Environmental and algal forcing of Daphnia production dynamics

Abstract

Recent studies of lakes suggest that the relative availability of light and phosphorus in the mixed layer helps determine the quality of phytoplankton consumed by herbivorous zooplankton. In turn, increases in algal quality positively affect zooplankton growth and reproductive rates. However, these studies have concentrated on explaining variation among lakes over relatively short time periods and have not evaluated the temporal dynamics in the physical-chemical forcing on zooplankton production. We investigated whether there were seasonal changes in the relationship between Daphnia fecundity and the physical-chemical environment, algal community characteristics, and Daphnia length using dynamic linear models (DLM) to analyze a high-resolution, 16-yr time series from Lake Washington. We used two metrics to describe algal quality: the percentage of blue-green algae and the light to total phosphorus (TP) ratio as an indirect metric for algal C:P. Chlorophyll a (Chl a) concentration and algal biovolume served as measures of algal quantity. As expected, both the light: TP ratio and the percentage of blue-green algae had negative effects on Daphnia fecundity, and Chl a concentration and biovolume had positive effects on fecundity. However, we found a comparatively stronger effect of Daphnia length on fecundity, although it was negative and therefore opposite to expectation. In all cases, we observed strong seasonal differences in environmental effects, but no temporal change in the fundamental relationships between our indices of algal quantity and quality, allometry, and Daphnia reproduction. Our results support a hypothesis of physical-chemical coupling to algal quality and zooplankton production but highlight the importance of temporal variation in this forcing.