Alleviation of phytoplankton light limitation by salinity intrusion in a CDOM rich, oligohaline estuary

Oligohaline zones of estuaries are often subject to harmful algal blooms. The Chowan River-Albemarle Sound is an oligohaline, eutrophic estuary in northeastern North Carolina. In recent summers, toxigenic cyanobacterial blooms have threatened the ecosystem and its value as an important commercial fishery. Typically, salinity is very low (0-2 psu) throughout what is normally a well-mixed water column. However, intrusions of saltier bottom water (4-8 psu) occur episodically during summer and autumn when river flow is low. High concentrations of colored dissolved organic matter greatly reduce light penetration in the water column. With such strong light attenuation, periodic salinity intrusion events that create a shallower mixed layer might be important for alleviating light limitation. In summer 2023, we measured photosynthetic rates under different irradiances to determine the light limitation status of phytoplankton in the Chowan River-Albemarle Sound and investigated the influence of a salinity intrusion event on light limitation. Phytoplankton production was light-limited even in summer when incident solar radiation was high. The salinity intrusion resulted in a 51% decrease in mixed layer depth and a 59% increase in mean water column irradiance within the mixed layer, thereby partially alleviating light limitation. The dominant algal genus changed from dominance by the high-light adapted cyanobacteria, Dolichospermum, during the salinity intrusion to the low-light adapted cyanobacteria, Pseudanabaena, after the salinity intrusion ended. Phytoplankton growth rates following the salinity intrusion were higher than during the salinity intrusion indicating that the effect of reduced light in the absence of salinity intrusion was offset by higher photosynthetic efficiency of the shade-adapted taxa. Understanding these community-level phytoplankton responses to physically-forced changes in light availability will help explain bloom dynamics and increase the predictability of blooms.

Presentation Preference: Oral

Primary Presenter: Mingying Chuo, University of North Carolina at Chapel Hill (mchuo@unc.edu)

Authors:

Mingying Chuo, University of North Carolina at Chapel Hill  (mchuo@unc.edu)

Nathan Hall, University of North Carolina at Chapel Hill  (nshall@email.unc.edu)