Characteristics of the spatial distribution of nitrogen and phosphorus nutrients in the upper waters of the northwest Indian Ocean during the dry season

The spatial distribution of nitrogen and phosphorus nutrients is a key factor regulating the structure and function of marine ecosystems. As a typical monsoon-driven sea area, the northwest Indian Ocean lacks systematic research on the distribution characteristics and dominant controlling mechanisms of nutrients in the upper waters during the dry season. From late December 2023 to mid-February 2024, an environmental survey was conducted in the northwest Indian Ocean (0.5°N-18°N, 59.5°E-67.5°E) with 102 survey stations set up. The seawater temperature, salinity, various forms of nitrogen and phosphorus nutrients, and chlorophyll a content were determined to analyze their horizontal and vertical distribution characteristics, and to explore the nutrient structure and the main factors controlling the distribution. The results showed that all nutrients in this sea area had clear spatial distribution patterns with significant vertical stratification. A subsurface maximum of nitrite nitrogen appeared at 70 m, and ammonium nitrogen was uniformly distributed in the upper water layer. The research results enrich the basic data of nutrients in the northwest Indian Ocean during the dry season and provide scientific support for understanding the biogeochemical processes and fishery resource management in this sea area.