Nutrient loads driving variation in Lake Erie anoxia

(June 2009)

The hypoxia team’s linked one-dimensional physical-dissolved oxygen model was calibrated to accurately describe the vertical temperature and dissolved oxygen structure in Lake Erie’s central basin.  This model, driven by climatological forcings and observed surface oxygen concentrations, was used to determine if the variation in thermal structure and mixing (i.e., the effects of climate) significantly controls interannual variation in hypoxia.  The modeled annual oxygen depletion rates that were required to match oxygen dynamics, after accounting for physical mixing, we analyzed to tease biological from physical effects.   Simulation for the period 1987-2005 demonstrated that biological oxygen depletion rates have change significantly over this period.  No significant relationships were found between these rates and the thermal properties, however; there was a significant correlation with soluble reactive phosphorus loading.  These results indicate that the climate variability, expressed as changes in thermal structure, does not account for all of the variation in anoxia.  Rather, variation in primary production of organic matter is a dominant driver, and that appears to have changed as a result of changes in phosphorus loads.