Phytoplankton assemblage shifts from whale shark off-season to in-season in Donsol, Philippines influenced by temperature and nutrients

Donsol, Sorsogon, Philippines experiences an annual aggregation of the whale sharks Rhincodon typus. This was anecdotally associated with high plankton prey, though the factors driving this are not understood. Here, we studied how phytoplankton and environmental conditions related to seasonal sightings of whale sharks in Donsol from 2017 to 2018. Sightings were categorized into “off-season” (September-November), and “in-season” (February-May). We collected and analyzed surface samples of phytoplankton and physicochemical parameters from twelve stations and used Hierarchical Modeling of Species Communities (HMSC) to discern potential associations of abiotic and biotic parameters. Environmental filtering largely structured the phytoplankton, with temperature and silicate as key variation predictors. Dinoflagellates emerged as off-season indicators, while diatoms represented in-season. The whale shark in-season was thus characterized by cooler temperatures, higher rainfall, high silicate concentrations, and an increase in diatom abundance and dominance. Riverine input may have supplied nutrients to the site favoring diatom growth.