Reassessment of copepod grazing impact based on continuous time series of in vivo gut fluorescence from individual copepods

Abstract

Copepod grazing is a fundamental link between autotrophs and heterotrophs, affecting the particle size spectrum, modifying trophic interactions for a wide range of organisms, and playing a potentially significant role in the carbon cycle. A considerable amount of work has been done to quantify and understand feeding in copepods from a behavioral, physiological and ecological standpoint. An impressive body of knowledge has accumulated as a result. However, some of the methods used to experimentally determine copepod grazing have known, un-quantified problems. The small size of individual copepods requires the use of multiple animals, thereby introducing poorly understood variability. Digestion and gut dynamics in copepods are not fully understood either. Here we propose an approach to measure grazing that aims to be complementary to past contributions. Using a novel planar laser imaging technique, we have quantified grazing (pellet evacuation rates and gut clearance rates, K) in feeding copepods (Calanus pacificus). Highly resolved time series up to 5 h long were obtained from individual copepods at three different temperatures. K was found to be temperature-dependent, to vary in time and among copepods, and to be significantly higher during feeding than non-feeding, invalidating some assumptions of previous methods. Previous studies not accounting for the observed variations in K may have underestimated copepod ingestion by 15–70%. This suggests that copepods may exert a greater grazing pressure on phytoplankton than previously estimated, implying a higher copepodmediated vertical carbon transport, and a greater transfer rate of energy to higher trophic levels.