Spatial and seasonal distribution of particulate phosphorous and nitrogen in the Persian Gulf: Nitrogen enrichment ties to diazotroph bloom in stratified warm waters

Abstract

Understanding the particulate organic and inorganic forms of nutrients and their elemental stoichiometry in suspended particulate matter of marginal seas has important implications in several areas. Here, we present the findings from two consecutive research expeditions conducted in September 2021 and January 2022 aboard the Persian Gulf Explorer research vessel, focused on the particulate P and N in the Persian Gulf and the Strait of Hormuz. The prominent features of spatial and seasonal distribution characteristics were nearshore-offshore decreasing gradients in particulate nutrients, the summer-to-winter declining trend in particulate nutrients more notably for particulate organic nitrogen, and higher biomass N:P ratio in summer than in winter with high spatial inhomogeneity. The trends were mainly governed by shallow remineralization of organic matter, resuspension of fine sediments, summer stratification/winter mixing of the water column, and inflow of the Indian Ocean Surface Waters from the Gulf of Oman. The contribution of inorganic N in the particulate pool of N (1.8% in summer and 1.3% in winter) was much less than the contribution of inorganic P in the particulate pool of P (23.0% in summer and 32.4% in winter), and hence the biomass N:P was higher than the corresponding bulk N:P by a factor of ∼1.5. Seasonal pattern in biomass N:P was similar to those observed at many sites being higher in the summer and lower in the winter. The formation of summer thermocline, characterized by high surface temperature (∼33 °C) and limited upward flux of remineralized inorganic nutrients, coincided with the proliferation of nitrogen-fixing cyanobacteria. Samples collected from the deep offshore region in summer showed significantly higher biomass N:P ratio, reaching up to 47, which is far beyond the classical Redfield ratio of 16. The central Persian Gulf experienced less seasonal variations than the deep region in the western Persian Gulf, the northwestern Arvand estuary, and the northwestern Gulf of Oman.