could not be explained solely by the availability of phytoplankton. The copepods seemed to feed at higher rates at the depths at which they were more abundantEcology, Evolution and Behavio