Application of phosphorus loading models to understand drivers of eutrophication in a complex rural lake-watershed system.

Several lakes in southwestern Nova Scotia, Canada have experienced reoccurring algal blooms and possess concentrations of phosphorus (P) that are in the eutrophic to hypereutrophic range (>35 μg/L). In this study a mass balance modeling approach was used to evaluate the relative contribution of P sources within these watersheds and lakes. Primary sources of P included land runoff, septic systems, agricultural activities including mink fur farming, aquaculture, as well as internal loading. These sources were assessed for three different study years (1983, 2008, and 2017), based on availability of water quality data, and to quantify the relative impact of the mink farming industry as it rapidly expanded during this time period. A novel method to estimate P loads from intensive fur farming operations was developed, using agricultural census, remote sensing, and publicly available spatial data. A suite of simulations were generated, using a steady state mass balance model, to examine lake P concentrations and sources for baseline conditions (no anthropogenic sources), no mink farming, and varying levels of P retention on mink farms (25%, 50%, 75%) scenarios. An additional scenario was also constructed which involved calibrating mink farm P retention coefficients using available water quality data. In the baseline scenario it was predicted that all study lakes would be oligotrophic, indicating that cultural eutrophication of these lakes has occurred. In the no mink farming scenario, it was predicted that all study lakes would be oligotrophic except for Hourglass Lake, which was predicted to be mesotrophic due to inputs from an aquaculture facility. Internal loading of P from lake sediments was also identified as an important potential P loading mechanism, which will likely be exacerbated by climate change. These findings indicate that the P from mink farms is the primary driver of cultural eutrophication in the study lakes, and highlights the need to consider these nutrient sources within watershed management plans. It is recommended that lake remediation efforts continue to focus on reducing P inputs from mink farms, and on controlling P loading from any new anthropogenic development in these watersheds.
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