In situ changes of tropical crustose coralline algae along carbon dioxide gradients

Abstract

Crustose coralline algae (CCA) fulfill important ecosystem functions in coral reefs, including reef framework stabilization and induction of larval settlement. To investigate in situ the effects of high carbon dioxide on CCA communities, we deployed settlement tiles at three tropical volcanic CO2 seeps in Papua New Guinea along gradients spanning from 8.1 to 7.4 pH. After 5 and 13 months deployment, there was a steep transition from CCA presence to absence around pH 7.8 (660 μatm pCO2): 98% of tiles had CCA at pH > 7.8, whereas only 20% of tiles had CCA at pH ≤ 7.8. As pH declined from 8.0 to 7.8, the least and most sensitive CCA species lost 43% and 85% of cover, respectively. Communities on upward facing surfaces exposed to high light and high grazing pressure showed less steep losses than those on shaded surfaces with low grazing. Direct CO2 effects on early life stages were the main mechanisms determining CCA cover, rather than competitive interactions with other benthic groups. Importantly, declines were steepest at near-ambient pH, suggesting that CCA may have already declined in abundance due to the recent seawater pH decline of 0.1 units, and that future severe losses are likely with increasing ocean acidification.

Enhanced macroboring and depressed calcification drive net dissolution at high-CO 2 coral reefs

“…One of the dominant encrusting taxa, CCA, is well known to be negatively influenced by OA [5,38], and several researchers have documented depressed coverage of CCA using settlement tiles deployed along natural CO 2 gradients [8,39,48]. In the light of these studies, the lack of a significant relationship between accretion and Upa-Upasina may, at first glance, appear incongruous.…”

“…3 m depth) using a single bolt and stable base pinned into bare coral rock (figure 1). Seawater samples were collected at each BAR site during four, two-week trips over the duration of the BARs' deployment ( n = 5–23 per BAR location, [8]), and pH (total scale) was measured using a high-accuracy glass electrode (InLab Expert Pro pH electrode, SG78 pH/temperature meter, Metler Toledo). Additional seawater samples were taken less frequently ( n = 2–18 per site, ) for analysis of total alkalinity ( A T , 855 Titrosampler, Metrohm) and dissolved inorganic carbon (DIC, Vindta 3C, Marianda), which were used to solve the carbonate system (Seacarb v. 2.4.8, ) and to confirm that pH was successfully describing the vent gradient.…”

Reduced heterotrophy in the stony coral Galaxea fascicularis after life-long exposure to elevated carbon dioxide

“…A spatial map of the seawater carbonate chemistry, along with a detailed description of the Upa-Upasina high CO 2 and control site can be found in Fabricius et al 5556. G. fascicularis colonies were collected near the seep site where seawater approximates 7.8 pH T (total scale), and from a control site with control CO 2 at ~8.1 pH T .…”

The Future of Coral Reefs Subject to Rapid Climate Change: Lessons from Natural Extreme Environments

“…A common feature across the various vent sites are changes in community structure, where only a few acidification-tolerant species thrive under high pCO 2 (e.g., Hall-Spencer et al, 2008;Fabricius et al, 2011Fabricius et al, , 2015Kroeker et al, 2011Kroeker et al, , 2013. There is also evidence that the structural integrity of any reef framework is compromised due to a loss of architectural complexity (Fabricius et al, 2011), increased rates of bioerosion (Enochs et al, 2016a,b), reductions in coral growth (Fabricius et al, 2011) and reduced skeletal density (Fantazzini et al, 2015;Strahl et al, 2015b).…”