Spatial and temporal variations of nutrients and chlorophyll a in the Indus River and its deltaic creeks and coastal waters (Northwest Indian Ocean, Pakistan)

Abstract Nutrient dynamics play an important role in mangroves and adjacent coastal ecosystems. This paper discusses the spatiotemporal variation of physicochemical factors in two monsoons and intermonsoons and their influence on chlorophyll a and the dissolved inorganic nutrient concentrations in selected creeks. The present study focused on dense mangrove cover creeks (Waddi Khuddi (WK) and Dabbo (DC)), sparse mangrove cover creek (Hajamro (HC)), and no mangrove to mixed terrestrial and agricultural vegetation (Khobar (KC)) along with the Indus River and adjacent NE Arabian Sea. Silicate (SiO32–) was the most abundant nutrient, and its concentration increased from the northeast to the southwest monsoon and decreased during the fall intermonsoon. Erosion of the riverbank catchment area, weathering, and sand mining are the major sources of elevated SiO32–. The highest concentration of nitrate (NO3–) was found in the main river channel (KC) throughout the year, with maximum concentration (37.42 ± 5.05 μM) in the river (KC) during the rainy season (SW monsoon) due to the agricultural runoff that resulted in an increase in chlorophyll a. The concentration of nitrite (NO2–) was high in the creek (DC) with mangroves and agricultural runoff input. The concentration of phosphate (PO43–) was high in mangrove-dominated creeks (WK and DC), suggesting litterfall as a major source of PO43− from the creeks. The river with no mangrove cover (KC) indicated that PO43– was a limiting factor. In the case of DSi:DIN ratios, SiO32–was not limited throughout the sampling period for primary production. The chlorophyll a concentration increased from upstream to downstream in the river and was positively correlated with SiO32– and NO3–, resulting in the dominance of diatoms during the SW monsoon. The study confirms that natural sources (river/terrestrial runoff and geomorphological factors) and anthropogenic factors (application of fertilizers, manure, construction of dams, and industrialization) significantly influenced the nutrients and chlorophyll a, both spatially and temporally. On a global scale, SiO32– concentrations in the Indus River during the southwest monsoon were lower than those in the Himalayan rivers (Brahmaputra, Ganges, Huanghe, and Yangtze Rivers). The NO3– concentration is comparable with that of Narmada, Tapti, and Pangani Rivers but lower than that for rivers originating from the Himalayas. Nutrient enrichment and eutrophication in coastal waters were mainly caused by urbanization and population increase, which affect the higher trophic level productivity and fish abundance in the coastal and shelf areas. Therefore, an integrated approach for the management and monitoring of upstream river systems is required for a better future.