, Volume 74, Issue 1, pp 115-130

Terrestrial vegetation and the seasonal cycleof dissolved silica in a southern New Englandcoastal river

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The Pawcatuck river watershed (797 km2) is located in southern Rhode Island and northeastern Connecticut. The predominant lithology of the area is granite, and over 60% of the watershed remains forested with mixed hardwoods (primarily oak) and eastern white pine. As part of a larger study of nutrient and sediment exports from the watershed to Little Narragansett Bay, we measured dissolved silica (SiO2) (DSi) concentrations at the river mouth over 70 times between January 14, 2002 and November 29, 2002. Annual export of DSi during our study was 40 × 106 mol or 50 kmol  km−2. The United States Geological Survey (USGS) obtained DSi concentrations at this site, at varying frequencies, from 1978 to the present, which allowed for a historical comparison of this study with previous years. River DSi concentrations exhibited a strong seasonal signal that did not vary in a regular way with water discharge or water temperature. DSi and dissolved inorganic nitrogen (DIN) concentrations were significantly related over the annual cycle (p<0.0001) and both decreased substantially during the spring. Dissolved inorganic phosphorus (DIP) did not covary at any time with silica or nitrogen, suggesting that in-stream biological uptake was not responsible for the seasonal decline in silica. The spring decline in river silica concentrations may be due to silica uptake by terrestrial vegetation. We estimate a net forest silica accretion rate of 41 kmol  km−2 y−1, a value that is stoichiometrically consistent with other measurements of net carbon accretion in nearby forests.