Phosphorus Dynamics in Southwestern Nova Scotia Lakes

Abstract

Eutrophication and algal blooms caused by anthropogenic sources of nutrients is an issue in freshwater systems all over Canada. Several lakes in southwestern Nova Scotia have experienced reoccurring algae blooms and possess elevated concentrations of phosphorus (P). In this thesis a mass balance modeling approach was used to understand the relative contribution of P sources within these watersheds and lakes. Primary sources of P included land runoff, septic systems, agricultural activities such as mink fur farming, aquaculture, and internal loading. These sources were quantified through literature and spatial analysis for three different study years: 1983, 2008, and 2017. These study years were chosen based on availability of water quality data, and to understand the relative impact of the mink industry as it rapidly expanded during this time period. Agricultural census data and spatial analysis were used to estimate the annual mass of P generated from mink farming in each watershed. A suite of loading scenarios was simulated using a steady state mass balance model, examining P concentrations and sources for baseline (no anthropogenic sources), no mink farming, and varying levels of P retention on mink farms (25%, 50%, 75%). An additional scenario was also constructed which involved calibrating mink farm P retention coefficients using available water quality data. Baseline modeling scenarios predicted that without anthropogenic sources of phosphorus, all lakes in the system 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. These findings suggest that the P from mink farms are the primary driver of cultural eutrophication in the study lakes. Results from the calibrated mass balance models demonstrated that for lakes with mink farms in their subwatersheds, the majority of P inputs are from the mink farms, and in lakes downstream of these subwatersheds, the majority of P loading originates from these upstream inputs. 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. Internal loading of P from lake sediments was also identified as an important P loading mechanism, which can be exacerbated by climate change. Further efforts should also focus on monitoring of internal loading dynamics.