Quantifying the major sinks and sources of phosphorus in Tabor Lake: implications for management and remediation

A study of Tabor Lake and its watershed during the open water season of 1995 was conducted in order to quantify the major sinks and sources of phosphorus, and the timing of phosphorus transfer between these sources and sinks. The results of this thesis are useful to help determine appropriate management strategies aimed at reducing phosphorus availability in Tabor Lake's water column and understanding some of the processes that control phosphorus availability in the lake. An exploratory modeling exercise was undertaken to evaluate morphometric and watershed variables in predicting spring phosphorus for 39 lakes in the region, including Tabor Lake. Two multiple linear regression models were constructed. The first model uses a stepwise technique to select statistically significant variables, and the second model uses variables found in the literature which have been shown to influence phosphorus loading. These two models account for 70% and 59% of the variation in spring phosphorus, respectively, and may provide managers with an expected phosphorus loading based on easily measured morphometric and watershed variables. The models predict that Tabor Lake should have a spring phosphorus concentration of 20 and 18 ug/L, respectively. The models are useful to establish the limits of water quality improvement for Tabor Lake. Another model was developed to estimate the quantity and timing of external sinks and sources of phosphorus in Tabor Lake for 1995. Phosphorus loading into Tabor Lake was estimated from runoff (136 kg) and atmospheric deposition (60 kg). Phosphorus output from Tabor Lake through the outlet creek was estimated at 188 kg for the entire year. Based on this input-output model, there was a net gain of 8 kg phosphorus into Tabor Lake during 1995. These results indicate that external phosphorus inputs and output is approximately balanced. Results from a prior monitoring program indicate that Tabor Lake experienced internal loading of phosphorus during periods of hypolimnetic anoxia. However, phosphorus loading also occurred when oxygen was present in the hypolimnion, suggesting another phosphorus loading mechanism was active in Tabor Lake. A specific effort was made to evaluate the role of senescing macrophytes (primarily Elodea canadensis) as a source of this phosphorus during the 1995 sampling season. Two experiments were conducted, one invitro and the other in-situ, to estimate phosphorus leaching from senescing macrophytes. The first in-vitro experiment used weeds collected August 8, 1995 and show that 87% (S.E.= 6%) of the total plant phosphorus is leached from these macrophytes, whereas the second in-vitro experiment, using samples collected September 22, 1995, show that only 9% (S.E.= 1%) of the phosphorus is leached from macrophytes. The difference between the two leaching estimates is believed to be the result of overwintering development and expected starch accumulation in the September samples. The in-situ study provided the best prediction of phosphorus leaching from senescing macrophytes, estimating 1958 kg of phosphorus (S.E.= 867 kg) was released from senescing macrophytes in 1995. The results from both the input-output model and the internal loading estimates show that Tabor Lake is dominated by the internal cycling of nutrients beginning in June and continuing until September. Furthermore, it quantifies two important phosphorus sources and the timing of phosphorus delivery from these sources. The quantification and timing of phosphorus delivery in Tabor Lake is useful to managers establishing remediation strategies which try to reduce the total phosphorus present in the water column. This research also shows that mechanical harvesting of macrophytes removes more phosphorus from the lake than exits through the outlet and can be useful in reducing the total phosphorus available to internal loading cycles in Tabor Lake.