The influence of watershed land use on lake N : P in a predominantly agricultural landscape

This study tests the hypothesis that lakes in watersheds dominated by row-crop agriculture (e.g., maize or soybeans) have systematically higher N : P than lakes in watersheds with large tracts of pasturelands. Current biogeochemical models of eutrophication suggest that agricultural nitrogen and phosphorus fluxes lead to a systematic decline in the N : P of receiving waters. In contrast, different agricultural activities (i.e., row-cropping vs. animal agriculture) use greatly divergent N and P amendments, and fluxes from agricultural watersheds diverge through a broad range of observed N : P (i.e., sub-Redfield to 100). Animal agriculture leads to low N : P fluxes and row-cropping to high N : P. The connection between agricultural watershed land use and lake nutrient stoichiometry was tested in a highly agricultural region of the United States (Iowa) on 113 lakes in watersheds with different amounts of row-crop (0%–95%) and pastureland (0%–36%). Multiple regression analysis shows that lakes in watersheds with large areas in pasturelands have low N : P, whereas lakes in watersheds dominated by row-cropping have systematically high N : P. Lakes in watersheds with 30% pasture had the lowest N : P, approaching Redfield levels. N : P was most frequently high ( 50 as atoms) in lakes with 90% of their watersheds in row-crop agriculture. The dynamics of agricultural practice necessitates the inclusion of real-world differences among agricultural systems in nutrient stoichiometric models. Intensive row-crop agriculture yields N : P stoichiometry at high levels usually observed in pristine headwaters and open oceans, whereas increased animal agriculture will drive N : P to low levels usually associated with cyanobacterial blooms. Agricultural activities are a major source of nutrients to freshwater (Howarth 1996) and marine (Downing et al. 1999b) ecosystems. Nitrogen and phosphorus have been identified as leading pollutants in lakes, rivers, and estuaries (Carpenter et al. 1998). Agricultural nutrients (e.g., commercial fertilizer and animal manure) are rich in nitrogen and phosphorus and enter water bodies through surface and subsurface flow. Since nitrogen and phosphorus are the principal production-limiting nutrients in freshwater and marine systems, excessive loading of these nutrients can adversely affect receiving waters. The impacts of agricultural nutrients on freshwater and marine eutrophication worldwide are now well documented (Kronvang et al. 1993; U.S. Environmental Protection Agency 1995; Howarth et al. 1996; Downing et al. 1999a). Both the quantity and stoichiometry of N and P influence aquatic primary production and community structure. Although N and P are essential to ecosystem function, the relative quantities (i.e., stoichiometry) of these elements are critical. When ambient nutrient supply ratios are extreme compared with biotic demand, ecosystem structure, function, and productivity are affected (Elser and Urabe 1999). In