Phytoplankton responses to tropical cyclone events: insights from discrete and continuous water quality monitoring in Louisiana estuaries

The effects of tropical cyclones on phytoplankton biomass and community structure in the coastal ocean vary with storm characteristics (i.e., wind v. runoff) and with prior conditions of the ecosystem (i.e., stratified v. well-mixed water column). A recent meta-analysis suggests that phytoplankton are sensitive to rainfall delivered to coastal ecosystems and show tradeoffs between resistance and resilience to these pulse disturbance events. Since 2019, monthly sampling data has been collected at 2 nearshore estuarine sites along the south-central Louisiana coast for water quality, phytoplankton biomass, and community composition. Since 2022, data has been collected from 5 estuarine sites via continuous sondes measuring abiotic variables (including nitrate at 1 site) and biomass of total phytoplankton (as chlorophyll-a) and freshwater cyanobacteria (as phycocyanin) every 15 minutes. Eleven named tropical cyclone systems have impacted the Louisiana coast since 2019, while additional flood (2019), unnamed storm (2024), and drought (2023) events also occurred. In 2024, Hurricane Francine made landfall in Terrebonne Parish. In the 1-2 days around landfall, a site 120 miles west showed a substantial, but temporary, increase in biomass. Conversely, a site 160 miles east showed little change during the storm, but biomass increased one week after landfall as falling salinity indicated runoff. Insights from individual pulse disturbances on phytoplankton dynamics, along with aggregated responses to disturbance characteristics, will be further discussed in this presentation.