Long-term spatiotemporal water quality and phytoplankton dynamics in Severn Sound, Ontario: a delisted great lakes area of concern

Abstract

Severn Sound is an embayment system located in south-eastern Lake Huron that was previously designated an Area of Concern (AOC) in 1987 as part of the Great Lakes Water Quality Agreement. The AOC designation was largely due to eutrophication, which has been a historical issue in Severn Sound extensively documented by a water quality and phytoplankton monitoring program since 1973. My thesis research analyzed this long-term dataset representing four embayments within Severn Sound as well as nearshore-offshore water quality in the embayments. I found statistically significant differences across stations in water quality parameters such as total phosphorus, total nitrogen, Secchi disk visibility, and water temperature. Overall decreases in total phosphorus and chlorophyll a and increases in Secchi disk visibility illustrate the influences of nutrient reduction strategies, and potential influences from invasive dreissenid mussels. Although an important driver of water quality changes, water temperature change was variable during the study, likely due to site-specific differences and inherent data variability. Generalized Additive Mixed Models showed decreases in total phytoplankton biovolume and different significant drivers across seven major phytoplankton divisions. Permutational Multivariate Analysis of Variance (PERMANOVA) revealed that station, along with period (before, during, and after delisting), and interaction of station and period were significant drivers of phytoplankton community composition. Using redundancy analysis, I found cyanobacterial community assemblages were significantly different both before, during, and after AOC listing, and before and after the zebra mussel invasion, as well as across stations, with period being the highest explanatory variable of community shifts. Generalized Additive Models revealed a significant difference in environmental drivers of three major cyanobacterial genera, Aphanizomenon, Dolichospermum, and Microcystis. Overall findings indicate that cyanobacterial communities shifted in response to nutrient mitigation, climate, and the zebra mussel invasion. In addition, I found statistically significant differences in nearshore-offshore chlorophyll a, conductivity, total ammonia/ammonium nitrogen, total organic nitrogen, and total phosphorus. A PERMANOVA also revealed that nearshore-offshore water quality was significantly different across region but found no statistical difference in embayments. Overall, these findings can be used to inform management decisions in the Severn Sound region, other Areas of Concern, and regions experiencing eutrophication.