Effect of a rainfall pulse on phytoplankton bloom succession in a hyper-Eutrophic subtropical lagoon

In the present study, we sought to understand the succession of phytoplankton species, after a natural nutrient pulse, in a subtropical lagoon located in southern Taiwan. The lagoon was surrounded by aquaculture ponds. The present study was performed during the wet summer season, before and after an episode of heavy precipitation. Before rainfall commenced, both the phosphate concentration and the level of phytoplankton were relatively low. After heavy precipitation, physical and chemical measurements indicated that significant amounts of dissolved inorganic nutrients had drained into the lagoon. A phytoplankton bloom occurred; organism levels reached 77.6×105 cells L-1. The dominant organism was Chaetoceros curvisetus (99.3%). After the bloom ceased, the levels of inorganic nutrients, especially silicate, fell. Phytoplankton became of low abundance once more. At the end of our study period, the ecosystem was dominated once more by diatoms (75.8%); this may have been caused by a low-level nutrient pulse following rainfall that occurred one day before final sampling. Overall, our results suggest that the bloom succession of phytoplankton species was principally dependent on nutrient dynamics in the lagoon, which was associated with nutrients discharged from drainage after heavy rainfall.