Spatial variations in dissolved inorganic nutrients in the groundwaters along the Indian coast and their export to adjacent coastal waters

Abstract

Submarine Groundwater Discharge (SGD) is one of the main external nutrient sources to the coastal waters. The concentrations of nutrients in groundwaters are a few folds higher than that of adjacent coastal waters; therefore, SGD enhances nutrients levels in the coastal waters and influences coastal biota. In order to examine the spatial and seasonal variability in nutrient concentrations and exchange to the coastal waters, groundwater samples were collected at ~ 90 locations along the Indian coast during the wet and dry seasons. This study revealed that dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphates (DIP) and urea were found to be high during the dry than wet period. Higher concentrations of DIN and DIP were observed during both wet and dry periods in the groundwater along the east than the west coast of India. The State-wise mean amount of fertilizer used during Kharif (wet) and Rabi (dry) period in each Indian State showed significant correlation with mean concentrations of DIN and urea. The observed linear relationship of DIN with bacterial respiration and inverse relationship with DO saturation and ammonium in groundwater suggested that decomposition of organic matter and nitrification contributed to the DIN pool in the groundwater. The mean rate of SGD fluxes varied between 1.6 � 104 m3/day and 1.75 � 1011 m3/day in the Indian coastal region. The annual mean SGD flux of DIN and DIP was estimated to be 0.103 � 0.02 and 0.021 � 0.01 Tg (1 Tg = 1012 g) to the western coastal Bay of Bengal (east coast of India) and 0.06 � 0.03 and 0.015 � 0.01 Tg/y to the eastern coastal Arabian Sea (west coast of India) respectively. The estimated SGD flux of DIN and DIP to the Indian coastal waters amounted to 0.163 � 0.04 and 0.036 � 0.02 Tg/y respectively, and it is almost close to that of nutrients discharged by rivers (0.22 � 0.05 and 0.11 � 0.03 Tg/y respectively). Among the external sources of nitrogen and phosphorus, such as river discharge, atmospheric deposition, the contribution by SGD is highly significant in the Bay of Bengal (30 and 17% respectively) than in the case of Arabian Sea (24 and 25% respectively).