Abstract

Coastal ecosystems are subject to multiple processes that drive pH change over time. Therefore, efforts to understand the variability in the coastal carbonate system are crucial to assess the marine system vulnerability to acidification. The variations of the carbon dioxide (CO2) system were studied, from December 2014 to January 2017, on 6 stations along a transect latitudinally crossing the northern Adriatic, from the Po River delta to the Istra Peninsula. The study aims to evaluate the influence of riverine inputs and other environmental drivers, such as temperature, air-sea CO2 exchanges and biological processes, on the carbonate system. Riverine discharges significantly affected the carbonate system, as they are an input of total alkalinity and nutrients. High alkalinity concentrations were measured in low salinity waters and a significant negative correlation between salinity and alkalinity was found. The influence of biological processes was underscored by the significant inverse correlation between pHT at a constant temperature (pHT25°C) and apparent oxygen utilization, and by the positive correlation between chlorophyll a and pHT25°C in samplings close to flood events. Moreover, thermic and non-thermic partial pressure (p) of CO2 in surface waters was evaluated. pCO2 was more strongly influenced by the thermal effect during summer, while the biological effect prevailed in the other seasons. The analysis of air-sea CO2 fluxes highlighted that the area acts as a sink of CO2 during winter, spring and autumn and as a source during summer. A biogeochemical simulation was used for bottom and surface waters to estimate future changes in northern Adriatic carbonate chemistry with the increase of anthropogenic CO2 and temperature, and to understand how biological processes could affect the expected trends. By 2100, under the IPCC scenario of business as usual and without the effect of biological processes, pHT is expected to decrease by ~0.3 and the aragonite saturation is expected to decline by ~1.3, yet not reach undersaturation values. Even though the northern Adriatic is characterized by high alkalinity buffering, pH seasonal variability will likely be more pronounced, due to the strong decoupling of production and respiration processes driven by stratification of the water column.

Highlights

  • IntroductionAtmospheric carbon dioxide (CO2) levels increased over 40% passing from 280 ppm (parts per million volume), before the Industrial Revolution, to 415 ppm in 2019, as is shown by the Keeling Curve

  • Atmospheric carbon dioxide (CO2) levels increased over 40% passing from 280 ppm, before the Industrial Revolution, to 415 ppm in 2019, as is shown by the Keeling Curve1

  • The oceanic uptake of CO2 has resulted in the acidification of the ocean (OA), since the beginning of the industrial era; the pH of ocean surface water has decreased by 0.1 units corresponding to a 26% increase in acidity, measured as hydrogen ion concentration (IPCC, 2014) and a further decrease of 0.4 is expected for the end of the century (Orr, 2011)

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Summary

Introduction

Atmospheric carbon dioxide (CO2) levels increased over 40% passing from 280 ppm (parts per million volume), before the Industrial Revolution, to 415 ppm in 2019, as is shown by the Keeling Curve. As a consequence of CO2 addition to the ocean: the concentration of bicarbonate ions (HCO3−) and the concentration of dissolved inorganic carbon (TCO2) increases, whereas pH, the concentration of carbonate ions (CO32−) and the saturation state of the carbonate minerals present in the seawater decreases These changes can have negative effects on the marine biota, especially on calcifying organisms (Kleypas et al, 1999). Coastal ecosystems are more complex and dynamic than that of the open ocean (Borges and Gypens, 2010; Cai et al, 2011; Ingrosso et al, 2016a), pH changes and long-term trends in coastal seas are usually considerably more complex (Duarte et al, 2013; Carstensen et al, 2018) These characteristics do not allow a direct transport of OA patterns from open oceans to coastal areas, leaving a remarkable lack in projections about acidification effects. The cumulation with other stressors as oxygen depletion and temperature increase (Wallace et al, 2014; Bednaršek et al, 2016), that are becoming more common in the last years, could worsen the impacts on marine organisms in the future

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