Can aquatic invaders overtake underwater habitats with minimal effects on energy flows? Probably not, but little information exists on food web effects of invasive macrophytes. This study examined effects of a dominant invasive Eurasian watermilfoil, Myriophyllum spicatum on lake trophic structure and energy flow. Stable isotope and dietary analyses were used to compare niche width and trophic levels of predatory fauna as well as trophic diversity in invaded and uninvaded lakes and macrophyte stands. Contribution of native and invasive macrophytes, their epiphyton and detritus to lacustrine food webs was partitioned using mixing models.

Results/Conclusions

Convex hull area analysis showed that overall trophic diversity was greater in milfoil-dominated lakes. Carbon signatures of macroinvertebrate consumers closely matched the signatures of plant stands from which they were collected. However, contribution of watermilfoil and its epiphyton to higher trophic levels was negligible, and littoral fish derived most of their energy from epiphyton and detritus of native emergent and floating macrophytes. Since our published data showed that fish feeding was not dependent on the relative abundance of invasive watermilfoil, it appears that fish may depend on remaining patches of native macrophytes, in which case the non-linear effects of invader abundance may be expected. Approach developed in this study can be used to determine the threshold density at which invaders compromise ecosystem function and to evaluate restoration success after their eradication.