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

Gut fluorescence attributable to algae caten 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 dcfecatcd and should also provide a calibration for the determination of evacuation rate in filtered seawater. The kinetics of gut filling and emptying in Temora Iongicornis 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-’ 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. The amount of carbon or energy consumed by particle-feeding zooplankton is an important unknown in the investigation of energy transformation by pelagic ecosystems. Animals used to measure zooplankton grazing are usually captured with a net, sorted by hand under a microscope or partitioned into size classes by sieving before being added to a representative portion of the natural environment or a laboratory facsimile, and their performance is then determined after a lengthy incubation in a closed container. The animals therefore are subjected to the stress of capture, sorting, and confinement often at concentrations much greater than in nature. Setting up, running, and analyzing such experiments are very laborious and the amount of data produced per unit of investigator’s effort is small. Furthermore, lengthy incubations may contribute to potential “containment” effects which can make the data unreliable or uninterpretable (Roman and Rublee 1980). Various factors also affect the feeding activity of copepods. Because marine environments are complex and variable, and the physiological state of animals may differ from time to time and from individual to individual, any attempt to estimate ingestion rate or ration in the field by using parameters obtained from laboratory experiments is risky. The best alternative would be to measure the feeding activity directly and in situ. Gut pigments, which are attributable to plants eaten, have been used as an index of feeding activity in herbivorous zooplankton in the field (Nemoto 1968, 1972; Mackas and Bohrer 1976; Boyd et al. 1980). The amount of gut pigment (G, in ng animal-‘) measured by fluorescence is the accumulation of plant pigments eaten (I, in ng animal-r t-l) during just previous “gut passage” or “turnover” time (T) or