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

The St. Johns River (SJR) is a dynamic system, spanning 310 miles of Florida’s eastern coast and provides ecological, recreational, and commercial benefit for the state. The lower SJR basin, located near Jacksonville, forms an estuarine environment where tidal flow from the Atlantic Ocean mixes with freshwater moving through the river toward the ocean. Anthropogenic influences, such as increased development, dredging, and wastewater discharge from a variety of sources has changed the water quality in the river over past years. In this study, a variety of water quality parameters and a suite of metals were measured in water samples collected from eight sites in the lower SJR from 2019 to 2022, continued from previous work which documented these parameters in the river from 2017 to 2019. Aquatic communities such as phytoplankton can be indicative of river health, therefore, phytoplankton were collected from each site and the diatom component identified. The total number of taxa in each sample ranged from 60 to 190, with 25 taxa accounting for the majority (64%). The species Thalassiosira wongii was first discovered in the lower SJR during this study, previously documented from southern Brazil and the North Pacific. Like water chemistry, seasonal fluctuations in phytoplankton abundance and diversity were observed, with a relative abundance of Skeletonema costatum and Skeletonema subsalsum in times of lowered diversity. Further, decreased phytoplankton diversity correlated with increased metal contamination in the lower SJR. This study provides new information about the impact of human disturbance on biotic communities.