Contribution of submarine groundwater discharge(SGD) to the marine carbonate biogeochemistry of the Western Irish Coastal Sea

Abstract

The main objective of this study was to investigate the coastal carbonate chemistry in the Irish coastal seas, and specifically to understand the role of Submarine Groundwater Discharge (SGD) in the coastal biogeochemical cycles as a driver of coastal metabolism. SGD was recognised from previous literature studies, as an important source of nutrients, Total Alkalinity (TA) and Dissolved Inorganic Carbon (DIC). This project, therefore, aims to identify the role of SGD to the coast and this was addressed with a comparative approach. Two areas with contrasting watershed input (Kinvarra Bay fed by SGD and Killary Harbour fed by rivers) were selected for the study. The two selected systems were sampled at different times of the year, in order to capture freshwater discharge variability. The question rising from the literature review on the influence of freshwater discharge in the TA:DIC ratio and therefore on the effect on coastal Net Community Production (NCP) was addressed. Carbonate system dynamics and NCP were analysed and compared. The freshwater input to both study areas decreases pH in the low salinity areas. Salinity-normalised TA-DIC slopes were compared, with reference to the open ocean. Freshwater inputs drove both systems into distinct metabolic (net autotrophy vs net heterotrophy) and biogeochemical (net CaCO3 precipitation vs net CaCO3 dissolution) states. The balance between organic and inorganic drivers of the TA:DIC ratio did not change in Killary Harbour with freshwater discharge or time of the year but did change in Kinvarra Bay. High resolution sampling strategy was applied on both freshwater sources in order to identify the main driver of short-term variability of carbonate system. The SGD spring carbonate system dynamics are driven by freshwater-seawater mixing, reversely the river system diel carbonate system dynamics are likely driven by physical processes, however not a clear trend was identified in the time-series analysis. A budget methodology was applied to quantify the NCP in both selected systems and to determine if are source or sink of organic and inorganic carbon. Killary Harbour showed positive NCP, and could be thus considered as an autotrophic system. On the other hand, in Kinvarra Bay during the autumn and winter surveys a net export of TA and DIC towards open ocean was quantified, and a negative NCP. These results suggest that freshwater inputs into Kinvarra Bay could seasonally drive the system into heterotrophy, intensifying current ocean acidification trends.