TitleWater quality implications from three decades of phosphorus loads and trophic dynamics in the Yahara chain of lakes
Publication TypeJournal Article
Year of Publication2014
AuthorsLathrop, RC, Carpenter, SR
JournalInland Waters

Trophic responses to phosphorus (P) loads spanning 29-33 years were assessed for the eutrophic Yahara chain of lakes: Mendota (area = 39.6 km(2), mean depth = 12.7 m, flushing rate = 0.23 yr(-1)); Monona (13.7 km(2), 8.3 m, 1.3 yr(-1)); Waubesa (8.5 km(2), 4.7 m, 4.3 yr(-1)); and Kegonsa (13.0 km(2), 5.1 m, 3.0 yr(-1)). During extended drought periods with low P loads, summer (Jul-Aug) total P (TP) concentrations declined substantially in all 4 lakes, with Mendota achieving mesotrophic conditions (<0.024 mg L-1). In years when P loads were high due to major runoff events, summer TP in the lakes was high (especially in shallower Waubesa and Kegonsa); in some summers dissolved inorganic P was elevated, indicating algae growth was not P limited. Summer TP returned to normal levels following both low and high load years, signifying the lakes were responsive to P load changes. The proportion of P input loads passed via a lake's outlet to the next lake downstream increased as flushing rates increased. Because Monona, Waubesa, and Kegonsa received 60, 83, and 76% of their surface water P load from the respective upstream lake's outlet, reducing P loads in Mendota's large watershed was predicted to produce significant water quality benefits downstream. Modeling indicated a significant grazing effect of Daphnia on summer TP and Secchi transparency readings for Mendota and Monona. Finally, using drought loads as targets, our study established P loading reductions needed to improve water quality in all 4 Yahara lakes.