Date of Award


Level of Access Assigned by Author

Campus-Only Thesis

Degree Name

Master of Science (MS)


Ecology and Environmental Sciences


Kevin S. Simon

Second Committee Member

Jasmine E. Saros

Third Committee Member

Stephen M. Coghlan, Jr.


Ecosystem linkages are ubiquitous across the ecological landscape and are critical for maintaining the structure and function of foodwebs in spatially connected ecosystems. Our knowledge of the factors and mechanisms that influence ecosystem linkages, however, are poorly understood. Diadromous fish represent an important link between marine and freshwater ecosystems via their migratory life histories. Inland migration of spawning adults contributes important subsidies of marine derived carbon and nutrients to freshwater foodwebs. However diadromous fish can also influence freshwater food webs and aquatic linkages via direct food web interactions. Here, I have investigated the influence of anadromous alewives (Alosa pseudoharengus) on structuring pelagic foodwebs in Maine lakes. I have expanded upon previous work on anadromous alewives by investigating the potential for lake trophic state to mitigate the effect of top-down trophic cascades on phytoplankton biomass by sampling lakes that span a large trophic gradient. Anadromous alewives decrease the average body size of crustacean zooplankton by ∼50% from spring to summer. The cascading effects of this feeding interaction result in increased summer phytoplankton biomass per unit phosphorus. However, the strength of those cascades and their potential to influence phytoplankton biomass differs along a trophic gradient. Trophic cascades are stronger in nutrient poor lakes than in eutrophic lakes, due to the greater relative importance of nutrient availability in regulating phytoplankton dynamics under high nutrient conditions. Additionally, I have addressed the potential for trophic interactions in lakes to influence the connectivity between lakes and their outflow streams. Lakes provide a constant flow of energy and nutrients to streams in the form of seston. Seston, which is comprised of suspended particles, such as zooplankton, is an important food subsidy that supports dense assemblages of filter feeding macroinvertebrates in lake outflow streams. I have found that alewives alter the flux of seston from lakes to streams on a seasonal basis by reducing the amount of zooplankton in the drift, via their consumptive effects on zooplankton in lakes. This results in a decrease in the concentration and quality of large food particles available for stream biota. However, there was no response in filter feeding invertebrate communities to changes in food availability, suggesting that aquatic macroinvertebrate communities in these streams are not food limited. This thesis highlights the need for a greater understanding of how trophic interactions between living organisms can influence the connectivity of ecosystems in the ecological landscape, by altering the timing and magnitude of spatial subsidies. Furthermore, this thesis increases our understanding of the influence of an important migratory fish species on freshwater foodwebs in Maine's coastal watersheds.