Pteropod time series from the North Western Mediterranean (1967-2003): impacts of pH and climate variability

Abstract

Environmental changes resulting from anthropogenic CO 2 emissions occur at global and local levels and have potentially harmful effects, particularly for calcifying taxa in the marine environment. A time series of pteropod abundance covering the period 1967−2003 was isolated from the Point B (northwestern Ligurian Sea) zooplankton time series. Inter-and intra-annual changes in the abundance of 3 families (Limacinidae, Cavoliniidae and Creseidae) were compared with the copepod time series to identify any differential effects driven by ocean acidification and temperature. pH values were hind-cast from total alkalinity estimated from local temperature and salinity measurements, and atmospheric CO 2 taken from the Mauna Loa time series. Although surface waters were supersaturated with respect to aragonite throughout the study period, it is estimated that pH declined by 0.05 units. All pteropod groups displayed a trend of increasing abundance, suggesting that any deleterious effect of declining pH T in the range of 0.05 units has not caused sufficient reductions in fitness as to decrease local abundances between 1967−2003. Pteropod populations are influenced by inter-annual changes in summer temperatures. Spectral analysis identified a ~14 yr periodic oscillation in sea surface temperature. Similarly timed oscillations in abundance are present for all pteropod families but not for copepods, indicating a possible influence of the North Atlantic quasi-decadal mode on pteropod populations. While laboratory studies have shown pteropods to be sensitive to changes in pH, this analysis suggests that local and regional scale drivers have had a greater effect on pteropod populations in the northwestern Mediterranean Sea in recent decades. It should be noted that pH changes in laboratory studies exceed 0.05 pH units and that the saturation state with respect to aragonite (Ω ar ) is usually much lower than that of the Mediterranean.

Simulation of Coastal Processes affecting pH with Impacts on Carbon and Nutrient Biogeochemistry

“…From the pre-industrial period until 2013 the pH in Mediterranean Sea has decreased by 0.055 -0.156 pH units (Hassoun et al, 2015) with an annual decreasing trend of 0.0044 ± 0.00006 (Flecha et al, 2015). Model simulations predict a further decrease in the pH of Mediterranean surface waters of 0.3-0.4 units by the year 2100 (Howes et al, 2015). In shallow nearshore Mediterranean areas, although seawater pH is influenced by various natural and anthropogenic processes other than CO 2 uptake (Borges & Gypens, 2010;Duarte et al, 2013;Hagens et al, 2015), long term data-based estimations at coastal Medit.…”

“…The penetration of anthropogenic CO 2 into the Mediterranean has led to lower saturation degree of calcium carbonate in relation to the preindustrial era (Hassoun et al, 2015). During the period 1967−2003, the estimated Ω ar in a coastal site in the western Mediterranean displayed a decreasing trend and fluctuated between the highest value of 4.3 observed in 1968 and a minimum of 3.1 in 2003 (Howes et al, 2015); the calculated Ω ar. , in the bottom waters of Elefsis Bay was found 2.19.…”

“…Sci., 19/2, 2018, 290-304 291 monitoring sites also indicate a shift towards more acidic conditions (e.g. Luchetta et al, 2010;Howes et al, 2015;Kapsenberg et al, 2017).…”

Spring distribution of shelled pteropods across the Mediterranean Sea

“…There are only two studies within the Mediterranean that focus solely on pteropod ecology (Howes et al, 2015;Manno et al, 2019) and both are limited to very restricted geographical regions. To our knowledge, there is no study on pteropod abundance and distribution across the Mediterranean Sea covering the whole basin and relatively large biogeochemical gradients.…”

Coastal ocean acidification and increasing total alkalinity in the northwestern Mediterranean Sea

“…For example, estimates of change in pH of bottom waters since the preindustrial era range between −0.005 and −0.06 (Palmiéri et al, 2015) and up to as much as −0.14 for full profile estimates (Touratier and Goyet, 2011). Techniques for estimating ocean acidification in the Mediterranean Sea thus far include the following: (1) hind-casting, using highresolution regional circulation models (Palmiéri et al, 2015), the TrOCA approach as applied to cruise-based profile data (Krasakopoulou et al, 2011;Touratier and Goyet, 2011;Touratier et al, 2016), and others (Howes et al, 2015); (2) partially reconstructed time series (Marcellin Yao et al, 2016); (3) comparative study periods (Luchetta et al, 2010;Meier et al, 2014); and (4) sensor-based observations over a short study period (Flecha et al, 2015). Ocean acidification time series of consistent sampling over many years are lacking for the Mediterranean Sea (The MerMex Group, 2011), particularly along the coast where river discharge influences the carbonate system (Ingrosso et al, 2016).…”