Flows of phosphorus on an agricultural landscape: Implications for eutrophication and restoration of Lake Mendota.

摘要

Phosphorus (P) pollution and the subsequent eutrophication of freshwater systems are serious and persistent environmental problems. Most commonly we parameterize our models of these flows with mean values or smooth linearized trends. However, at the watershed scale, P dynamics may be discontinuous and difficult to predict. This thesis includes insights from a simple linear model of P flow in the Lake Mendota watershed and a consideration of certain discontinuities and surprises.; The linear model couples in-lake and upland processes to assess the long-term impacts of various management strategies. Policies that lead to balancing the P budget in the watershed are critical for long-term restoration of Mendota. However, altering the flow of P across riparian zones would slow the accumulation of P in the lake, and biomanipulation and alum application can reduce algal densities.; In a comparison of two years of daily P-yield (mug ha-1day -1) from six Southeast Wisconsin watersheds with contrasting riparian buffer attributes, mean daily P-yield was most closely correlated with the variability in riparian patch size. Variability in P-yield was most closely correlated with riparian buffer characteristics.; To evaluate the impact of tile drainage on water quality, I compared P concentrations during high flow events in two consecutive stream reaches. Phosphorus loads from tiles were highest during heavy rains and, during the two heaviest rain events, in-stream phosphorus concentrations were markedly higher in the tiled reach.; Zebra mussels are likely to invade Lake Mendota. I coupled mussel-mediated phytoplankton removal estimates with chlorophyll production estimates in a dynamic model developed to predict the impact of zebra mussel invasion on water clarity. With frequent epilimnetic circulation, the percentage of bloom condition days decreased. However, with infrequent epilimnetic circulation greater mussel density was required for increased water clarity.; My work suggests long-term restoration of the Lake Mendota watershed will require balancing the upland P budget by reducing P import in fertilizer and feed. It suggests that discontinuous shifts resulting from buffer configuration, drain tiles, or species invasion may produce rapid changes in the lake that would not be anticipated from a linear analysis of the P cycle.