Climate-related changes in total alkalinity as a key to understanding ocean acidification in the coastal zone

Rising CO₂ concentrations in the atmosphere have a multidimensional influence on marine ecosystems. Through amplifying the greenhouse effect they lead to seawater temperature increase, which initiates cascade changes in the environment. In addition to this climatic pathway, increasing atmospheric CO₂ concentrations cause also an overall increase of CO₂ concentrations in surface seawater and, consequently, a pH decrease – a mechanism called Ocean Acidification (OA). OA is already fairly well understood and traceable in the open ocean waters, where large-scale projects and actions supply an enormous amount of observations and experimental data and where the magnitude of OA is to a large extent thermodynamically consistent with the increase in atmospheric pCO₂. In the coastal and shelf seas, OA is still a considerably understudied phenomenon despite their high socio-economic importance and potentially great vulnerability of these regions to acidification due to often lower salinity and corresponding lower buffer capacity of waters as compared to open ocean.

In the present study, we underline the importance of total alkalinity (TA) as the key factor shaping the OA dynamics and pH fields in the coastal and shelf seas. The extensive research performed in 2018-2022 extended from the brackish Baltic Sea through the open waters of the eastern Fram Strait to the Spitsbergen fjords affected by the high inflow of meltwaters. It revealed extremely high variability in the marine CO₂ system structure and significantly different influences of freshwater input in the investigated regions. The observed overall spatial and temporal (seasonal) variability in TA extended in the broad range between <350 and 4,320 µmol kg^-1. This makes TA a fundamental variable for studying the large-scale pH and pCO₂ changes and forecasting the development of OA in the coastal zone in the future high-CO₂ world. This large-scale study can be considered as the reference for future OA research in the investigated regions and simultaneously calls for action to include TA in the routinely observed parameters in the coastal waters.