Dynamics of gut pigment in the copepod Temora longicornis and the determination of in situ grazing rates

Abstract

Gut fluorescence attributable to algae eaten by an herbivorous zooplankter should be a function of chlorophyll consumption minus loss of pheopigments in defecation. The amount of chlorophyll ingested, as determined in a laboratory grazing experiment, minus the gut pigment content at the end in the grazers, should equal chlorophyll defecated and should also provide a calibration for the determination of evacuation rate in filtered seawater. The kinetics of gut filling and emptying in Temora longicornis have been studied at different food concentrations. Neither is constant with time. Filling rate is positively correlated with food concentration but gut pigment content may be maximal at lower concentrations when food supply is presumably still limiting. Evacuation rates, while not constant, are independent of initial gut chlorophyll content or food concentration in the experimental environment over a range from 0.5 to 8 mm3 liter−1 of Coulter Counter volume. Ingestion rates determined from gut pigment concentration and gut clearance time and those determined from filtration rate were directly correlated (slope = 1) but showed a systematic difference apparently caused by undetected loss of gut pigment. An improved method for determining in situ grazing rates from gut pigment measurements is proposed.