High biomass phytoplankton blooms are recurrent in the Sea of Oman; some of these blooms can be harmful for marine life as well as for human health. Eutrophication has been suggested as one of the factors involved in promoting these blooms through inputs of nitrogen and phosphorous. In some oceanic areas described as "high-nutrient-low-chlorophyll", iron and other micronutrients may also be limiting phytoplankton growth. More recently, large areas of the world ocean were showing deficiency of B vitamins. This suggests that vitamin limitation could control phytoplankton production in coastal and open ocean environments. The aim of this work was to investigate experimentally the limiting nutrient(s) for natural phytoplankton community growth in the coastal waters of the Sea of Oman particularly in Al Bustan area (Muscat) during two different durations, spring bloom and post spring bloom. The factorial and fractional factorial designs were employed to test the role of inorganic nutrients, trace metals and vitamins in 500 ml of pre-filtered (180 um) seawater incubated under natural light (sunlight) for 48 hours. Phytoplankton response was evaluated by the measurement of in vivo chlorophyll-a. Results showed different limiting nutrients during the spring and after spring blooms. Nitrogen appeared to be the most significant factor after the spring bloom, whereas metals and vitamins were significant during the spring bloom. Phytoplankton development was driven by different compositions of nutrients. Results support the hypothesis that at some time throughout the year (during the spring), although basic nutrients (N, P, Si) are abundant, HABs species are not promoted easily because other nutrients are limiting. Before the spring bloom phytoplankton were limited by B12 or Cobalt and since most HAB species are auxotrophs for B12 (cannot synthesize de novo); HABs are thus unusual although nutrients concentrations may be high. After the spring bloom, the bacteria degraded the organic matter produced and provided a pool of B12, auxotrophic HAB species can grow and get nutrients below the thermocline (mobile species), although nutrients concentrations may be low. The fractional factorial design allowed the experimental identification of all 7 interacted nutrients alone or in pairs, using only 32 individual bottles. This study elaborated the need to conduct further investigations to assess the importance of vitamin B12 and its relation with cobalt as a growth factor for phytoplankton specially HABS