Long-term phosphorus mass-balance of Lake Erie (Canada-USA) reveals a major contribution of in-lake phosphorus loading

Abstract

While early efforts to reduce external phosphorus (P) loads to Lake Erie eliminated the worst eutrophication symptoms in the 1970s and 1980s, the observed intensification of these symptoms in recent decades is difficult to explain by changes in the external P loading alone. We hypothesized that, in addition to external inputs, in-lake processes that release P to the water column may be contributing to the re-eutrophication of Lake Erie. Directly measuring these internal inputs is challenging and their scaling up to the entire lake system remains fraught with uncertainty. To constrain the magnitude of the net P input associated with in-lake processes, we generated a long-term annual total P (TP) budget for the Lake St. Clair–Lake Erie system, averaged over a 14-year period, from 2003 to 2016. The budget shows that the TP output from the system substantially exceeds the sum of the external TP inputs. To balance the budget, we estimate that in-lake processes add a net internal TP load of 3783 metric tons per year (MTA) to the water column. In comparison, the mean yearly external TP load to the Lake St. Clair–Lake Erie system is 11,703 MTA. Thus, overall, in-lake processes account for about a quarter of the total (external plus internal) TP input. The internal input is not evenly distributed among the system's basins, however, with Lake St. Clair and Lake Erie's western, central, and eastern basins receiving 6, 13, 21 and 60% of the total internal P input, respectively. Our results highlight the importance of accounting for the internal P loading in nutrient cycling modeling and load reduction strategies for large lake systems.