Effects of ocean acidification and global warming on reef bioerosion—lessons from a clionaid sponge

Abstract

Coral reefs are under threat, exerted by a number of interacting effects inherent to the present climate change, including ocean acidification and global warming. Bioerosion drives reef degradation by recycling carbonate skeletal material and is an important but understudied factor in this context. Twelve different combinations of p CO 2 and temperature were applied to elucidate the consequences of ocean acidification and global warming on the physiological response and bioerosion rates of the zooxanthellate sponge Cliona orientalis -one of the most abundant and effective bioeroders on the Great Barrier Reef, Australia. Our results confirm a significant amplification of the sponges' bioerosion capacity with increasing p CO 2 , which is expressed by more carbonate being chemically dissolved by etching. The health of the sponges and their photosymbionts was not affected by changes in p CO 2 , in contrast to temperature, which had significant negative impacts at higher levels. However, we could not conclusively explain the relationship between temperature and bioerosion rates, which were slightly reduced at both colder as well as warmer temperatures than ambient. The present findings on the effects of ocean acidification on chemical bioerosion, however, will have significant implications for predicting future reef carbonate budgets, as sponges often contribute the lion's share of internal bioerosion on coral reefs.

Combined Effects of Experimental Acidification and Eutrophication on Reef Sponge Bioerosion Rates

“…Results regarding higher rates at increased pCO 2 are comparable to previous studies on other clionaid species (C. orientalis and C. celata) (Wisshak et al, 2012(Wisshak et al, , 2013(Wisshak et al, , 2014Fang et al, 2013a). Sponges and other borers are assumed to benefit from eutrophication (Holmes, 2000;Carballo et al, 2008).…”

“…Therefore, the sum of chemical and mechanical bioerosion should be considered as being conservative. Here, total bioerosion rates yielded from buoyant weight measurements are regarded as more reliable and are comparable to results estimated from previous studies (Fang et al, 2013a;Wisshak et al, 2013). Rates of chip production did not differ significantly between pCO 2 scenarios, day/night and eutrophication treatments.…”

“…Chemical bioerosion was determined using the alkalinity anomaly technique (Smith and Key, 1975;Chisholm and Gattuso, 1991) involving measured changes in A T (µmol kg −1 ) associated with dissolution in seawater during 6 h incubation periods and correcting for changes in the concentrations of ammonium, nitrate and phosphate (Jacques and Pilson, 1980;Wisshak et al, 2013). The amount of mass of dissolved calcium carbonate [∆ M (CaCO3) , in µg] was calculated using Equation (1) (Zundelevich et al, 2007;Nava and Carballo, 2008):…”

“…Schönberg (2008) provided a first glimpse of a pH gradient toward the etching sites using micro-sensors which may indicate that sponges do indeed alter the chemistry at the site of bioerosion. Although the underlying physiological and mechanical processes employed by the sponges to erode are currently unknown, a number of studies have shown that an increase in pCO 2 of the ambient water results in increased bioerosion rates (Wisshak et al, 2012(Wisshak et al, , 2013Duckworth and Peterson, 2013;Fang et al, 2013a), suggesting that changes in seawater chemistry directly affect the saturation state at the site where the coral aragonite is dissolved.…”

Sponge bioerosion accelerated by ocean acidification across species and latitudes?

“…Rützler and Rieger 1973;Zundelevich et al 2007;Nava and Carballo 2008;see Wisshak et al 2013 for an overview), we proposed that it is mainly the chemical dissolution which is sensitive to ambient pH and will thus be accelerated in a more acidic environment . The rate of mechanical extraction of sponge chips, however, remains relatively constant in case of C. orientalis (figure 7 in Wisshak et al 2013). This line of reasoning was supported most recently by Fang et al (2013) who found a significant increase in chemical, but stable mechanical sponge bioerosion rates under a series of combined acidification and warming scenarios.…”