Nutrients and chlorophyll-a in the Gulf of Oman: High seasonal variability in nitrate distribution

Abstract

As part of comprehensive oceanographic studies on the Persian Gulf and Gulf of Oman, this article documented the seasonal and vertical distribution of nutrients, chlorophyll-a, and nitrogen deficiency in the Strait of Hormuz and northern part of Gulf of Oman. During the three consecutive research cruises of RV Kavoshgar Khalij Fars (Persian Gulf Explorer) in September–October 2018, December 2018, and May 2019, 7–18 stations were surveyed. During the southwest monsoon season, the water column is characterized by a 10–20 m shallow mixed layer and high spatial variation in nutrients and their ratio in the upper 100 m. Shallow remineralization of nitrogen-rich organic matter and lateral advection from the Arabian Sea upwelling system lead to nutrient accumulation, and enrichment of nitrate-nitrogen compared to P and Si in the euphotic zone with some sign of Si limitation. On the other hand, deepening of the mixed layer due to the cool convective mixing during the northeast monsoon, brings nutrients to the surface and stimulate the phytoplankton blooms (i.e. with surface chlorophyll-a maximum of 1.46 ± 0.57 μg l⁻¹). So, the primary production is likely to be controlled by grazing rather than nutrient availability during the northeast monsoon. In the spring inter-monsoon, the multilayer structure of the water column with a less well-characterized mixed layer has a surface layer severely depleted in nitrite and nitrate with low yet detectable concentrations of silicate (0.77 ± 0.44 μM) and phosphate (0.29 ± 0.09 μM). The distribution of surface chlorophyll-a concentration was patchy with generally low concentrations of 0.37 ± 0.10 μg l⁻¹ with highly variable subsurface maxima occurring at 10–25 m depth level. Primary production seemed to be potentially limited by inorganic nitrogen availability in spring inter-monsoon, as severe nitrogen deficiencies extended from the surface to the bottom layer (17.54 ± 4.39 μM at 400–1000 m depth level).