A baseline study of food-web structure and function began in 2009 to contribute to monitoring of
the ecological impacts of impending dam removals on the Penobscot River watershed and nearshore marine environment. The study was led by Dr. Karen Wilson of the University of Southern Maine and Dr. Graham Sherwood of the Gulf of Maine Research Institute.
Freshwater and marine species were collected and sampled for nitrogen (15N) and carbon (13C) isotope signatures which provide informationon trophic position and energy source (i.e., feeding location). The results of this analysis pre-dam removal revealed valuable insights:
- Stable isotope signatures, particularly of carbon, were markedly different, as expected, among freshwater and marine consumers.
- Change in 13C from freshwater to marine indicated little connectivity between these adjacent systems prior to dam removal (with the exception of a few taxa).
- A handful of species showed high levels of connectivity between freshwater and marine systems. For example, mackerel captured in Penobscot Bay had intermediate 13C values indicative of feeding on freshwater-derived prey (assumed to be out-migrating juvenile river herring; mackerel, a highly mobile predator, may have consumed these prey from another bay).
- In the nearby Kennebec River watershed, which has significant river herring spawning populations, a strong shift towards marine C signatures, suggesting that river herring restoration efforts in this river system has resulted in the incorporation of marine derived nutrients into the riverine food web. Alewife captured in the river had entirely marine C signatures indicating that these species can function as a vector for marine-derived carbon into the freshwater system.
Small mouth bass from the lower Penobscot with adult alewife in its throat. Spring 2011, Photo by Ian Kiraly.