Global Change Biology (2016) 22, 1225–1234, doi: 10.1111/gcb.13023

Framework of barrier reefs threatened by ocean acidification S T E E V E C O M E A U 1 , 2 , C O U L S O N A . L A N T Z 1 , 2 , P E T E R J . E D M U N D S 1 , 2 and R O B E R T C . CARPENTER1,2 1 Department of Biology, California State University, 18111 Nordhoff Street, Northridge, CA 91330-8303, USA, 2ARC Centre of Excellence in Coral Reef Studies, The University of Western Australia, School of Earth & Environment & Ocean’s Institute, Western Australia 6009, Australia

Abstract To date, studies of ocean acidification (OA) on coral reefs have focused on organisms rather than communities, and the few community effects that have been addressed have focused on shallow back reef habitats. The effects of OA on outer barrier reefs, which are the most striking of coral reef habitats and are functionally and physically different from back reefs, are unknown. Using 5-m long outdoor flumes to create treatment conditions, we constructed coral reef communities comprised of calcified algae, corals, and reef pavement that were assembled to match the community structure at 17 m depth on the outer barrier reef of Moorea, French Polynesia. Communities were maintained under ambient and 1200 latm pCO2 for 7 weeks, and net calcification rates were measured at different flow speeds. Community net calcification was significantly affected by OA, especially at night when net calcification was depressed ~78% compared to ambient pCO2. Flow speed (2–14 cm s 1) enhanced net calcification only at night under elevated pCO2. Reef pavement also was affected by OA, with dissolution ~86% higher under elevated pCO2 compared to ambient pCO2. These results suggest that net accretion of outer barrier reef communities will decline under OA conditions predicted within the next 100 years, largely because of increased dissolution of reef pavement. Such extensive dissolution poses a threat to the carbonate foundation of barrier reef communities. Keywords: calcification, coralline algae, corals, flow, fore reef, ocean acidification, reef pavement Received 31 January 2015 and accepted 29 May 2015

Introduction Ocean acidification (OA), which is the direct consequence of human CO2 emissions, is threatening marine organisms and ecosystems and particularly those producing structures made of calcium carbonate (Gattuso & Hansson, 2011). A large number of studies on the effects of OA have focused on coral reefs, which are the largest calcium carbonate structures in coastal environments that are dependent on the capacity for rapid deposition of CaCO3 (Milliman, 1993). Most studies have been conducted at the organismal scale (Erez et al., 2011), and a few have addressed the larger biological scale of communities (Jokiel et al., 2008; Andersson et al., 2009; Dove et al., 2013; Comeau et al., 2015). As part of the effort to evaluate the community-level response of coral reefs to OA, it is important to evaluate the type of community being considered and the habitat in which it might be found (i.e., fringing reef, back reef, or outer barrier reef). Given the strong differences among coral species in their responses to OA (Chan & Correspondence: Steeve Comeau, tel. +61 8 6488 3644, e-mail: [email protected]

© 2015 John Wiley & Sons Ltd

Connolly, 2013; Comeau et al., 2013a, 2014a) and the effects of physical divers in modulating these responses (e.g., Dufault et al., 2013; Suggett et al., 2013; Comeau et al., 2014b), it is likely that the response of coral reef communities to OA will differ among communities with varying species assemblages and among habitats characterized by different suites of physical conditions. Although back reef habitats represent an important part of most coral reefs, the outer barrier reef habitat is characterized by the highest biological diversity, and the most rapid rates of coral calcification that contribute to the foundational role of outer reef habitats in reef construction (e.g., Vecsei, 2001), yet there is virtually no empirical information on the ways in which benthic communities (or the component organisms) in this habitat will respond to OA. Critically, this degrades our ability to predict how outer barrier reefs – best known from eastern Australia, Mesoamerica, and numerous smaller barrier reefs surrounding islands such as Moorea and Tahiti in the south Pacific – will respond to OA. This information gap is associated with the logistical challenges of working in a reef habitat characterized by deep water and challenging hydrodynamic conditions where classical methodological approaches for measuring rates of calcification (e.g., the alkalinity 1225

