Exploring How Invasive Species Have Altered Biomass Flow and Food Web Properties in Lake Huron

Invasive species have resulted in dramatic changes in the Lake Huron food web since the beginning of the twentieth century. One unwelcome change to sport fishers during the past decade is the degradation of the most popular Chinook salmon sport fishery following the collapse of the alewife population. The alewife collapse likely resulted from bottom-up effects caused by invasion of dreissenid mussels and from top-down effects caused by over-abundance of introduced Chinook salmon. We used the Ecopath modeling approach to investigate changes in biomass flows and food web properties in Lake Huron in the period between 1984 and 2003, a period when dressenid, Bythotrephes, and round goby invaded. We conducted a mixed trophic impact analysis to quantify the relative importance of dreissenids consumption and salmonines predation to the collapse of alewife population. Two biomass-balance models were built to represent the food webs during the pre-dreissenid period and the period of alewife population decline. We identified 28 functional groups in the Lake Huron food web (25 and 28 groups in the first and second periods, respectively). We used biomass estimates from 1984 and 2003, and assembled ecological information including diet composition, production rate, and consumption rate for each functional group for the two models. Our results showed that these invasive species and introduced salmonines have resulted in a significant simplification of the Lake Huron food web. This was reflected by lower total system biomass flow, mean food chain length, biomass supported by unit net primary production, and mean residence time of biomass. Results from mixed trophic impact analysis showed that salmonines had stronger negative effects on alewife population than dreissenids, and these two groups had the highest trophic effects in the food web.