Climatological variations of total alkalinity and total dissolved inorganic carbon in the Mediterranean Sea surface waters

Abstract

Abstract. A compilation of data from several cruises between 1998 and 2013 was used to derive polynomial fits that estimate total alkalinity (A T ) and total dissolved inorganic carbon (C T ) from measurements of salinity and temperature in the Mediterranean Sea surface waters. The optimal equations were chosen based on the 10-fold cross-validation results and revealed that second-and third-order polynomials fit the A T and C T data respectively. The A T surface fit yielded a root mean square error (RMSE) of ± 10.6 µmol kg −1 , and salinity and temperature contribute to 96 % of the variability. Furthermore, we present the first annual mean C T parameterization for the Mediterranean Sea surface waters with a RMSE of ± 14.3 µmol kg −1 . Excluding the marginal seas of the Adriatic and the Aegean, these equations can be used to estimate A T and C T in case of the lack of measurements. The identified empirical equations were applied on the 0.25 • climatologies of temperature and salinity, available from the World Ocean Atlas 2013. The 7-year averages (2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012) showed that A T and C T have similar patterns with an increasing eastward gradient. The variability is influenced by the inflow of cold Atlantic waters through the Strait of Gibraltar and by the oligotrophic and thermohaline gradient that characterize the Mediterranean Sea. The summer-winter seasonality was also mapped and showed different patterns for A T and C T . During the winter, the A T and C T concentrations were higher in the western than in the eastern basin. The opposite was observed in the summer where the eastern basin was marked by higher A T and C T concentrations than in winter. The strong evaporation that takes place in this season along with the ultra-oligotrophy of the eastern basin determines the increase of both A T and C T concentrations.

Increase of dissolved inorganic carbon and decrease in pH in near-surface waters in the Mediterranean Sea during the past two decades

“…Monthly surface samples collected at the Dyfamed time series station between 1998 and 2013 indicate an increasing DIC trend of 1.35 µmol kg −1 yr −1 . This value is uncertain (r 2 = 0.05) because of the large seasonal variability displayed in the monthly samples (Gemayel et al, 2015). Nevertheless, this value is closer to the trend we calculated between the two periods, 1993-1995 and 2013-2015 (1.40 µmol kg −1 yr −1 ) than to the trend inferred from the atmospheric increase (1.15 µmol kg −1 yr −1 ).…”

“…These changes have complex direct and indirect impacts on marine organisms and ecosystems (Gattuso and Hansson, 2011). Empirical methods to estimate the anthropogenic CO 2 penetration in the ocean since the industrial revolution have improved over the past few decades (Chen and Millero, 1979;Gruber et al, 1996;Sabine et al, 2008;Goyet, 2004, 2009;Woosley et al, 2016). As the concentration of anthropogenic carbon, C ant , cannot be distinguished from the natural background of DIC through total DIC measurements, these methods are based on the analysis of different chemical properties of the water column.…”

Attributing seasonal pH variability in surface ocean waters to governing factors

“…Contrary to our results, they did not identify a clear seasonality in surface‐water TA. The seasonality in DIC these authors estimated (105 µmol kg −1 ) is similar to the seasonality of ~100 µmol kg −1 determined in more recent studies spanning the 1998–2013 and 1998–2011 periods, respectively [ Gemayel et al ., ; Marcellin Yao et al ., ]. All of these values are, however, almost twice as high as the peak‐to‐peak amplitude of DIC determined in this study.…”

“…All of these values are, however, almost twice as high as the peak-to-peak amplitude of DIC determined in this study. This may be attributed to either the longer timespan of the current data set compared to Copin-Montégut and , thereby including interannual variability, or to the different methodology this study uses to determine the seasonal mean compared to Gemayel et al [2015] and Marcellin Yao et al [2016].…”

In situ abundance and carbon fixation activity of distinct anoxygenic phototrophs in the stratified seawater lake Rogoznica

“…These values are at least 1 mmol l −1 higher than measured and modelled DIC concentrations in Adriatic surface seawater (e.g. Cossarini et al ., ; Gemayel et al ., ). Carbonate mineral (calcite and dolomite) dissolution could have been an important DIC source in Lake Rogoznica, as it has been previously reported for other karst and epikarst hosted surface water bodies and aquifers in Croatia and elsewhere (e.g.…”