A re-evaluation of the empirical relationships connecting dissolved oxygen and phosphorus loading after dreissenid mussel invasion in Lake Simcoe

Lake Simcoe has had many stressors in the past decades. High total phosphorus (TP) loads into the lake led to end-of-summer volume-weighted hypolimnetic dissolved oxygen (MVWHDO) levels that were lethal to coldwater fish species. Additionally, dreissenid mussels became established in the lake around 1996, and may have affected nutrient cycling and phytoplankton abundance. Using data averaged over two time periods following dreissenid mussel establishment, we re-evaluated empirical relationships connecting TP loading rate and MVWHDO concentration that were developed with pre-dreissenid data. Oxygen depletion rate predicted chlorophyll (Chla) well within the range of other Ontario lakes. Measured Chla corresponded well with Chla predicted from lake TP concentration except at the shallowest station, indicating that the lake may becoming more spatially heterogeneous. Interestingly though, the Chla:TP temporal trend increased post-dreissenids when it is often observed to decrease (e.g., Great Lakes). Measured lake TP concentration was better predicted by TP loads when averaged over the ice-free season compared with the spring. The final model predicted MVWHDO directly from TP load very closely except for the 2002-2006 time period when the end-of-summer date was September 15 rather than September 30, suggesting that the date when MVWHDO is determined should be continuously re-evaluated. Additionally, trend analyses on each variable from 1980 to 2008 did not identify any changes that coincided with dreissenid establishment. Given that this model appears to be sound for the post-dreissenid period, an estimated TP load of 44 tonnes/yr was predicted that would yield the current MVWHDO target level of 7 mg/L. Crown Copyright(C) 2011 Published by Elsevier B.V. on behalf of International Association for Great Lakes Research. All rights reserved.