Living on the edge:: feeding of subtropical open ocean copepods

The objective of this study was to provide quantitative information on environmental feeding rates of warm water oceanic epipelagic copepods. We determined clearance rates at 23 degrees C for various particle size ranges in shipboard studies in the western oligotrophic subtropical Atlantic Ocean for females of the calanoid species Clausocalanus furcatus and Mecynocera clausii. These in situ clearance rates were then applied to the various particle size ranges of environmental particle spectra of auto- and heterotrophs at different depths from three stations in the western Atlantic. After calculating the metabolic demands of each of these two copepod species and applying an assimilation efficiency of 90%, we determined that C. furcatus meets its metabolic demands in all six cases, and M. clausii in two of six cases. Clausocalanus furcatus would also meet its energy demands at 25 degrees C, where it is often found, while M. clausii at 20 degrees C, where it is regularly found, would cover its metabolic needs in four of six cases. It is hypothesized that these species, and most likely most of the other co-occurring copepod species, are limited in their abundance by food availability, or, better said, are 'living on the edge' in relation to food abundance.