Gross growth efficiencies of protozoan and metazoan zooplankton and their dependence on food concentration, predator-prey weight ratio, and taxonomic group

A comprehensive datasct on the gross growth efficiency (GGE) of planktonic protozoans and metazoans was gathered from the literature in order to (1) identify typical ranges of values, (2) to reexamine the taxon specificity of GGE, and (3) to evaluate the impact of food concentration, predator-prey weight ratio, and temperature on GGE. All taxa (i.e. nano/microAagellates, dinoflagellates, ciliates, rotifers, cladocerans, and copepods) were found to have mean and median GGE of -2O-30%. Contrary to the common practice of using different values of GGE for ciliates and crustaceans, I found that the GGE hardly differed between taxa. Variability within all taxa was high and could only partially be attributed to the independent variables mentioned above. The dependency of GGE on food concentrations was the most reliable relationship identified by multiple regression. Establishing further generalizations regarding the dependency of GGE on other factors was hampered by methodological differences among studies and taxa and the lack of information on other potentially important factors such as the clemental composition of prey items. Future studies of GGE should recognize the importance of these factors. Knowledge of fluxes of matter and energy within ecosystems is a prerequisite for the understanding of food web regulation and of the role that oceans and lakes play in the global carbon cycle (Longhurst 199 1). However, quantifying carbon fluxes reliably in particular ecosystems is hampered by many difficulties. The complexity of aquatic food webs outpaces our capacity to make all the necessary measurements at any one site (Vkzina and Platt 1988). This fact forces ecologists to infer unknown fluxes from those that have been measured. A relative straightforward and therefore common approach is to estimate ingestion rates, Z, of a group of organisms (of a guild) from measured growth rates, G, and a fixed gross growth efficiency, GGE: Z = G/GGE. The crucial ratio GGE (G/Z) is the fraction of prey carbon consumed converted to predator carbon. Despite its importance and abundant use in numerous models and applied studies, e.g. on the estimation of fish yield, the literature on GGE is not well developed and provides no or only a few weak generalizations. Searching the literature of carbon flux modeling, one finds that one generalization in particular has reached modelers’ ears: planktonic protozoans are thought to achieve higher GGE than do planktonic metazoans. Modelers are quite aware of high protozoan GGE (sensu Caron et al. 1990a) and usually use protozoan GGE 240% in their models. On the other hand, the conclusion of Calow (1977)-that “Metazoa can achive the best possible levels of efficiency preAcknowledgments This study was performed within the Special Collaborative Program (SFB) 248 “Cycling of matter in Lake Constance” supported by Deutsche Forschungsgemeinschaft. This manuscript benefited greatly from the discussion and critical reading by Ursula Gaedke. Willi Nag1 provided statistical advice. Lcwi Stone, Dvora Hart, Karl-Otto Rothhaupt, and Hans Glide greatly improved earlier drafts of this paper. Sincere thanks to Walter Geller, Alan Tessier, and one anonymous reviewer for constructive comments on this manuscript. dieted by theory and may, in this respect, be more efficient than isolated bacterial and protozoan cells”-is ignored. This generalization has been (possibly uncritically) applied by, among others, Fasham (1985) using a protozoan GGE of 40% vs. a metazoan GGE of 15%; Vkzina and Platt (1988) lo-60 vs. O-40%; Weisse et al. (1990), 40 vs. 25%; Nielsen and Kiorboe (1991), 40 vs. 33%; Leakey et al. (1992), 40 vs. 25%; Lignell et al. (1993), 40-50 vs. 25%; Nielsen et al. (1993), 40 vs. 33%; and Stone et al. (1993), 40-50 vs. 20%. The assumed taxon specificity is the only generalization on GGE of planktonic organisms used in these models. Individual studies have shown that GGE will depend on food concentration (Verity 1985; Urabe 1991), temperature (Sherr et al. 1983; Rassoulzadegan 1982), and food quality. One aspect of food quality, the effect of relative prey size on GGE, has not yet been addressed systematically in individual studies. However, pelagic predators are well known for their size-selective feeding behavior (Hansen et al. 1994). Furthermore, the dependence of GGE on relative prey size plays a crucial role in models on the trophic transfer efficiency along size gradients (Borgmann 1982; Gaedke 1993). Another important aspect of food quality is the biochemical composition of the food (Checkley 1980; Nakano 1994; Sterner and Hessen 1994). Unfortunately, lack of data prevents further analysis of the effect of biochemical prey composition on GGE. The purpose of this contribution is twofold. First, the experimental evidence for taxon-specific GGE of planktonic consumers is reexamined. Second, the dependence of GGE on food concentration, ambient temperature, and predatorprey weight ratios is analyzed to provide a new and critical basis for the use of GGE in models.