Overview of decadal ecosystem changes in the Western Arabian Sea and the occurrence of algal blooms

Gradual decadal changes have taken place in the Western Arabian Sea over the last 50 years. Here we syntheses available evidences and reveal the trends pronounced in directly measured and remotely sensed parameters. We show that decadal changes have affected wind speeds, atmospheric and sea surface temperature, thermohaline stratification, shoaling of the oxycline, and dust/iron inputs. A decrease in nitrate supply of the photic layer have caused an increase in annual frequency of harmful algal blooms and fish kills. Along with that, a decrease in diatom biomass and a shift from red Noctiluca to green Noctiluca during the northeast monsoon was observed during the last two decades, Even though these are the same species they have very different nutritional modes. The red one is a heterotroph with a preference for grazing diatoms, while the green one has a symbiont and thus it is a mixotroph. Recent results suggest that this shift may be caused by the shoaling oxycline since the green Noctiluca grows better under low oxygen because the symbiont produces oxygen for its host. The western Arabian Sea is temporally and spatially complex. With the recent advances in remote sensing of the ocean, a further understanding of the mesoscale spatial-temporal variability of Noctiluca blooms can be gained through analyzing frequent images with opportunistic ground-truthing.