Phytoplankton-zooplankton inter-relationships in tropical waters - Grazingand gut pigment dynamics

An examination of the feeding-mechanisms in two taxonomically dissimilar cl asses of tropical planktonic herbivores Evadne tergestina (Cladocera) and A crocalanus spp. (Copepoda) revealed a distinct rhythmicity of food intake. Brief bouts of phytoplankton intake were interspersed with breaks during w hich grazing activity ceased probably due to the onset of digestive enzyme activity. Over 90% of the chl a ingested was rapidly degraded during its pa ssage through the digestive tract of the animals suggesting a high assimila tory and metabolic capacity within tropical planktonic herbivores as compar ed to temperate and polar forms. When subject to prolonged periods of starv ation, 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 rei ngestion of fecal pellets when quantifying and computing energy transfer in the food chain.