The Influence of Nutrient (N and P) Enrichment and Ratios on Phytoplankton Abundance in Keunekai Waters, Weh Island, Indonesia

The Keunekai waters around Weh Island, Indonesia, have become a prone coastal area because of degradation from climate anomalies and anthropogenic pressure. The high level of coral mortality caused by mass bleaching and fish bombing several years ago may have led to the deterioration of the water conditions and the disruption of the biogeochemical cycle, resulting in potential nutrient enrichment and algae blooms (eutrophication). This study aimed to determine the influence of nutrient ratios on phytoplankton abundance and monitor the existing water conditions around Keunekai. Spectrophotometry analysis was used to determine the concentrations of the nutrients, and the Sedgewick-Rafter counting method was used to identify the species and abundance of the phytoplankton. A large variation in N content (ranging from 1.1 to 1.6 mg N/L) and, particularly, P content (ranging from 0.02 to 0.18 mg P/L) most likely reflected differences in the supply ratios of N and P, rather than differences in absolute N and P availability. Three taxa of phytoplankton and their relative abundance were identified in the study area: Bacillariophyceae (diatom) (72%), Cyanophyceae (3%), and Dinoflagellate (25%). It was found that P-limitation mostly controls potential algae blooms, which support the density of Dinoflagellates that may endanger the water