ANADROMOUS ALEWIVES IN LINKED LAKE STREAM ECOSYSTEMS: CAN TROPHIC INTERACTIONS IN LAKES INFLUENCE STREAMS?

ANADROMOUS ALEWIVES IN LINKED LAKE STREAM ECOSYSTEMS: CAN TROPHIC INTERACTIONS IN LAKES INFLUENCE STREAMS?

By Lee Michael Demi

Thesis Advisor: Dr. Kevin Simon

A Lay Abstract of the Thesis Presented
in Partial Fulfillment of the Requirements for the
Degree of Master of Science
(in Ecology and Environmental Science)
August 2010

            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, or migratory, 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.

            I investigated the seasonal influence of anadromous anadromous alewives (Alosa pseudoharengus) on pelagic foodwebs in lakes by characterizing zooplankton communities and phytoplankton biomass during the spring and summer in 8 Maine lakes, four of which received spring alewife runs.  Young of the year alewives decrease the average body size of crustacean zooplankton by ~50% from spring to summer, by preying selectively on large bodied individuals.  The cascading effects of this feeding interaction result in increased summer phytoplankton biomass per unit phosphorus compared to non-alewife lakes.  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, which includes zooplankton and other suspended particles.  This subsidy often supports dense communities of filter feeding macroinvertebrates in outflow streams.  Alewives reduce the quantity and quality of large seston particles available for stream biota from spring to summer by selectively consuming large bodied zooplankton.  However, filter feeding macroinvertebrates communtities in the outflows of alewife lakes did not respond to seasonal changes in seston availability, suggesting that those communities are not limited by the availability of large, nutritious food particles, such as lake-derived zooplankton.