Results Suggest Considerable Variation in P Input-Export Relationship
Nutrient budgets that account for all inputs and outputs to a watershed provide an estimate of net nutrient additions, which in turn are an indicator of potential pollution. Haejin Han, Nathan Bosch and David Allan estimated net anthropogenic phosphorus inputs (NAPI) to 18 Lake Michigan (LM) and 6 Lake Erie (LE) watersheds for five agricultural census years from 1974 to1992. Fertilizer was the dominant phosphorus input overall, varying by three orders of magnitude among the 24 watersheds, but detergent was the largest input in the most urbanized watershed.
On average, rivers of the LE watersheds exported about 10% of NAPI, whereas LM watersheds exported 5% of estimated NAPI. Our results together with findings from the Chesapeake Bay region suggest that regional variation in P input-export relationships is considerable and in need of further study to determine the range of fractional export estimates and the conditions that may result in this variation.
- The amount of phosphorus exported by rivers is an approximately linear function of net anthropogenic phosphorus inputs (NAPI) to individual watersheds. Lake Erie and Lake Michigan data points are from Han, Bosch and Allan (in review) and represent values for individual watersheds averaged over 5 agricultural census years (1974, 1978, 1982, 1987, 1992). Chesapeake Bay data points come from Russell et al. (2009).
- (October 2009)