labs_title

Long-Term Phosphorus Loading and Springtime Temperatures Explain Interannual Variability of Hypoxia in a Large Temperate Lake

D. Del Giudice,, Y. Zhou, E. Sinha and A.M. Michalak

Anthropogenic pressures in the form of nutrient enrichment of riverine waters and more indirectly global warming can have serious consequences on the health of aquatic ecosystems, including the formation of oxygen depleted areas, a.k.a. hypoxic zones. In this study we sought to better understand which environmental variables linked to riverine nutrient loads and climate are most associated with lacustrine hypoxia. Our results show that decadal nutrient loads are much more strongly linked to hypoxic areas than the annual loads previously studied. We also observe that climate, in the form of springtime temperatures, plays a substantial role in exacerbating oxygen depletion by controlling the time of onset of thermal stratification of a temperate lake. Overall, these newly identified drivers point to the importance of taking into account longer horizons and climate change when setting nutrient load targets to manage lake water quality.


Abstract

Anthropogenic eutrophication has led to the increased occurrence of hypoxia in inland and coastal waters around the globe. While low dissolved oxygen conditions are known to be driven primarily by nutrient loading and water column stratification, the relative importance of these factors and their associated time scales are not well understood. Here, we explore these questions for Lake Erie, a large temperate lake that experiences widespread annual summertime hypoxia. We leverage a three-decade data set of summertime hypoxic extent (1985− 2015) and examine the role of seasonal and long-term nutrient loading, as well as hydrometeorological conditions. We find that a linear combination of decadal total phosphorus loading from tributaries and springtime air temperatures explains a high proportion of the interannual variability in average summertime hypoxic extent (R2 = 0.71). This result suggests that the lake responds primarily to long-term variations in phosphorus inputs, rather than springtime or annual loading as previously assumed, which is consistent with internal phosphorus loading from lake sediments likely being an important contributing mechanism. This result also demonstrates that springtime temperatures have a substantial impact on summertime hypoxia, likely by impacting the timing of onset of thermal stratification. These findings imply that management strategies based on reducing tributary phosphorus loading would take several years to reap full benefits, and that projected future increases in temperatures are likely to exacerbate hypoxia in Lake Erie and other temperate lakes.

Del Giudice, D., Y. Zhou, E. Sinha, A.M. Michalak (2018) "Long-Term phosphorus loading and springtime temperatures explain interannual variability of hypoxia in a large temperate lake," Environmental Science & Technology, 52 (4), 2046-2054, doi:10.1021/acs.est.7b04730.