Spatial distribution of nutrients and chlorophyll a across the Persian Gulf and the Gulf of Oman

Despite the economic and political importance of the Persian Gulf and the vast amount of marine trade carried on, mainly in tanker traffic, chemical oceanographic properties of the area, particularly nutrients and chlorophyll a had been measured only a few times in the past (expeditions of the Research Vessels Meteor (1965), Atlantis II (1977), Umitaka-Maru (1993-1994), ROPME (2000, 2001 and 2006)) and no observations after 2006 (ROPME cruise) were taken. The present study aims to investigate the hydrographic characteristics, nutrients and chlorophyll a on 20 stations between 25 degrees N and 27 degrees N across the Persian Gulf and the Gulf of Oman during the PGE 1801 cruise in January 2018. Seawater samples were collected at the surface, 10 m, 25 m, 50 m, 75 m and upper depths, depending on each station's total depth (the deepest sample was taken at 180 m). The results showed that chlorophyll a is concentrated in the surface layer and decreases with increasing depth, which may be associated with a decrease in the phytoplankton population in the lower layers. The inflowing surface water is high in phosphate (0.74-1.10 mu M) due to the north-east monsoon's upwelling. The N/P ratio was lower than the standard Redfield ratio of 16/1 suggesting nitrogenous nutrients as limiting elements. In nearly-enclosed basins with minimal exchanges with open oceans similar to the Persian Gulf, the primary production generally is limited by nitrogen. Nitrate was the highest and ammonium was the lowest in the Persian Gulf and Gulf of Oman waters. The results show the penetration of surface water with a salinity of 36.7 psu into the Persian Gulf which is carried along the Iranian Coast, gradually increasing in salinity to more than 38.2 psu.