Ken Johnson¹ and Steve Kahl²

¹Senator George J. Mitchell Center for Environmental and Watershed Research, University of Maine, Orono.
²Center for the Environment, Plymouth State University, Plymouth, NH

Atlantic salmon (Salmo salar) have been returning to Maine rivers in very small numbers for many years. A number of potential explanations have been offered. The National Academy of Sciences (2003) report on Atlantic salmon highlighted water chemistry as a concern for the recovery of Atlantic salmon in Maine. This factor is currently being evaluated by several studies, including ours. We report here on the first systematic water chemistry survey of Maine salmon rivers. Samples were collected from approximately 70 stations on 13 rivers in spring, summer and fall of 2003 and 2004. In each time period, samples were collected from the full set of stations on the same date.

The rivers we studied fall into two main categories based on their water chemistry (Figures 1 and 2). Rivers east of the Penobscot have low base cation concentrations, a typical condition for waters in landscapes dominated by acidic bogs and wetlands. These waters are characterized by high dissolved organic carbon (DOC), low pH, low base cation concentrations, and low acid neutralizing capacity (ANC). Rivers to the west of the Penobscot have opposite chemistry; low DOC, higher pH, more base cations, and a higher ANC. These differences are driven by the geology of these areas of Maine. The bedrock geology of eastern Maine can be simplified into two types of terrain separated by a line running from the Penobscot River near Winterport northeast towards Topsfield (Norumbega Fault). North and west of this line (rivers west of the Penobscot River) the rocks are mostly derived from former marine sediments with some rocks containing a fraction of carbonate minerals. South and east of this line (rivers east of the Penobscot River) the rocks are derived from volcanic rocks and more recent intrusive igneous rocks. These rock types differ in their chemistries (especially calcium, magnesium, aluminum, and iron) and resistance to erosion or dissolution.

Figure 1: Box plot showing the differences in pH, calcium, and DOC for rivers east and west of the Penobscot. Rivers to the west have a higher pH and lower DOC. These rivers also have a larger range of calcium, perhaps due to agricultural activities in addition to geologic contributions. Rivers to the east have a much larger range of DOC concentrations due in part to the greater number of bogs and wetlands feeding those rivers. (Note: ClpH = closed-cell pH)

Figure 2: Box plot showing differences in ANC and aluminum for rivers east and west of the Penobscot. Less soluble bedrock east of the Penobscot contributes less ANC and more aluminum than the bedrock formations west of the Penobscot.

References:

Johnson, K.J. and J.S. Kahl. 2005. A systematic survey of water chemistry for Downeast area rivers. Senator George J. Mitchell Center for Environmental and Watershed Research, University of Maine, Orono.

National Academies of Science. 2003, Atlantic Salmon in Maine, The National Academies Press, Washington, D.C.