Abstract
The marine planktonic copepod, Macrosetella gracilis, ingested from 90 to 126% of its body carbon per day when feeding on the blue‐green alga Trichodesmium. Comparisons of laboratory ingestion rates with field measurements of M. gracilis abundance and the autotrophic activity of Trichodesmium indicate that M. gracilis could convert much of the carbon and nitrogen fixed by Trichodesmium into secondary production.