Long-term impact of Central Basin hypoxia and internal phosphorus loading on north shore water quality in Lake Erie

Abstract

ABSTRACTWe hypothesized that north shore water quality of Lake Erie’s Central Basin is impacted by Central Basin hypoxia and, by extension, related to redox-dependant internal phosphorus (P) loading from the profundal sediments. To evaluate this hypothesis, we first quantified Central Basin hypoxia as hypoxic factor (HF, days in Aug–Sep that a sediment area equal to the Central Basin surface area was hypoxic) from published annual hypoxic (<2.0 mg/L dissolved oxygen) areal extent and duration. From 1985 to 2012, mean HF values were 15.2 (SE 1.45) d/Aug–Sep and ranged between 0 (1996) and 34.3 (2012) d/Aug–Sep. Second, we estimated internal load from an areal P release rate of 8 mg/m2/d, multiplied by HF. The estimate of 122 (SE 11.6) mg/m2/Aug–Sep (1985–2012, n = 28) is supported by an independent estimate of 136 (SE 20.3) mg/m2/summer (1970–1986, n = 15) computed from total P concentration increases at fall turnover and by an estimate for 1970. Third, 2013 temperature and oxygen profiles demonstrated periodic upwelling on the north shore of the Central Basin, in agreement with other studies. Fourth, we analysed P, chlorophyll a (Chl-a) and phytoplankton species data at 2 north shore drinking water intake stations. HF and internal loading were significantly correlated with summer soluble reactive P, August–October Chl-a concentrations, and cyanobacteria abundance at the Central Basin site but not at the control site in the Western Basin. While these correlations are weak, suggesting other effects including benthic productivity, they may be partially caused by hypolimnetic water circulated toward the shoreline.