Probable loss of chlorophyll-derived pigments during passage through the gut of zooplankton, and some of the consequences

By use of biogenic silica as an inert marker, it is shown that chlorophyll a and its derivatives can be destroyed or absorbed during passage through the gut of a herbivorous copepod. This observation is contradictory to the hypothesis that chlorophyll a is converted to pheophorbide a with 100% molar efficiency. The currently used equations for measuring chlorophyll and pheopigment by fluorescence cannot be used to give concentration of phcopigment. Determining quantitatively the role of zooplankton as intermediates in material and energy transfer in aquatic food chains remains a formidable challenge for biological oceanographers. Several kinds of in vivo or in situ techniques have been developed to avoid the lengthy incubations usually required to measure grazing rates in the laboratory, with their attendant problems of interpretation and application. Probably the most widely used of these is the gut chlorophyll method (Mackas and Bohrer 1976). The usefulness of this technique to examine short term variability in zooplankton feeding has been frequently demonstrated (Boyd et al. 1980; Dagg and Grill 1980; Dagg and Wyman 1983; Nicolajsen et al. 1983; Head et al. 1984). On the other hand, there have been relatively few attempts to relate ingestion based on gut pigment contents with ingestion derived from conventional grazing experiments and, when this has been attempted, the results have been contradictory. Kiorboe et al. (1982) carried out a parallel series of grazing experiments and gut chlorophyll ingestion measurements under the same conditions of food and temperature, but at different times, and then used somewhat different models to describe their results, concluding that the agreement was good enough to make (p. 192) “the prospects for application of the indirect method (i.e. gut pigment method) to field situations promising.” Wang and Conover (1986) addressed the same problem by measuring the accumulation of gut pigment and the rate of its removal by grazing simultaneously, but they found that more chlorophyll was ingested than could be accounted for by that found in the gut and that disappearing from the animal during defecation. Several possible explanations for the discrepancy between the ingestion rates generated by the two methods in Wang and Conover’s ( 1986) studies come to mind. First, to convert the level of gut pigment in an animal to a rate of ingestion requires an estimate of gut turnover or evacuation rate, which are assumed to be equivalent. Evacuation rate is usually determined from loss of gut pigment in previously fed animals after transfer to filtered seawater (Mackas and Bohrer 1976; Dagg and Grill 1980; Kiorboe et al. 1982; Wang and Conover 1986). However, an evacuation rate for starving animals might underestimate turnover rate for those that are feeding regularly. Or alternatively there could be loss of pigment, perhaps through exposure to light or carbon dioxide or as a consequence of freezing (Nicolajsen et al. 1983). Or conceivably, a portion of the pigment actually ingested is digested or destroyed in the gut. Wang and Conover (1986) examined these several possibilities and concluded that pigment destruction by the digestive process was the most probable explanation. Here we test this hypothesis. In their original description of the gut pigment method, Mackas and Bohrer (1976)