Water-column Controls on the Carbonate Cycle and Potential ocean acidification Impacts in the Gulf of Maine

Chair: Samantha Siedlecki

Zhaohui Aleck Wang (1), Cynthia Pilskaln (2), Gareth Lawson (3), Amy Maas (4)
1 Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
2 School for Marine Science and Technology, University of Massachusetts Dartmouth, New Bedford, MA 02744, USA
3 Biology, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
4 Bermuda Institute of Ocean Sciences, St. George’s GE 01, Bermuda

Background
Recent studies indicate that the U.S. Northeast continental shelf region, particularly the Gulf of Maine (GoME), may be susceptible to ocean acidification. However, in-situ studies to assess water-column drivers of the CaCO 3 cycle are particularly lacking for shelf systems. It is largely unknown how CaCO3 saturation states impact CaCO3 dissolution, its fluxes, and pelagic calcifiers in the GoME. It is thus challenging to project how future changes in carbonate chemistry will affect the CaCO3 cycle and organisms. Yet ever increasing interest in ocean acidification (OA) in the region demands process studies in this regard.

Methods
Water column sampling of carbonate chemistry was conducted during seasonal cruises over two years. To investigate impacts of potential CaCO 3 dissolution, pteropods distribution was also sampled in situ using Multiple Opening and Closing Net, with an Environmental Sensing System (MOCNESS). These data were combined with historical CaCO3 particle fluxes and CO2 data to examine water column controls on the carbonate cycle and potential impacts of CaCO3 dissolution.

Findings and Conclusions
The data show sub-surface waters in the basins of the GoME already experience under-saturation with respect to aragonite in spring and summer. Water-column aragonite dissolution may occur throughout the year, even when aragonite is slightly over-saturated. The dissolution process may be the dominant control on the CaCO 3 budget of the lower water column in the basins and their vicinity of the GoME. Surface production, remineralization at depth, and sediment resuspension all play a role in affecting water column carbonate chemistry, thus the CaCO3 budget. However, these primary drivers vary with seasons, creating seasonal variability in aragonite saturation states. These findings are surprising for a non-upwelling shelf system and likely have important implications to the CaCO3 cycle, shell-building organisms, such as Pteropods, and the GoME ecosystem and its fisheries. This study is the first comprehensive study to examine in-situ water-column CaCO3 dissolution and its major drivers in the OA sensitive Gulf of Maine.