Shifts in zooplankton species composition affect nutritional food quality for fish

Background/Question/Methods

Degradation of carrying capacity and consequent food limitation has been proposed as one of the key factors driving the recent decline of pelagic fishes in the upper San Francisco Estuary. Dramatic drop in fish biomass, however, was not accompanied by equivalent decrease in zooplankton carbon, the key food source for threatened and endangered fish species. It is hypothesized that shifts in zooplankton species composition associated with the establishment of invasive species has reduced the nutritional prey quality for secondary consumers. We tested whether essential nutrient concentrations vary among native and introduced zooplankton species. Fatty acid (FA) associated food quality is a critical factor that regulates the energy transfer between primary producers and consumers, and plays an important role in growth, development and reproduction success in heterotrophs. We compared the fatty acid profiles of seven dominant native and invasive zooplankton species collected in the Estuary. Lower concentrations of essential FAs in invasive species would suggest negative changes in the food quality for fish.

Results/Conclusions

Our analyses show substantial differences in long-chain polyunsaturated FAs (PUFAs) across zooplankton taxa. The invasive cyclopoid Limnoithona that currently dominates plankton communities had the lowest concentration of eicosapentaenoic acid (EPA). We also detected substantially lower accumulation of monounsaturated fatty acids (MUFAs) by Limnoithona, potentially associated with their carnivorous diet. Higher accumulation of EPA but no accumulation of docosahexaenoic acid (DHA) in cladocerans was confirmed in our analysis of Daphnia. These taxonomical differences in the FA composition altered nutritional quality of the whole community, because native cladocerans significantly declined and invasive Limnoithona increased after their introduction in the early 1990s. Observed shifts in the essential FAs of the whole zooplankton community likely modified growth and survival of secondary consumers. Temporal dynamics in the zooplankton nutritional status is calculated and compared to the observed declines of pelagic fishes in the Estuary. Our study highlights the importance of nutritional quality to fully understand the underlying processes that limit production at higher trophic levels.