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Ordinarily increases or decreases in lake phosphorus result from changes in the inputs of phosphorus to the lake. The inputs may come from various sources which can be measured or estimated. The following is an assessment of the present phosphorus inputs to Lake Owasso for a normal rainfall year: Source Water (acre-feeUyear) Phosphorus ( ounds/mar) Runoff 1,426 (62%)2 829 (37%)1 Atmosphere 862 (38%)3 268 (12%)4 Internal na 1,146 (51%)5 Total 2,288 acre-feet /year 2,243 pounds /year The numbers above are used as a basis for discussion. Estimates of runoff phosphorus made for earlier years6 were higher because the American Legion Pond had not yet been implemented. The American Legion Pond has reduced the amount of phosphorus entering Lake Owasso and the lake's phosphorus content has declined. Taken together, these sources of water and the phosphorus they carry, account for the phosphorus content in Lake Owasso. A model that predicts the phosphorus concentration in lakes estimates a phosphorus concentration, based on a `normal' condition (= 27 inches of precipitation per year) to be 37 ppb'. The lake's phosphorus budget includes a significant internal phosphorus input, which has been estimated conservatively5. Changes in the management of runoff account for the some of the decreasing trend in lake phosphorus. The American Legion Pond project accounts for the largest benefit. The Barr report found that the watershed draining through the American Legion Pond contributed about two thirds of the total phosphorus input from runoff to Lake Owasso. Assuming that the American Legion Pond captured the total amount of runoff and removes 50% of the phosphorus in the nutoff, the amount of phosphorus removed. would be (1,450 pounds x 67% x 50%) 486 pounds per year. This is about 60% of the phosphorus being currently contributed to Lake Owasso from runoff (and about 20% of the phosphorus from all sources). The 1991 Barr report also included a calculation that the American Legion Pond would remove 35 kg (=77 pounds). In either case, this is a significant reduction in phosphorus delivered to Lake Owasso and accounts for some portion of the observed decrease in lake phosphorus. Terry Noonan, a Ramsey County litnnologist, has been studying and observing Lake Owasso throughout this entire period. According to Terry (personal communication) it is "very likely that the increased macrophyte (rooted aquatic plants) abundance resulted from improved water clarity during the extended drought (1987-1989), and associated biological components such as attached algae, zooplankton and fish, basically in-lake processes, have promoted improved water quality." Terry added, "Reductions in external (runoff) phosphorus loading would act to stabilize the (improving) condition. In other words, the 1987-1989 drought resulted in lower lake levels and less phosphorus inputs in runoff which in turf resulted in clear water and an expansion of aquatic plant growth in Lake Owasso. The event (the drought) caused Lake Owasso to switch from a condition of turbid water to a condition of clear water maintained by more abundant aquatic plants. The watershed improvements that have been made in the ensuing years have served to stabilize this improved condition. This explanation is consistent with the shallow lake bonus (Appendix A). It is unclear whether the present condition in Lake Owasso is stable. The observed phosphorus concentration in the lake is 22 and 32 ppb in 1998 and 1999. The predicted concentration is 37 ppb, which approximates the lake concentration over the past five years. Another indication that Lake Owasso benefits from the shallow lake bonus is the fact that there is less algae in Lake Owasso than expected based on the known patterns in other lakes. Normally, as lake phosphorus concentrations increase, the atnount of algae increases in turn. Tlus is a predict<~ble pattern. A model8 has been developed that is used to predict algae abundance (as measured by chlorophyll) from a lake's phosphorus concentration. By examining the actual phosphorus and chlorophyll measurement from recent years, we see that the amount of algae growth in Lake Owasso is less than other metro lakes with similar levels of phosphorus (see table below):