1226 S . C O M E A U et al. anomaly method) cannot readily be applied (Bosscher & Schlager, 1992; Vecsei, 2001, 2004). Outer barrier reefs are characterized by large variations in the light environment (quantity and quality) and flow conditions as a function of depth (Sebens & Johnson, 1991), both of which potentially can affect the response of coral reef organisms to ocean acidification (e.g., Dufault et al., 2013; Suggett et al., 2013; Comeau et al., 2014b). Light intensity was shown to modulate the response to OA of both scleractinian corals (e.g., Dufault et al., 2013; Suggett et al., 2013; Comeau et al., 2014b) and coralline algae (e.g., Martin et al., 2013; Comeau et al., 2014b), likely by stimulating photosynthesis (e.g., Comeau et al., 2013b). Flow speed and flow direction in fore reef habitats varies greatly among depths (Sebens & Johnson, 1991) and is a primary physical force modulating the success of benthic taxa such as corals and algae (e.g., Patterson et al., 1991; Williams & Carpenter, 1998; Finelli et al., 2006). Increased flow speeds also have been suggested to attenuate the negative effects of OA on the calcification of back reef organisms and communities (Anthony et al., 2013; Comeau et al., 2014c). On outer barrier reef communities, hard substrata typically consists mostly of live coral and reef pavement (i.e., cemented calcium carbonate substratum covered by an assemblage of turf and coralline algae), with pavement typically accounting for a large proportion of the benthos following strong physical disturbances such as storms that dislodge corals and scour the substratum. Such disturbances are predicted to increase in intensity and become more common as global climate continues to change (Knutson et al., 2010), and thus, it is likely that the abundance of reef pavement on coral reefs will increase as coral reef communities are increasingly disturbed (Karlson & Hurd, 1993; Kayal et al., 2012). These trends have important implications in addition to depleting the size of scleractinian populations, because reef pavement is susceptible to dissolution and erosion (Tribollet et al., 2009; Andersson & Gledhill, 2013). However, despite the strong potential for the relative and absolute cover of reef pavement to increase on outer barrier reef communities in the next few decades, there has been no quantification of these effects in previous studies of how coral reef communities will respond to climate change and ocean acidification. A few studies have evaluated dissolution of tropical benthic substrata under OA conditions, and of these, most have focused on sediments (Andersson et al., 2007; Andersson & Gledhill, 2013; Cyronak et al., 2013) rather than coral communities or reef pavement. In the case of reef pavement, there are reasons to expect the response to OA to be large and negative, because laboratory experiments have shown that the bioerosion and disso-

lution of dead carbonate skeletons are enhanced by OA (Tribollet et al., 2009; Silbiger & Donahue, 2014). This study investigated the response of an outer barrier reef benthic community from the tropical South Pacific Ocean to OA. We focused on outer reef communities in Moorea, French Polynesia, where most benthic substrata to at least 17 m depth is hard and forms a massive barrier around the island that is separated from the back reef by an emergent reef crest. In addition to the aforementioned knowledge gap regarding the effects of OA on fore reef communities, we chose to study this habitat in Moorea because its biological features (e.g., percentage cover of coral, macroalgae, and reef pavement) and physical conditions (e.g., light, temperature, and flow speed) are extensively described through time series analyses of the Moorea Coral Reef Long Term Ecological Research program (http:// We quantified the response of the outer barrier reef community to OA under different flow conditions representing those naturally encountered on the reefs of Moorea (Washburn L of Moorea Coral Reef LTER 2014; Fig. S1), as previously we have shown that flow speed (2–10 cm s 1) affects the response of back reef communities to OA (Comeau et al., 2014c). Coral reef communities composed of scleractinian corals, coralline algae, and reef pavement were constructed in four outdoor flumes (described in Comeau et al., 2015) with percentage cover for each group matched to that recorded in 2006 on the outer barrier reef of Moorea at 17 m depth (Edmunds PJ of Moorea Coral Reef LTER, 2014). Experimental communities were maintained in the flumes for 7 weeks under ambient (400 latm) and elevated pCO2 (1200 latm). To estimate the contribution of each component of the community to the calcification budget, calcification rates were determined for the entire community, and then separately for reef pavement, corals, and coralline algae.

Materials and methods

Collection and sample preparation This experiment was carried out from August 24 to October 11, 2014, in Moorea, using organisms collected from the outer barrier reef of the north shore at ~15–17 m depth. Coral reef communities were constructed in replicate flumes, and the community composition adjusted to match the planar cover of the fore reef of Moorea in 2006 (Carpenter, 2014; Edmunds, 2014). We assembled communities replicating the outer barrier reef in 2006 because the present-day community remains affected by an outbreak of the corallivorous seastar Acanthaster planci (crown of thorns), which was at peak abundances from 2007 to 2009 in Moorea and was followed by the damaging effects of Cyclone Oli in 2010. Together, these disturbances

© 2015 John Wiley & Sons Ltd, Global Change Biology, 22, 1225–1234

B A R R I E R R E E F V U L N E R A B I L I T Y T O P C O 2 1227 reduced coral cover on the outer reef of the north shore to ~5% in 2010 (Kayal et al., 2012; Adam et al., 2014), but despite this decline in coral cover, the current population trends suggest that scleractinian corals will cover a similar percentage of the benthos to that occurring in 2006 within the next few years (as has occurred with previous disturbances [Adjeroud et al., 2009]). As our objective was to evaluate the effects of OA on a benthic community representative of outer reef habitats, we reasoned that the community found in 2006 was more representative of the decadal-scale average benthic cover than that present in 2014. In 2006 (as well as presently), macroalgae were not abundant in this community (

Framework of barrier reefs threatened by ocean acidification.

To date, studies of ocean acidification (OA) on coral reefs have focused on organisms rather than communities, and the few community effects that have...
479KB Sizes 5 Downloads 9 Views