In vivo determination of Daphnia feeding rates using PAM fluorometry

Abstract

We examined the utility and sensitivity of PAM fluorometry as a tool for estimatingin vivo gut fluorescence and phytoplankton ingestion rates by Daphnia.KEYWORDS: ingestion; PAM fluorometry; zooplankton; gut pigmentsThe quantity, composition and rates of crustacean zoo-plankton feeding are used to quantify the trophic role ofthese animals in aquatic food webs. The most widelyused field and laboratory methods measure: (i) rate ofchange of the prey field (Gauld, 1951) and (ii) concen-tration of phytoplankton in the zooplankton gut(Mackas and Bohrer, 1976). Each approach has advan-tages and disadvantages (see Ba˚mstedt et al., 2000) andthe choice of method must consider the trade-offbetween information gained and implementationdifficulty.Clearance rate estimations are attractive becauseanimals need not be sacrificed and they can yield infor-mation on selectivity in terms of composition of theprey field. Recent methodological advances haveemployed tools including the FlowCAM (Ide et al.,2008) and the PhytoPAM (Lu¨rling and Verschoor, 2003)to reduce time-consuming microscopic analyses.However, these methods still indirectly measure materialactually ingested and are prone to error associated withovercrowding and sloppy feeding. On the other hand,direct measures of gut contents such as the gut fluor-escence technique are practical and simple: ingestionrate is calculated as the product of gut transit time(GTT) and content. Ingested phytoplankton biomass isestimated with solvent extraction followed by fluorome-try and GTT estimated as the rate of change of gutcontent for animals incubated in filtered water. Dietcomposition can further be measured using pigment-specific analysis (e.g. HPLC; Kleppel and Peiper, 1984;Quiblier et al., 1994). This approach has been widelyused in marine studies but criticized regarding thecorrect interpretation of the photosynthetic pigmentdegradation products in the gut (Ba˚mstedt et al., 2000).Although the gut fluorescence method is relativelysimple and thus attractive, tissue requirements may behigh (e.g. HPLC analyses; Kleppel and Peiper, 1984).Furthermore, solvent treatment is prohibitive when theobjective is to relate feeding to physiological statemeasurements (e.g. enzyme activity). An ideal methodwould permit accurate repeated measurement offeeding activity on the same individuals (or subset) asused for physiological measurements. Recent studieshave pursued non-destructive gut fluorescence measure-ments and demonstrate that in vivo (as opposed toextracted) measures of gut fluorescence represent poten-tially rapid and reliable estimates. Such methods havebeen used to estimate gut pigment content and GTTon