Seasonal variations of ocean acidification in Korean coastal waters

Abstract

Since the beginning of the Industrial Revolution, large quantity of CO2 has been emitted into the atmosphere, mainly as a result of the burning of fossil fuels, cement production and extensive land use change. Around two-thirds of these emissions have remained in the atmosphere with its concentration increasing from around 280ppm in 1800 to 400ppm in 2016. Remaining a third of these emissions appears to be absorbed by the ocean, indicating the oceanic sink as the major regulator of the atmospheric CO2. However, the absorption of excessive atmospheric CO2 by the oceans is leading to severe consequences on ocean biogeochemistry both regionally in coastal waters and globally in open oceans. The most prominent consequence is the ocean acidification which has caused the average surface ocean pH to decrease by 0.1 units relative to the preindustrial values (pH 8.1) and is predicted to change patterns of biogenic carbonate formation and may have further impacts on other ocean biogeochemical cycles. In coastal regions, the additional inputs of freshwater from rivers combined with the high-magnitude and high-frequency of natural variability as well as anthropogenic chemical perturbations further complicates our understanding of variations in ocean in space and time. However, a long-term oceanographic data is lacking in marginal seas including Korea to provide sufficient information on changes of ocean acidification. Therefore, it is important to extend our understanding of coastal waters through careful investigations to detect and attribute trends in ocean acidification and thus further evaluate the resulting implications for coastal marine ecosystems. This paper is structured in the following two parts. Chapter 2 is the study of impact of borate alkalinity on calculation of ocean acidification parameters in coastal waters of Korea and Iceland and Chapter 3 presents the study of seasonal variations of ocean acidification in Korean coastal waters. The first study investigated for any discrepancies in boron (B) concentration in coastal waters, thereby boron to salinity (B/S) ratio compared to the previously established B/S ratio from open ocean studies and thus verify possible impacts on calculation of ocean acidification parameters. The accurate measurement of B concentration in seawater is essential for the calculation of carbonate alkalinity, and thereby in estimating the seawater buffering capacity to increasing anthropogenic CO2 in the atmosphere. The analysis of B conducted covered the coastal regions of Korea (n=154) and Iceland (n=249). The results yielded the mean B/S ratio of 0.1322±0.0007 mg kg-1 ‰-1 regardless of sampling location or water depth. The mean B/S ratio estimated in the present study produced ±0.0014 mg kg-1 ‰-1 discrepancy from the B/S ratio established by Lee et al. (2010) leading to an error of less than ±1 μmol kg-1 in AB estimation. This is a negligible difference and will have an insignificant effect on the accuracy of Ac estimation. Therefore, this indicates the near-constant B concentration and thus B/S ratio in seawater regardless of known processes in coastal waters including atmospheric deposition, ocean biology, ice and river discharges. In particular, the present study indicates no apparent anthropogenic impacts despite the enhanced fluxes of B in seawater due to anthropogenic activities. The second study explored seasonal variations of ocean acidification in coastal waters of Korea. This study presented the beginning part of ocean acidification research started in April 2015 in coastal waters of Korea with big data produced from samples collected and analyzed during the first two years (n=1371). The results showed distinctive variations of ocean acidification parameters (Ωarg and pH) with respect to location and season. The marked changes were mostly observed in ocean surface while little or no variations were detected in the near-bottom waters over the study period due to slower response of deeper ocean to changes in the surface. Large fluctuations in surface carbonate parameters especially in Ωarg imply potential impacts of ocean acidification on calcifying marine organisms by altering their calcification rates. The spatiotemporally non-uniform variations in carbonate chemistry were attributed to different hydrographic features of each region. In particular, the west and south coasts of Korea are in close proximity to the regions of highest population density and receive riverine discharges from the major rivers. Influence of freshwater plume from Changjiang River in Southeast China also caused large temporal variations in surface ocean acidification parameters. The study period was too brief to show interannual variations in surface or near-bottom seawater ocean acidification parameters at this point. However, the present study found marked spatiotemporal variations over 2015 and 2016 which provides an idea to lead a long-term research in investigating changes in coastal ocean acidification and further evaluation for the resulting implications for marine ecosystems.