Harmful algal blooms (HABs) are a growing concern in coastal Georgia, posing threats to oyster aquaculture industries and larger ecosystem health. Until recently, Georgia has lacked a monitoring framework to address the timing, duration, and environmental drivers of these events. We began the state’s first high-resolution HAB monitoring campaign in the Skidaway River Estuary to target Akashiwo sanguinea, a HAB-forming dinoflagellate known for its damaging effects on marine life in temperate waters globally. The dataset spans two years and contains information on phytoplankton community composition via fluid imaging, physicochemical parameters, and nutrient concentrations. Findings from 2023 and 2024 reveal correlations between Akashiwo abundance and warmer waters with low ammonium and nitrate/nitrite levels, suggesting the species may have a competitive advantage when nitrogen concentrations are low.
To put this seasonal dataset in a broader context, these findings are compared to qualitative assessments performed by citizen scientists as part of NOAA’s Plankton Monitoring Network (PMN) on Skidaway Island over a 15-year period. PMN data correlated well with our results, as Akashiwo was observed during high temperature and mid-salinity conditions. Elevated Akashiwo levels in the estuary, as seen in PMN records and our dataset, align with records of oyster population crashes in the nearby UGA Marine Extension Shellfish Research Lab.
A regional survey was conducted via boat sampling, from the ocean break to the upper estuary. This spatial analysis revealed that Akashiwo blooms are most prominent in the brackish mid-estuary waters. The spatial distribution indicated that Akashiwo favors moderate salinity zones, with lower abundance observed closer to the ocean and in more riverine settings. While broad trends were evident, small differences in temperature and salinity between sites often corresponded with significant variations in abundance, suggesting that multiple environmental factors—such as temperature, turbulence, and nutrient availability—interact in complex ways to drive the spatial distribution of Akashiwo.
Hourly sampling with depth during periods of high abundance showed a mix of rapid fluctuations in total Akashiwo abundance through the water column and relatively stable concentrations throughout the diel cycle. A strong daily vertical migration signal was not apparent. Instead, rapid changes in abundance suggest that blooms are highly patchy, even in the well-mixed, turbulent environment of the Skidaway River Estuary. These insights highlight the complexity of Akashiwo sanguinea growth dynamics and will be valuable for informing bloom prediction models and coastal management strategies.