LAKE ERIE TROPHIC STATUS COLLABORATIVE STUDY (LETS): INVESTIGATING MECHANISMS AND EXTENT OF INTERNAL PHOSPHORUS LOADING

We report here the results of the recently completed LETS study - a project which involved 27 scientists studying distribution and flux of biomass and nutrients to clarify mechanisms and extent of internal phosphorus movement, especially relating to DO depletion in central L. Erie. Open water and nearshore attributes vary at different scales and respond to different proximal factors. Total phosphorus (TP) loadings since 1990 have been determined mainly by regionally regulated annual variation in tributary discharges. Central basin TP concentrations have tended to rise since the 1990s. TP concentration and hypolimnetic oxygen depletion (HVOD) rates correlate with loadings of the previous year but not the current year. When values are adjusted for these variations in loadings, TP and HVOD are not different than values of the 1980s prior to arrival of dreissenids. Basin and seasonally- averaged chlorophyll a concentrations are poorly or uncorrelated with TP levels. In contrast, nearshore processes appear to reflect local effects of benthic-pelagic coupling among water clarity, nutrients, benthic primary production, dreissenid distribution and abundance, and round goby distribution and production. These features vary on shorter timescales and are affected by the adjacent landscape. Models of L. Erie dynamics should track and integrate processes in nearshore zones with open water compartments.