Cooperative Fish and Wildlife Research Units Program: Maine
Education, Research and Technical Assistance for Managing Our Natural Resources

Characterizing the role of marine derived nutrients in the Penobscot River has important implications for management of the watershed. This is a topic central to discussions concerning the planned removal of two main-stem dams. The ecological role of historically prominent migratory species is controversial in both scientific and political arenas because of the paucity of historical data. This pilot study may improve our understanding of this system’s basic ecology. The ratios of naturally-occurring levels of carbon (13C and 12C) and nitrogen (15N and 14N) isotopes in biota are influenced by a number of factors including trophic level and environment. Marine systems generally have greater proportions of heavier isotopes (Craig 1953; Peters et al.1978; Owens 1987). These “signatures” are also observed in anadromous fish that migrate between marine and fresh water environments (Bunn et al. 1989; Doucett et al. 1999a; 1999b; McCarthy and Waldron 2000). Anadromous fish runs can deliver considerable marine-derived biomass to the food webs of fresh water systems, leaving detectable “marine signatures” in the ecosystem, including the plant community (Helfield and Naiman, 2001). Importantly, trees can chronicle fluctuations in this signature by the growth of annual rings. Considerable runs of anadromous fish (e.g., American shad, alewife, sea lamprey and Atlantic salmon) occurred in the Penobscot River prior to the construction of main stem dams in the early 1800’s and declined precipitously afterwards. There is uncertainty, however, over specific characterizations of this occurrence, as much of the evidence is anecdotal. Long lived trees offer a tremendous opportunity to chronicle marine derived nutrient contributions to fresh water ecosystem in the Penobscot River watershed. Those few large trees whose growth has spanned periods previous to and subsequent to dam construction may carry quantifiable signatures of marine derived nutrients through a historic time series. These data may also help in interpreting the spatial extent of this contribution throughout the Penobscot River drainage. Objectives and Approach Objective 1: Identify areas in the Penobscot drainage with old growth trees that would have been exposed to marine derived nutrients prior to mainstem dam construction (early 1800’s). Objective 2: Obtain dendrochronological cores from old, riparian trees. Objective 3: Measure ratios of marine to freshwater-derived carbon and nitrogen isotopes in annual growth rings corresponding to before and after construction of main stem dams in the Penobscot River.