Abstract
Environmental changes resulting from anthropogenic CO2 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 com- pared 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 CO2 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 pHT 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 tempera- tures. Spectral analysis identified a ~14 yr periodic oscillation in sea surface temperature. Simi- larly 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 sug- gests 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 labo- ratory studies exceed 0.05 pH units and that the saturation state with respect to aragonite (Ωar) is usually much lower than that of the Mediterranean.
Highlights
Anthropogenic activities such as the burning of fossil fuels, cement production and deforestation are causing an increase in atmospheric concentrations of CO2 at rates that are potentially unparalleled in the past 300 million years (Hönisch et al 2012)
The extreme high values that occurred in September 2009 were not replicated by the model (Fig. 2A); it is likely that these values were related to local events that were not possible to replicate with Mauna Loa pCO2atm data
Comparison of calculated surface pHT values to measured values (2007−2012) using linear regression produced an r2 of 0.49; as the model failed to accurately predict the extreme low observations, these observations corresponded to a peak in pCO2sw during 2009 that was not predicted by the pCO2sw model
Summary
Anthropogenic activities such as the burning of fossil fuels, cement production and deforestation are causing an increase in atmospheric concentrations of CO2 at rates that are potentially unparalleled in the past 300 million years (Hönisch et al 2012). One fourth of atmospheric CO2 is absorbed by the world’s oceans, leading to a lowering of pH and a reduction in carbonate ion concentration ([CO32−]) (Caldeira & Wickett 2003). The reduction in [CO32–] caused by ocean acidification has potentially negative effects for calcifying organisms, as it decreases the calcium carbonate saturation state (Ω). In a system of pure Ca2+ and CO32−, if Ω is 1, the mineral phase is supersaturated and precipitation will occur. Three different polymorphs of CaCO3 are precipitated by marine calcifiers: high Mg calcite, low Mg calcite and aragonite. Calcifying organisms have been observed to exhibit the strongest negative responses to ocean acidification (Kroeker et al 2013). The effects of decreased pH can interact with other stressors, such as elevated temperatures, often increasing organism sensitivity (Kroeker et al 2013)
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