Nutrient dynamics and buffering capacity of coastal groundwater-dependent microbialite ecosystems

Groundwater discharge is an important terrestrial source of nutrients to the coastal ocean and influences water quality and primary production. This study estimated the nutrient loads from coastal groundwater discharge along a semi-developed coastline in South Africa. It investigated an extensive network of supratidal spring-fed living microbialite ecosystems (SSLiME) that develop on rocky shores and how they change the nutrient chemistry of groundwater before it reaches coastal waters. SSLiME require groundwater discharge to actively accrete calcium carbonate minerals through microbially mediated processes. The microbial biofilm doing this requires nutrients for primary production and, therefore, it is hypothesised that the SSLiME might significantly reduce nutrient loads from the coastal aquifers before discharge to the ocean. Land use of the hinterland was a significant predictor of DIN with maximum concentrations observed in the austral winter and at developed sites (i.e., downstream from a coastal village). Developed sites were also characterized by high DIN:DIP ratios indicative of sewage-impacted groundwater. Nevertheless, nutrient inputs from coastal groundwater discharge are small compared to surface water sources for the area. The overall nutrient fluxes decrease down the elevation gradient as the groundwater flows through the SSLiME and is likely attributed to the utilisation of nutrients by primary producers. These systems attenuate nutrients by decreasing the loads to coastal waters but are also sensitive to further coastal development and nutrient inputs.