Food quality determinants for Daphnia growth in P-limited lakes

The growth ofherbivorous zooplankton is crucial for production at higher trophic levels in lakes. Among the environmental factors affecting growth rates of planktonic herbivores, food availabiliry has received much attention because the abundance and production of phytoplankton vary gready in nature. In addition, growing evidence suggests that zooplankton grazers potentially suffer nor only from insufficient food quantity but frequently also from inadequate food quality (ELSER er al. 2000, HESSEN & FAAFENG 2000). While a large number of studies have addressed food quantity effects on herbivore growth, the relative importance of food quality and quantity has rarely been assessed in nature (STERNER & SCHULZ 1998). Other than toxiciry, size and morphology, relarive contents ofN, P and some bio-chemicals have been proposed as determinants of food quality for herbivores (HESSEN 1992, URABE & WATANABE 1992, MüLLER-NAvARRA 1995a, AHLGREN et al. 1997, .K!LHAM et al. 1997). Much of this work has focused on Daphnia, a key herbivore in lake food webs (CARPENTER & .K!TCHELL 1993). MDLLER-NAVARRA (1995a) showed that Daphnia growth in a German lake was most strongly correlated with the content of fatty acids, especially eicosapentaenoic acid (20:5ro3, EPA) in seston. In contrast, ELSER et al. (2001) demonstrated that P amendment to seston from P-limited lakes stimulated Daphnia growth, independent of any changes in fatty acids. These inconsistent results suggest that the nutritional substance determining food quality for Daphnia growth is site specific. If this is the case, it is important to determine when and where a particular nutritional substance is involved in determining food quality (STERNER 1997). In the present study, somatic growth rates of Daphnia individuals were measured using seston from P-limited lakes and from manipulated enclosures experiencing different light and P-supply regimes. Since production and chemical composition of algae vary widely depending on light and nutrient regimes (URABE & STERNER 1996, STERNER et al. 1998), field enclosures with different light and nutrient regimes provided a good opportunity to examine the effects of various nutritional substances in food on Daphnia growth. Specific goals were to evaluate (I) the relative importance of food quality and quantity, and (2) the relative importance of various putative substances in determining food quality for Daphnia growth.