Inland Waters, Vol 4, No 1 (2014)

Water quality implications from three decades of phosphorus loads and trophic dynamics in the Yahara chain of lakes

Richard C Lathrop, Stephen R Carpenter
Pages: 1-14


Trophic responses to phosphorus (P) loads spanning 29–33 years were assessed for the eutrophic Yahara chain of lakes: Mendota (area = 39.6 km2, mean depth = 12.7 m, flushing rate = 0.23 yr−1); Monona (13.7 km2, 8.3 m, 1.3 yr−1); Waubesa (8.5 km2, 4.7 m, 4.3 yr−1); and Kegonsa (13.0 km2, 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.