Abstract
Abstract. The calcareous tubeworm Spirorbis spirorbis is a widespread serpulid species in the Baltic Sea, where it commonly grows as an epibiont on brown macroalgae (genus Fucus). It lives within a Mg-calcite shell and could be affected by ocean acidification and temperature rise induced by the predicted future atmospheric CO2 increase. However, Spirorbis tubes grow in a chemically modified boundary layer around the algae, which may mitigate acidification. In order to investigate how increasing temperature and rising pCO2 may influence S. spirorbis shell growth we carried out four seasonal experiments in the Kiel Outdoor Benthocosms at elevated pCO2 and temperature conditions. Compared to laboratory batch culture experiments the benthocosm approach provides a better representation of natural conditions for physical and biological ecosystem parameters, including seasonal variations. We find that growth rates of S. spirorbis are significantly controlled by ontogenetic and seasonal effects. The length of the newly grown tube is inversely related to the initial diameter of the shell. Our study showed no significant difference of the growth rates between ambient atmospheric and elevated (1100 ppm) pCO2 conditions. No influence of daily average CaCO3 saturation state on the growth rates of S. spirorbis was observed. We found, however, net growth of the shells even in temporarily undersaturated bulk solutions, under conditions that concurrently favoured selective shell surface dissolution. The results suggest an overall resistance of S. spirorbis growth to acidification levels predicted for the year 2100 in the Baltic Sea. In contrast, S. spirorbis did not survive at mean seasonal temperatures exceeding 24 °C during the summer experiments. In the autumn experiments at ambient pCO2, the growth rates of juvenile S. spirorbis were higher under elevated temperature conditions. The results reveal that S. spirorbis may prefer moderately warmer conditions during their early life stages but will suffer from an excessive temperature increase and from increasing shell corrosion as a consequence of progressing ocean acidification.
Highlights
Atmospheric carbon dioxide (CO2) is a primary substrate for life on Earth but is a major driver of global-scale environmental change, causing ocean acidification (Greene et al, 2012), controlling climate variability (Retallack, 2002; Galeotti et al, 2016) and initiating mass extinctions (Jaraula et al, 2013; Veron et al, 2009)
At the end of the summer and autumn experiments (September and December respectively) we found numerous unstained living S. spirorbis specimens on the Fucus thalli which had shell diameters < 1.3 mm
Elevated pCO2 levels, lowered pH and calcium carbonate saturation states expected for the end of the 21st century had no significant impact on tube extension rates
Summary
Atmospheric carbon dioxide (CO2) is a primary substrate for life on Earth but is a major driver of global-scale environmental change, causing ocean acidification (Greene et al, 2012), controlling climate variability (Retallack, 2002; Galeotti et al, 2016) and initiating mass extinctions (Jaraula et al, 2013; Veron et al, 2009). At the same time surface seawater pH decreased. S. Ni et al.: Impact of pCO2 and warming on S. spirorbis growth by 0.1 units, corresponding to 30 % increase in the hydrogen ion concentration (Raven et al, 2005; Cao and Caldeira, 2008). It is predicted to further decrease by 0.3 to 0.4 pH units until the year 2100 when atmospheric pCO2 levels may reach 950 μatm (IPCC, 2013). By the end of this century, the average surface ocean pH could be lower than it has been for more than 50 Myr (Caldeira and Wickett, 2003) with severe consequences for marine calcifying organisms (Orr et al, 2005; Andersson et al, 2008; Erez et al, 2011)
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