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This overview compares algal blooms and pelagic fisheries of the Arabian Gulf with the Sea of Oman. The data consist of remotely sensed characteristics, directly sampled and modeled. Elucidated seasonal trends were based on 15-year seasonal means, as well as weekly time series of physical parameters. The environmental characteristics (namely photosynthetically available radiation, atmospheric pressure, temperature, wind, aerosol optical thickness, surface currents, surface temperature, salinity, concentration of dissolved oxygen, nitrates, phosphates, chlorophyll-a, net primary production, phytoplankton, zooplankton biomass, fish larvae abundance, small and large pelagic fish catches) were compared between regions. In Sea of Oman, high concentrations of chlorophyll-a were associated with relatively high concentrations of nitrates and phosphates, as well as kinetic energy of surface currents which exceeded that in Arabian Gulf. The dinoflagellate Noctiluca scintillans is one dominat alga in the Sea of Oman, whereas diatom species are more common in the Arabian Gulf blooms. In general, the phytoplankton and zooplankton species diversity during winter was higher than in summer periods. Catches of small pelagic fish (in particular sardines) in the Sea of Oman exceeded that in the Arabian Gulf. This might be associated, in part, with differences in trophic levels interactions. The turnover rate of the net primary production through zooplankton in Sea of Oman was found to be much higher than in Arabian Gulf waters.

<p>Seasonal variability is a powerful component of the spatio-temporal dynamics of plankton communities, especially in the regions with oxygen-depleted waters. The Arabian Sea and the Black Sea are typical representatives of these regions. In both, the dinoflagellate Noctiluca scintillans (Macartney) Kofoid & Swezy, 1921, is one of the abundant plankton species which forms algal blooms. Sampling on coastal stations in the upper mixed layer by the plankton nets with the 120-140 µm mesh size was carried out in 2004-2010. Monthly data were averaged over years. A comparison of seasonal patterns of Noctiluca abundance pointed to the persistence of a bimodal seasonal cycle in both regions. The major peak was observed during spring in the Black Sea and during the winter (Northeast) monsoon in the Arabian Sea. The timing of the second (minor) peak was different over regions as well. This peak was modulated by advection of seasonally fluctuating velocity of coastal currents which transport waters enriched by nutrients by coastal upwelling. The abundance of Noctiluca of the major peak (with the concentration around 1.5*10<sup>6</sup> cells m<sup>-3</sup>) was from one to two orders as much high in the western Arabian Sea compared to the northern Black Sea. The remotely sensed chlorophyll-a concentration during the time of the major seasonal peak exhibited a fivefold difference over these regions. In terms of nutrient<sub></sub>concentration in the upper mixed layer (in particular, nitrates and silicates), a difference of about one order of magnitude was observed.</p>

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