Dinoflagellate responses to nutrients and mangrove leaf organic matter in the bioluminescent Mangrove Lagoon, St. Croix, U.S. Virgin Islands

Bioluminescent bays (biobays) are uncommon coastal ecosystems featuring dense populations of dinoflagellates, often found in mangrove-ringed lagoons with long water retention times and high organic matter (OM) levels. Although mangroves are associated with high dinoflagellate abundance, it remains uncertain how inorganic nutrients compare with mangrove-derived organic matter in shaping phytoplankton communities. We performed short-term in situ bioassays in Mangrove Lagoon, St. Croix, U.S. Virgin Islands, to compare phytoplankton community responses to additions of inorganic nutrients (N and P) and senescent mangrove leaf additions. Phytoplankton composition was assessed using high-performance liquid chromatography (HPLC) pigment analysis combined with CHEMTAX, and data were analyzed with compositional and centered log-ratio principal component analyses (CLR-PCAs). Nutrient additions primarily increased total phytoplankton biomass and favored diatom-dominated communities, consistent with N limitation, but did not significantly alter community composition. Conversely, adding senescent mangrove leaves consistently shifted the community toward higher abundances of peridinin-containing dinoflagellates, without significantly increasing the abundance of other phytoplankton groups. Multivariate analysis showed that leaf additions shifted the community along a continuous compositional spectrum rather than creating distinct assemblages. These findings suggest that OM (leachate) from senescent mangrove leaves, rather than inorganic N and P enrichment alone, is a plausible mechanism for promoting dinoflagellate dominance in Mangrove Lagoon. Our results underscore the potential role of mangrove OM input in sustaining dinoflagellate-dominated environments in some tropical lagoons, demonstrating a mechanism by which mangroves may regulate phytoplankton community composition in adjacent waters.