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Ordinarily increases in lake phosphorus result from changes in the inputs of phosphorus to the lake. The inputs may come from various sources. These 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-feet/year) Phosphorus (pounds/year) Runoff 1,571 (65%)~ 1,450 (51%)3 Atmosphere 862 (35%)2 268 ( 9%)4 Internal na 1,146 (40%)s Total 2,433 acre-feet /year 2,864 pounds /year The numbers above are used as a basis for discussion. Other estimates have made for earlier years, but these estimates are not substantially different. These estimates of water and phosphorus inputs represent a reasonable basis for the purposes in this report and until direct measurements of runoff water quality are actual made, it is probably not productive to further refine these estimates. 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 47 ppb6 . This is approximately the level observed throughout much of the late-1980s and 1990s. There have been land use changes in the watershed and in the management of runoff that could account for the decreasing trend in lake phosphorus. Two main factors are evaluated: 1) the impacts of the American Legion Pond project and 2) the impact of wet or dry years on the phosphorus inputs. The Barr (1991) 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 remove 50% of the phosphorus in the runoff, the amount of phosphorus removed would be (1,450 pounds x 67% x 50%) 486 pounds per year. This 10% of the total amount of phosphorus being contributed to Lake Owasso. The Barr (1991) report also included a calculation that the American Legion Pond would remove 35 kg (=77 pounds). This is an even smaller impact. This improvement would result in a proportional improvement in Lake Phosphorus concentration, or about 5 ppb. Assuming that the amount of phosphorus carried in runoff varies in proportion to the annual amount of precipitation, an estimate of increased (wet years) or decreased (dry years) phosphorus inputs to Lake Owasso can be made. By examining the annual amounts of precipitation form the years 1984 through 1997, the `wettest' was about 37 inches or 37% more than normal and the driest was about 21 inches or 22% less than normal. The analysis of these two factors shows that neither can totally account for the decrease in lake phosphorus concentration observed in Lake Owasso. Terry Noonan, a limnologist for Ramsey County, 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 word, the 1987-1989 drought resulted in lower lake levels and less phosphorus inputs in runoff which in turn 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