Temperature-Dependent Bifurcated Seasonal Shift of Phytoplankton Community Composition in the Coastal Water off Southwestern Korea

Coastal water around the archipelago off southwestern Korea has been bathed in an unprecedented pool of warm water in summer in recent years. Here, we examined phytoplankton community responses to amplified seasonal temperature fluctuation around the archipelago based on chemotaxonomic analysis in association with physicochemical properties in the shallow coastal water from June 2016 to March 2018. Concentrations of dissolved inorganic nutrients were significantly higher during the warm season than during the cold season. Canonical correspondence analysis revealed that water temperature was a main driver controlling the phytoplankton community. Generalized additive models denoted that small phytoplankton groups (cryptophytes, cyanobacteria, and prasinophytes) were more resistant to high temperatures (>25 degrees C) compared with diatoms. Indeed, dominance of diatoms was significant (80%) during the cold season and sharply declined (to 49%) during the warm season. Small phytoplankton were dominant (62%) as the water temperature exceeded 22 degrees C. Our results highlight the effects of temperature-dependent bifurcated seasonality on phytoplankton communities in coastal waters. Furthermore, unprecedentedly high summer temperatures in August 2016 coincided with a remarkable increase in the importance (similar to 70%) of small phytoplankton. Accumulating evidence provides indications regarding future scenarios: while small phytoplankton will probably account for summer blooms, diatom blooms will shift to winter.