Temporal and Spatial Changes in Water Quality and Phytoplankton Populations in the Lower St. Johns River, Florida

The St. Johns River (SJR) is an ecologically and economically important estuarine river system undergoing extensive anthropogenic change. In this study, water quality parameters (dissolved oxygen, temperature, salinity, pH, hardness, alkalinity, ammonia-N, nitrate-N, and nitrite-N) and a suite of metals (cadmium, copper, lead, nickel, silver, and zinc) were measured in water samples collected from eight sites in the lower SJR from 2019 to 2022. This project was continued from previous work that documented these parameters in the river from 2017 to 2019. Aquatic communities such as phytoplankton can be indicative of river health; therefore, phytoplankton were also collected from each site, and the diatom component was identified. The total number of taxa in each sample ranged from 60 to 190, with 25 taxa accounting for the majority (64%). Similar to water quality, seasonal fluctuations in phytoplankton abundance and diversity were observed, with an increased relative abundance of Skeletonema costatum and Skeletonema subsalsum in times of lowered diversity. Furthermore, decreased phytoplankton diversity correlated with increased metal concentrations in the lower SJR. Multivariate analyses highlighted significant interactions among phytoplankton diversity and water quality variables. Significant parameters affecting phytoplankton biodiversity included salinity, pH, temperature, copper hazard quotient, and the nickel hazard quotient. This study provides new information about the impact of human disturbance on biotic communities and the complexity in predicting population changes.