Chapter 9 - Submarine groundwater discharge as a significant source of nutrients in the coastal zone

Abstract

This book chapter includes the study of submarine groundwater discharge (SGD) across various coastlines and related nutrient fluxes to the coastal oceans. SGD represents an essential pathway of materials like carbon, nitrate, phosphate, silicate, and trace metals between land and sea. Global DOC and DIC Fluxes through SGD studied across different mangrove climatic zones indicate that the area of tropics accounted for 71% and 81% of DIC and DOC fluxes, respectively. Subtropics contributes 29% and 19% respectively, while the contribution of temperate region is even less than 1% for both the parameters. An increase of about 38% in SGD nitrogen input between 1950 and 2000 has been observed in global coastal water and is estimated to increase by 22% between 2000 and 2050. The rise in nitrogen is not followed by an equivalent increase in phosphate and silicate, suggesting that nitrogen is the primary nutrient affected by SGD. The increase in nitrogen is mainly due to the anthropogenic contamination of the certain aquifer, which in turn modify the Redfield ratio where the N/P ratio exceeds the river system revealing that human activities can change N-limited coastal primary production to P-limited one. The Indian coastline is mainly composed of red, yellow, and alluvial soils, which are poor in nutrients. Hence Indian farmers use excessive fertilizers to obtain a better yield. Which plays a significant role in controlling nutrient dynamics in groundwater. SGD-derived trace metals are also equal to or higher than the river fluxes and contribute approximately 10% of total trace elements to the oceans.