Modeling the Effects of Climate Change, Nutrients, and Invasive Species on Lake Huron Food Webs.

Abstract

I used ecological models to investigate potential effects of climate change, variation in nutrients, and invasive species on food webs of Saginaw Bay and the main basin of Lake Huron. In Chapter 2, I used bioenergetics models to investigate climate change-induced effects of increased temperatures on the growth and consumption by three salmonines: Chinook salmon, lake trout, and steelhead. Results showed that future growth and consumption by these salmonines will increase substantially if prey availability is not limited but will decrease if prey availability remains at current levels. Of these three salmonines, results indicated that Chinook salmon required the greatest increase in prey consumption to maintain current growth under increased temperatures. In Chapter 3, I analyzed impacts of changing nutrients, and invasive dreissenids and alewives on the Saginaw Bay food web. I found that nutrient loads were positively correlated with biomass of most food web groups and that alewife biomass was negatively correlated with biomass of most fish and zooplankton groups. Modeled dreissenid impacts were most severe on lower trophic levels but were minor on fish groups relative to nutrient and alewife effects. Results suggest that the target level of nutrient reduction established in 1978 may not sustain current fishery harvests in Saginaw Bay, given food web changes caused by invasive dreissenids. In Chapter 4, I assessed effects of Chinook salmon, invasive dreissenids, and declines in nutrients on the collapse of alewives in the Lake Huron main basin. Results suggested that the collapse of alewives was caused by combined effects of relatively high Chinook salmon biomass, invasion of dreissenids, and decline in nutrients. Although the top-down control from Chinook salmon biomass decreased in years previous to the alewife collapse, the bottom-up control increased as dreissenid mussels expanded and nutrient loads declined. Under current conditions of low nutrient loads and high quagga mussel biomass, results showed that it is unlikely that alewives will recover. Overall, results indicate that given the current trend toward oligotrophication in Lake Huron and rising temperatures due to climate change, sustaining a desirable level of Chinook salmon population and catch is unlikely.