Phytoplankton-zooplankton inter-relationships in tropical waters - Grazing and gut pigment dynamics

Abstract

An examination of the feeding mechanisms in two taxonomically dissimilar classes of tropical planktonic harbivores Evadne tergestina (Cladocera) and Acrocalanus spp. (Copepoda) revealed a distinct rhythmicity of food intake. Brief bouths of phytoplankton intake were interspersed with breaks during which grazing activity ceased probably due to the onset of digestive enzyme activity. Over 90% of the chl a ingested was rapidly degraded during its passage through the digestive tract of the animals suggesting a high assimilatory and metabolic capacity within tropical planktonic herbivores as compared to temperate and polar forms. When subject to prolonged periods of starvation, both animals exhibited a clear tendency to reingest their own fecal pellets. In tropical oligotrophic waters where an invariably limiting size of phytoplankton food stock imposes constraints on zooplankton growth, the ability of tropical planktonic herbivores to resort to coprophagic feeding assumes particular importance as existing models make no allowances for reingestion of fecal pellets when quantifying and computing energy transfer in the food chain.