Characterizing algal blooms in a shallow & a deep channel

The outbreaks of algal blooms occur in both shallow and deep-water bodies with varied frequencies in different seasons. To investigate the occurrences of algal blooms in these two different water bodies, we consider the Arabian Gulf (shallow) and the Sea of Oman (deep) as examples for this study. In this work, we have used a recent unique and comprehensive in-situ dataset of the frequent algal blooms collected over the last decade in the Arabian Gulf and Oman Sea. This data includes algal blooms' frequencies as well as seawater properties including sea surface temperature (SST), Dissolved Oxygen (DO), salinity and pH. In addition, we have used satellite SST and chlorophyll-a (Chl-a) data, ocean reanalysis data of water currents and bathymetry data. These data have been analyzed through statistical methods including descriptive analysis, trend analysis, and empirical orthogonal function. The results obtained demonstrate a general decreasing trend of the algal bloom events from 2010 to 2018 in the shallow waters while in the deep waters the trend is increasing. We reveal a clear seasonality with the highest frequency and Chl-a concentration of algal blooms during winter and spring. The frequent occurrences of algal blooms during winter is due to favorable SST between 24 °C and 32 °C in shallow waters and up to 28 °C in deep waters. Although salinity differs in the shallow waters (~39 psμ) and deep waters (~37), but algal blooms are found to be tolerant to these different salinity ranges as well as at pH of 8 in both regions. Chl-a commonly exceeds 10 mg m−3 in the shallow waters (100 m) and at water currents exceeding 0.2 m/s. Even at optimum levels of SST and water depth algal blooms cannot occur if there is insufficient supply of nutrients. [Display omitted] •Algal blooms commonly occur in deep waters when the sea temperature is below 28 °C.•In the shallow waters, the algal growth is enhanced when SST approaches 24 °C and 32 °C.•During summer, Chl-a levels in deep waters are low due to higher water temperatures and reduced upwelling.•The algal blooms frequently occur in deep and shallow water at a pH of 8.•Chl-a levels commonly exceed 5 mg.m-3 where the water depth is less than 100 m.