Dual nitrogen and phosphorus reductions are needed for long-term mitigation of eutrophication and harmful cyanobacterial blooms in the hydrologically-variable San Francisco Bay Delta, CA

Cyanobacterial harmful algal blooms (CyanoHABs) are a major concern for water quality, public health and viability of aquatic ecosystems. Increased inputs of nutrients, i.e., nitrogen (N) and phosphorus (P), are known to amplify the occurrence, severity, and duration of CyanoHABs. There is growing concern that CyanoHABs are proliferating along the freshwater to marine continuum, including throughout estuaries. We assessed the influence of nutrient enrichment on the abundance and composition of CyanoHABs and accompanying phytoplankton communities in the San Francisco Bay Delta (SFBD) estuarine ecosystem, a vital resource for California's water supply, fisheries, and recreation. In situ nutrient addition bioassays were conducted in June and September 2022, at the end of a record three-year drought period, and May and August 2023, an extremely high rainfall and discharge year. Water was collected from two locations in the SFBD recognized for having CyanoHAB issues, Discovery Bay (DB) and the Stockton Channel (STK). Both sites showed the highest proportion of cyanobacteria in the total phytoplankton community biomass during summer months, and this was particularly noticeable at STK. In June 2022, additions of N and N+P were both shown to increase overall phytoplankton biomass in DB and N+P specifically stimulated cyanobacteria. P alone was not stimulatory. In September 2022, NH₄ promoted the growth of cyanobacteria faster than NO₃, particularly in DB communities. A similar set of responses to N occurred in 2023 in DB, despite major differences in freshwater input between years. In 2022, nutrient additions had no significant stimulatory effects on STK phytoplankton communities, suggesting nutrients were replete throughout the bloom season. However, in 2023 N limitation became more evident in STK, likely due to a dilution effect from the very high freshwater discharge from a record snowpack and reservoir releases, ultimately changing the availability of inorganic N during the CyanoHAB growth period. The combined effect of high flow and nutrient dilution in 2023 was responsible for the reduced CyanoHAB potential. By examining these key differences between seasons in these hydrologically contrasting years, it appears that internal supplies of "legacy P" ensure P availability throughout the summer bloom season regardless of hydrologic variability, while N enrichment plays a key role in stimulating algal production and CyanoHABs under hydrologically variable conditions. Once N was added, P further stimulated biomass production in some cases, indicating potential N+P co-limitation. We conclude that under varying hydrologic conditions, long-term dual N and P input reductions are needed to control eutrophication and CyanoHAB outbreaks throughout the SFBD.