Comments submitted by CSI Executive Director Stephen Penningroth, Ph.D., to NYSDEC (Comments on Draft Total Maximum Daily Load (TMDL) for Phosphorus in Cayuga Lake, New York, submitted on July 2, 2021; available on our Public Events and Presentations page) address the amounts of dissolved and particulate phosphorus that the Draft Total Maximum Daily Load (TMDL) estimates are being loaded each year to Cayuga Lake from non-point, mostly agricultural, sources (Draft Total Maximum Daily Load (TMDL) for Phosphorus in Cayuga Lake, NYSDEC, 2021). Averages of results from volunteer-CSI stream monitoring partnerships with two studies in the scientific literature indicate that streams annually transport approximately 62 tons of dissolved bioavailable phosphorus and 53 tons of soil-bound, mostly inert phosphorus to Cayuga Lake. By contrast, the Draft TMDL estimates that 17 tons of dissolved bioavailable phosphorus and 190 tons of soil-bound phosphorus are loaded to Cayuga Lake annually. The Draft TMDL estimates, which are based on a SWAT modeling protocol, are statistical outliers compared to the three independent studies. The large difference in the loading estimate for dissolved phosphorus, 17 tons/year vs. 62 tons/year, is particularly significant because dissolved, bioavailable phosphorus is widely believed to be the main driver of cultural eutrophication in freshwater lakes. The Draft TMDL’s emphasis on soil-bound phosphorus encourages stakeholders to implement best management practices (BMPs) that miss the mark for reducing the risk of cultural eutrophication. The Draft TMDL’s underestimate of dissolved phosphorus loading can likely be attributed to the fact that the SWAT model profile for Fall Creek, a much-studied southern tributary stream, was extrapolated to the entire Cayuga Lake watershed without performing phosphorus measurements to validate the model for tributaries to the north. Evidence that dissolved phosphorus concentrations in the northeastern part of the watershed average seven times higher than Fall Creek was published by O’Leary et al in 2019 (O’Leary, et al, 2019; availabe to read on CSI’s Publications page). Evidence of high dissolved phosphorus concentrations in nine Cayuga Lake tributary streams in the northern half of the watershed may be found in the Cayuga Lake section of CSI’s public Water Quality Database. The most effective approach to preventing eutrophication of Cayuga Lake is to withdraw the lake-wide TMDL, delist Cayuga Lake for phosphorus (a legal requirement under the 1972 federal Clean Water Act to avoid a TMDL), and develop Nine Element Plans for tributaries with disproportionately high dissolved phosphorus and nitrogen loads by building on long-term nutrient data sets funded by local governments and available at in the Cayuga Lake section of our public Water Quality Database at database.communityscience.org.
