Documenting the duration and chlorophyll pigments of an allochthonous Karenia brevis bloom in the Loxahatchee River Estuary (LRE), Florida

In Fall 2017 a large bloom of the toxic dinoflagellate Karenia brevis developed in the Gulf of Mexico. After persisting for months, in Fall 2018 wind and water circulation patterns drove K. brevis towards the east coast of Florida. On September 29, 2018 Palm Beach County, FL beaches were closed due to respiratory and gastrointestinal issues associated with brevotoxins, and effects of brevotoxins were reported from within estuarine segments of the Loxahatchee River Estuary (LRE). This was the first apparent report of a K. brevis bloom impacting inshore portions of the LRE prompting us to question the longevity of K. brevis within a relatively shallow, well-flushed coastal-estuarine system. Within 3 days (October 1, 2018) of the first reported effects of toxins, K. brevis reached over one million cells/L and chlorophyll-a concentrations peaked at 13 µg L^-1. Within 11 days (October 9, 2018) both K. brevis and chlorophyll pigment concentrations significantly (p-perm ≤ 0.05) dropped to an average of ≤ 30,000 cells L^-1 and < 4 µg L^-1 chlorophyll-a, indicating that the bloom had diminished. Using distance-based linear modeling (DistLM) K. brevis abundance alone explained 66% of the variation in a multivariate measure of chlorophylls (driven by carotenoids and chlorophyll-c pigment concentrations), supporting a K. brevis dominated bloom. Following the K. brevis bloom, additional HAB species K. mikimotoi and Pseudo-nitzschia spp singularly explained 6% of the variations in the multivariate measure of chlorophylls. The low explanatory power of individual HAB species, including K. brevis (≤ 0%), signifies the recovery of the phytoplankton population, where non-HAB species likely contributed to the variability in the multivariate measure of chlorophylls and overall chlorophyll-a concentrations (average of 2 µg L^-1 during non-bloom conditions). Finally, we evaluated ambient and historical water quality data to assess how these parameters changed before, during, and after the 2018 K. brevis bloom. Temperature, salinity, and nutrients in the LRE were comparable to reports of other K. brevis bloom events along the west coast of Florida. Reduced ammonia-nitrogen (NH₃-N) concentrations and increased tidal amplitude coincided with the end of the bloom in 2018. More work is needed to understand the specific mechanisms constraining K. brevis blooms in tidal estuaries. We suggest that future research focus on water residence times along with nutrient availability in controlling allochthonous HABs in lotic and tidally flushed estuaries. Also, we anticipate this work may stimulate additional efforts to characterize HABs using in situ observations coupled with multivariate measures of chlorophylls, though we recognize much work remains to fully define the value of this approach.

Keywords: Chlorophylls; Dinoflagellates; HAB; Nutrients; Red tide; Tidal flushing.