Chemical oceanography of Irish waters, with particular emphasis on ocean acidification

Abstract

Strategically positioned along the western margin of the North Atlantic, Irish shelf and offshore waters play a crucial role in the global thermohaline circulation and regional and global climate cycles. The main objective of this study was to investigate the biogeochemical characteristics of the main water masses in the region to generate information on how the marine environment is changing with time. Dissolved oxygen, nutrient and carbon data, collected across the Rockall Trough in February 2009 and 2010, proved useful as chemical tracers of water masses in the region and highlighted processes that could not have been identified using hydrographic data alone. Inorganic carbon data from 2009 and 2010 were compared with WOCE data collected across the Trough in the 1990s to assess the temporal evolution of anthropogenic carbon (Cant) in the region over 2 decades. Two methods were used to calculate Cant between surveys, CT-abio and extended multiple linear regression, both of which resulted in similar rates of increase in Cant through the water column, with subsequent decrease in pH and saturation state of calcium carbonate minerals. Between 1991 and 2010, pH in subsurface waters has decreased by 0.040±0.003 units and by 0.029±0.002 units in Labrador Sea Water. Net community production (NCP) was calculated along the western shelf edge between 49.8-55.4ºN. Generally maximum NCP was measured in surface waters over the 500-750m contours, decreasing in both offshore and shallower on-shelf surface waters. Where calculated, there was a net CO2 uptake from the atmosphere suggesting this region is a CO2 sink during the productive season. Due to its influence on the buffer capacity of the surface ocean, the distribution of total alkalinity (AT) in Irish coastal and shelf waters was investigated. The AT distribution in outer estuarine and coastal waters is more complex than along the western shelf and through the centre of the Irish Sea due to varying river inputs. Rivers with limestone bedrock catchments had relatively high AT concentrations which influence the buffer capacity, and hence rate of pH change, of the surrounding coastal waters. Results indicate that the algorithm produced by Lee et al. (2006) to calculate AT from temperature and salinity should not be used in Irish coastal waters due to variable but substantial riverine inputs of AT.