Date of Award


Degree Type


Degree Name

Master of Science (MS)


Forestry and Natural Resources

First Advisor

Paris D. Collingsworth

Second Advisor

Tomas O. Hook

Committee Chair

Paris D. Collingsworth

Committee Co-Chair

Tomas O. Hook

Committee Member 1

Barry M. Lesht


Nutrient abatement programs, originally enacted to counter eutrophication have successfully suppressed nutrient loads and relative primary production in marine and freshwater systems. Recently, the additive impacts of invasive filter feeders have further reduced offshore primary production in several aquatic systems throughout the world. It has been hypothesized that the biological activity of these invasive species may sequester nutrients within the nearshore benthic environment, creating steep gradients in primary production between nearshore and offshore habitats. In the Laurentian Great Lakes, past studies of oligotrophication have primarily focused on food web responses in offshore habitats. Organisms at higher trophic levels have been impacted by reduced productivity in offshore waters and may now rely more heavily on nearshore production pathways. This study analyzed long-term changes in surface chlorophyll a concentrations along a nearshore to offshore gradient and the subsequent responses in physiological condition of the secondary consumer Bythotrephes longimanus (hereafter referred to as Bythotrephes). First, we examined long-term (1998-2013) seasonal and regional trends in surface chlorophyll a concentrations in the main basins of lakes Michigan and Huron using satellite imagery. Contrary to the pronounced declines in offshore areas, nearshore chlorophyll a concentrations increased or remained stable throughout time for most seasons. While no regional trends were observed in Lake Huron, nearshore trends were not homogeneous across regions in Lake Michigan. From 1998 to 2013, Chlorophyll a concentrations increased in nearshore areas in the southeast and northwest quadrants of Lake Michigan, most likely due the influence of large adjacent agricultural watersheds and warmer surface water temperatures that characterize these regions. These results were in contrast to the southwest region where nearshore chlorophyll a concentrations declined over the course of 16 years. Increases in nearshore chlorophyll a concentrations and subsequent increase in prey availability, may corroborate the recent reliance of nearshore resources by some fish species in the Great Lakes. The second aspect of this study focused on whether increased nearshore food web subsides would affect the condition of Bythotrephes along a nearshore to offshore gradient and regionally in Lake Michigan. No nearshore to offshore differences in lengths, RNA content, and relative fatty acid content were observed; however, regional differences were, again, prominent. Regional variation in surface water temperature facilitated larger sizes and overall condition of Bythotrephes on the eastern shores of Lake Michigan during the spring and summer. By the fall, surface water had warmed sufficiently lake-wide to allow for comparable size and condition in Bythotrephes across southern Lake Michigan. Overall, responses to oligotrophication at lower trophic levels have differed in both time and space in lakes Michigan and Huron. Further description of how lower trophic levels respond to oligotrophication can help improve our understanding of how nutrient abatement programs and the establishment of invasive filter feeders are influencing food webs across the Great Lakes.