David vs. Goliath: the Importance of Local Processes in Mediating the ocean acidification Signal in Shelf Seas

Chair: Samantha Siedlecki

Yuri Artioli (1)*, Sarah Wakelin (2), Jason Holt (2), Susan Kay (1), John Bruun (1), Jeremy Blackford (1)

1 Plymouth Marine Laboratory, Plymouth, PL13DF, United Kingdom
2 National Oceanographic Centre, Liverpool, L35DA, United Kingdom

Background
Changes in the carbonate chemistry due to the increasing concentration of atmospheric CO2 are locally mediated by other drivers, including biological ones like primary production. Although the atmospheric driver is clearly the major one, in productive regions the magnitude of Ocean Acidification and the variability of the carbonate chemistry are significantly affected by the local drivers. Impacts of Ocean Acidification on biogeochemical processes can further increase the importance of the local drivers.

Methods
The coupled marine ecosystem model NEMO-ERSEM has been run to project the state of the North Western European Shelf until 2100 under the IPCC RCP 8.5 scenario. Outcomes have been analysed applying Box-Jenkins time series analysis methods to emphasise the structure of the Ocean Acidification signal, in particular the differences in trend and seasonal cycle. The tool has been applied at different spatial and temporal scales to study the impact of resolution on the assessment of Ocean Acidification, and to different variables to investigate what drives the variability in response to Ocean Acidification.

Findings
Significant spatial differences in the global trend of surface pH have been observed, with areas with higher net primary production projected to experience a lower acidification (up to 30% lower). Changes in the pH seasonal cycle have also been observed, with larger amplitude (up to 40%) in areas where primary production is projected to increase, and a shift in phase correlated to changes in plankton phenology. These changes are exacerbated when a positive feedback of Ocean Acidification on primary production is considered in the model.

Conclusions
This study highlights that local processes are important to assess the signal of Ocean Acidification and its impact on organisms, since this is driven by local condition of the carbonate chemistry.