Trophic relationships in Dutch reservoirs recently invaded by ponto-caspian species: Insights from fish trends and stable isotope analysis

Abstract

Invasive species can directly or indirectly alter (a)biotic characteristics of ecosystems, resulting in changing energy flows through the food web. This can potentially affect bottom-up or top-down control on resident species. The food web structure in the Biesbosch reservoirs (The Netherlands) was examined after recent invasions of the quagga mussel (Dreissena rostriformis bugensis) and round goby (Neogobius melanostomus), by means of stable isotope analysis. Quagga mussels reached relatively high densities after invasion, replacing the zebra mussel (D. polymorpha). The δ13C signatures in the food web showed two distinct basic signatures of primary producers: 1) phytoplankton and Elodea nuttallii, and 2) more enriched submerged macrophytes (i.e. Potamogeton sp.). Quagga mussel was found to rely on zooplankton and phytoplankton. Mussel detritus seemed to be of importance for the gammarid Dikerogammarus spp. (both D. villosus and D. haemobaphes). δ15N as a proxy for trophic level revealed that the largest specimens of ruffe (Gymnocephalus cernuus), pike-perch (Sander lucioperca) and perch (Perca fluviatilis) formed the top of the food chain. A stable isotope mixing model estimated that 43% of round goby diet consisted of Dikerogammarus spp. and approximately 27% of quagga mussels. Ruffe diet consisted of 29% of quagga mussels and only 12% of Dikerogammarus sp. Quagga mussels were less important as a food source for round goby than often has been described. Besides being a food source, mussel beds also provide a suitable habitat for macroinvertebrates (i.e. Dikerogammarus spp.). The invasive round goby and native ruffe seemed to compete for food in the benthic zone, where round goby possibly forced ruffe to use less nutritive or favourable food sources. Likely, this has contributed to the decline in ruffe abundance after the invasion of the highly competitive round goby. The altered and recent new links between species have changed the food web. The successful invasion in the reservoirs by the benthic invaders — quagga mussel and round goby — changed the benthic-pelagic coupling and has most likely increased the importance of the benthic food web in these reservoirs.