St. Clair-Detroit River system: Phosphorus mass balance and implications for Lake Erie load reduction, monitoring, and climate change

On the Role of a Large Shallow Lake (Lake St. Clair, USA‐Canada) in Modulating Phosphorus Loads to Lake Erie

“…We showed that during 1 March 1 to 31 October, the lake was a net sink for phosphorus with an average TP retention under different observed meteorological forcing (Table ) of about 18% (Table ). This retention rate estimate is similar to the 1998–2016 average annual retention of 20% estimated with a TP mass balance based on measured loads into and out of the lake (Scavia et al, ), and the variability with meteorological conditions (Table ) may explain the substantial interannual variability (4–34%) Scavia et al () reported. The small difference between our estimate and that of Scavia et al () may be because our simulation period did not include ice‐covered season when the lake surface is sheltered from the effects of surface wind stress and settling rates should dominate resuspension.…”

“…Located in the connecting channel between Lakes Huron and Erie, the lake processes water from the upper Great Lakes (Superior, Michigan, and Huron) via the St. Clair River, as well as from its proximate 15,400 km 2 watershed that is roughly 63% in Canada and 37% in the United States (Table 2). In addition to receiving P from the upper Great Lakes and the watersheds of the St. Clair River, it receives P from many direct tributaries, including significant loads from the Clinton, Thames, and Sydenham rivers, as well as point source discharges (Scavia et al, 2019). While the lake's theoretical flushing time is roughly 9 days, that flushing time (or water residence time [WRT]) varies seasonally and, more significantly, spatially (Bocaniov & Scavia, 2018) such that during summer, water in the southeastern part of the lake flushes more slowly than water in the northwestern part.…”

“…The plans will undoubtedly address loads from all sources, but because the Detroit and Maumee rivers contribute 41% and 48% of the total P (TP) load and 59% and 31% of the dissolved reactive P (DRP), respectively (Maccoux et al, 2016;Scavia et al, 2016Scavia et al, , 2019, they will likely get special attention. Several efforts are in place to assess the relative contributions of, and potential controls of, P loads from the Maumee River watershed (e.g., Kalcic et al, 2016;Muenich et al, 2016;Scavia et al, 2017).…”

“…Several efforts are in place to assess the relative contributions of, and potential controls of, P loads from the Maumee River watershed (e.g., Kalcic et al, 2016;Muenich et al, 2016;Scavia et al, 2017). Similar efforts are in place for the binational watersheds of the connecting channel between Lake Huron and Lake Erie (Bocaniov & Scavia, 2018;Dagnew et al, 2019;Hu et al, 2018;Scavia et al, 2019), but unlike the Maumee watershed, much of the connecting channel P loads do not flow directly into Lake Erie. Much of it must pass through the relatively large Lake St. Clair (Figure 1a) that has its own sizable watershed consisting of both highly urbanized areas (e.g., Detroit, Windsor, and London) and watersheds (e.g., Clinton River basin), as well as intensive agriculture in the Thames and Sydenham river basins.…”

“…The average annual relative phosphorus loads to Lake St. Clair for 2013-2015 are 71.5%, 4.8%, 12.1%, and 5.4% from the connecting channel (St. Clair River) and the three major tributaries, Clinton, Thames, and Sydenham rivers, respectively (Scavia et al, 2019). Because these are substantial inputs to the overall system and because Scavia et al (2019) estimate that Lake St. Clair retains, on average, 20% of its TP inputs, it is important to understand how P load reductions from individual tributaries to Lake St. Clair will correspond to the reduction in load to Lake Erie via the Detroit River.…”