Trophic relationships in Dutch reservoirs recently invaded by Ponto-Caspian species: insights from fish trends and stable isotope analysis

摘要

Invasive species can directly or indirectly alter (a)biotic characteristics of ecosystems,resulting in changing energy flows through the food web. This can potentiallyaffect bottom-up or top-down control on resident species. The food web structure inthe Biesbosch reservoirs (The Netherlands) was examined after recent invasions ofthe quagga mussel (Dreissena rostriformis bugensis) and round goby (Neogobiusmelanostomus), by means of stable isotope analysis. Quagga mussels reachedrelatively high densities after invasion, replacing the zebra mussel (D. polymorpha).The δ 13 C signatures in the food web showed two distinct basic signatures ofprimary producers: 1) phytoplankton and Elodea nuttallii, and 2) more enrichedsubmerged macrophytes (i.e. Potamogeton sp.). Quagga mussel was found to relyon zooplankton and phytoplankton. Mussel detritus seemed to be of importance forthe gammarid Dikerogammarus spp. (both D. villosus and D. haemobaphes). δ 15 N asa proxy for trophic level revealed that the largest specimens of ruffe (Gymnocephaluscernuus), pike-perch (Sander lucioperca) and perch (Perca fluviatilis) formed thetop of the food chain. A stable isotope mixing model estimated that 43% of round gobydiet 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 hasbeen described. Besides being a food source, mussel beds also provide a suitablehabitat for macroinvertebrates (i.e. Dikerogammarus spp.). The invasive round gobyand native ruffe seemed to compete for food in the benthic zone, where round gobypossibly forced ruffe to use less nutritive or favourable food sources. Likely, thishas contributed to the decline in ruffe abundance after the invasion of the highlycompetitive round goby. The altered and recent new links between species havechanged the food web. The successful invasion in the reservoirs by the benthic invaders— quagga mussel and round goby — changed the benthic-pelagic coupling and hasmost likely increased the importance of the benthic food web in these reservoirs.