全球变化知识资源中心(globalchange): Shift from coral to macroalgae dominance on a volcanically acidified reef

Coral reefs contain the highest concentration of biodiversity in the marine realm, with abundant flora and fauna that form the backbone of complex and dynamic ecosystems⁶. From an anthropocentric standpoint, coral reefs provide valuable goods and services, supporting fisheries and tourism, and protect shorelines from storms⁷. Recently, widespread coral mortality has led to the flattening of reef frameworks and the loss of essential habitat⁴. This trend will be accelerated by ocean acidification (OA), as calcification is impaired, and dissolution is accelerated^{8, 9}. Furthermore, experimental evidence suggests that OA could enhance the growth¹⁰ and competitive ability of fleshy macroalgae¹¹. This OA-induced shift in the competitive balance between corals and algae could exacerbate direct effects of OA on calcifying reef species¹² and lead to ecosystem shifts favouring non-reef-forming algae over coral^{4, 5}. Understanding the individual responses of taxa to OA, as well as alteration of multi-species assemblages, is therefore critical to predicting ecosystem persistence and managing reef health in an era of global change.

At present, much of what is known concerning the impacts of OA on coral reef biota has been laboratory-based experimental work focused on the responses of select taxa². This has been expanded to mesocosm-based studies, allowing manipulation of groups of organisms and investigation of community responses¹³. Although these multi-species experimental studies are vital, they cannot recreate the variability (physical, chemical, biological) of real-world reef systems¹⁴. In an effort to overcome the limitations of laboratory studies, real-world low-saturation-state (Ω) sites have been investigated. In the eastern Pacific, nutrient and CO₂-enriched upwelled waters impact coral calcification and the precipitation of carbonate cements, influencing the distribution of reefs¹⁵. In Mexico, freshwater springs depress Ω, influencing coral calcification and species distributions¹⁶. In Palau, restricted circulation and biological activity contribute to elevated p_CO₂, with little impact on reef communities¹⁷. These locations provide insight into community-scale responses to OA; however, variation in other environmental parameters can complicate conclusions.

Volcanic enrichment of CO₂ from submarine vents has been shown to impact the structure of temperate and sub-tropical ecosystems, including seagrasses¹⁸, rocky-shore and rocky-reef communities^{19, 20}, soft sediments²¹ and vermetid reefs²². The occurrence of CO₂ vents near coral reef ecosystems is rare and, at present, only two regions have been studied: Papua New Guinea (PNG; ref. 23) and a sub-tropical system at Iwotorishima Island, Japan²⁴. High-p_CO₂ vent communities in Japan, comparable to conditions projected for the end of the century (pH ≈ 7.8) are dominated by soft corals, whereas nearby control sites (pH ≈ 8.1) are dominated by hard scleractinian corals²⁴. In PNG, coral cover is not significantly different at a pH of 7.8, although species composition changes and diversity is reduced²³. Given the different ecosystem responses observed at the previously described sites and the paucity of temporally and spatially explicit data sets, further work is necessary to examine the multifarious influences of OA on coral reefs.

Here we identify and characterize a CO₂ vent impacting a tropical coral reef ecosystem in the Commonwealth of the Northern Mariana Islands (CNMI; Fig. 1). We use high-accuracy instrumentation to characterize the carbonate chemistry of this system and investigate its relationship with reef community composition (Methods).

Figure 1: Map showing the location of the study site at Maug.


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