Effects of increasing nutrient disturbances on phytoplankton community structure and biodiversity in two tropical seas.

Statistical analysis of rainfall data from 2005 to 2015 showed that atmospheric deposition supplied large amount of dissolved inorganic nitrogen (38-155 mg·m·month) in N-deficient South China Sea and Eastern Indian Ocean. To understand marine ecosystem responses to increasing nutrient disturbances, we implemented field mesocosm experiments to study phytoplankton community structure and biodiversity responses to nutrient treatments with nitrate, phosphate and iron across tropical seas. Our results showed that DIN supply would change phytoplankton community structure and stimulated the regime shift from cyanobacteria to diatoms (relative dominance R > 0). Phytoplankton communities were dominated by diatoms (relative abundance >50%) accompanied by high chlorophyll a content with 1.58-39.27 μg·L in DIN-added cultures, whereas cyanobacteria dominated communities (relative abundance >60%) with low biomass of 0.12-0.18 μg·L in undisturbed cultures. Simultaneously increased DIN loading from atmospheric deposition would decrease ecological diversity of tropical seas owing to species competition and succession (Shannon diversity H' decreased to <1).