Documenting phytoplankton community responses to hydrological resetting events in eutrophic low-inflow estuaries

The impact of hydrological events on the estuarine phytoplankton community was studied in two permanently eutrophic low-inflow estuaries. The study area along the warm temperate south coast of South Africa experienced a drought from 2015 which was followed by a period of high rainfall in 2023. Sampling of abiotic and phytoplankton parameters was conducted monthly from November 2023 to March 2024 to document the impact of the freshwater pulse on the dynamics of harmful algal blooms (HABs), particularly Heterosigma akashiwo, in the Swartkops and Sundays estuaries. Both estuaries experience HABs due to nutrient enrichment, albeit from different sources. The Sundays Estuary is subject to agricultural return flow, whereas the Swartkops Estuary receives enriched baseflows from three upstream wastewater treatment works. The return of H. akashiwo was observed 4 and 5 months after the initial freshwater pulse events in the Sundays and Swartkops estuaries, respectively. The return of H. akashiwo to the Sundays Estuary was faster than anticipated given that typically unfavourable freshwater-dominated conditions presided in the estuary one month prior to its reestablishment. Based on these results, the future management of both estuaries cannot rely solely on freshwater pulses to remove H. akashiwo. Instead, management interventions should focus on both hydrological restoration and nutrient reduction strategies to mitigate against HABs into the future.