Abstract
The impact of simulated seawater acidification and warming conditions on specimens of the mussel Mytilus galloprovincialis locally adapted to very distinct, widely separated sites in the Mediterranean Sea (Tunisia) and Atlantic Sea (Galicia, NW Spain) was evaluated in relation to key behavioural and eco-physiological parameters. Over the 2-month exposure to the experimental conditions, mussels were fed optimally to ensure that there are no synergistic interactions between climate change drivers and energetic status of the individuals. In general, regardless of origin (Atlantic or Mediterranean), the mussels were rather resilient to acidification for most of the parameters considered and they were able to grow in strongly acidified seawater through an increased feeding activity. However, shell strength decreased (40%) consistently in both mussel populations held in moderately and highly acidified seawater. The observed reduction in shell strength was not explained by slight alterations in organic matter, shell thickness or aragonite:calcite ratio. The combined effects of high acidification and warming on the key response of byssus strength caused a strong decline in mussel performance, although only in Galician mussels, in which the valve opening time decreased sharply as well as condition index (soft tissue state) and shell growth. By contrast, the observed negative effect of highly acidified scenario on the strength of Tunisian mussel shells was (partly but not totally) counterbalanced by the higher seawater temperature. Eco-physiological and behavioural interactions in mussels in relation to climate change are complex, and future scenarios for the ecology of the species and also the feasibility of cultivating them in Atlantic and Mediterranean zones are discussed.
Highlights
One-third of the total anthropogenic CO2 emissions have been absorbed by the ocean (Sabine et al, 2004), in turn triggering a process called ocean acidification (OA) (Orr et al, 2005; Gattuso et al, 2015)
The clearance rate (CR) was significantly higher in mussels from the Mediterranean Sea (Tunisia) as compared with the Atlantic Sea (Galicia) especially for highly acidified environments regardless temperature value (0.97–1.08 L.h−1 vs. 0.56–0.57 L.h−1 for both Tunisian and Galician mussels, respectively; see interaction term treatment × origin; Fig. 1A)
Regarding the behavioural and eco-physiological parameters considered, an interesting range of responses was observed in the mussel M. galloprovincialis, originally from very different, widely separated and heterogeneous environments (Tunisia, in the Mediterranean Sea and NW Spain in the Atlantic Ocean)
Summary
One-third of the total anthropogenic CO2 emissions have been absorbed by the ocean (Sabine et al, 2004), in turn triggering a process called ocean acidification (OA) (Orr et al, 2005; Gattuso et al, 2015). Anthropogenic activities have almost doubled the pre-industrial concentrations of atmospheric CO2 (280 μatm pCO2) to around the actual 408 μatm pCO2 (https://www.noaa.gov/). The rise in pCO2 that has caused OA is measured as a decline in seawater pH, accompanied by a decrease in both the carbonate ion (CO32−) concentration and the saturation states (Ω) of various calcium carbonates forms (Zeebe and Westbroek, 2003). Contemporary surface ocean pH has decreased by 0.1 units since pre-industrial time (Raven et al, 2005). The pCO2 is expected to rise above 1100 μatm by 2100, which would lead to a decrease in seawater pH of approximately 0.3– 0.4 units (IPCC, 2014). The IPCC (2014) has highlighted the fact that the upper 75 m of oceans has warmed at an average rate of ∼0.1◦C per decade over the past 40 years. Modelling studies have predicted that the mean global ocean temperature may increase within 1–4◦C by 2100 (Doney et al, 2012), trends may differ regionally, seasonally and inter-annually
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