Abstract:In view of the current global warming trends, China has a "double carbon" goal, which reflects China′s initiative in assuming the responsibility of dealing with global climate change. Oceans play an important role in achieving carbon neutrality. Oyster reefs are typical coastal ecosystems that contain huge carbon reserves and strong carbon sequestration ability. Due to overfishing, coastal engineering construction, environmental pollution, and climate change，the global oyster reef is in a seriously degraded state and urgently needs to be restored and protected. Oyster reefs released CO2 to the atmosphere in the processes of calcification and respiration and also bury large volumes of carbon during biological and physical deposition, which makes it uncertain whether oyster reefs are a source or sink of CO2. To explore the carbon source and sink function of oyster reefs, we summarized the research status of the carbon source-sink functions by oyster reefs, analyzed the key ecological processes affecting it, and discussed the characteristics of oyster reef carbon source-sink functions in different conditions. We aim to provide opinions and suggestions for research, restoration, and protection of oyster reefs. Until recently, few studies have reported the carbon source-sink functions of oyster reefs. A study of an oyster reef in Rachel Carson Reserve of North Carolina found oyster reefs have different carbon source-sink characteristics under different environmental conditions. The reefs on intertidal sandflats were net sources of CO2 [(710±120) g C/(m2·a)], whereas shallow subtidal reefs [(–100±40) g C/(m2·a)] and saltmarsh-fringing reefs [(–130±40) g C/(m2·a)] were net carbon sinks. The concentration of seston, water temperature, depth, hydrodynamic regime, oyster density, individual size, age, reef size and structure, and sediment are important factors affecting the carbon source-sink function of an oyster reef. The oyster calcification, biological deposition, biosynthesis, and respiration processes, sediment resuspension and decomposition processes, and the physical sedimentation of oyster reefs are the key ecological processes affecting the carbon source-sink function of an oyster reef. In the process of calcification, oysters absorb bicarbonate to form calcium carbonate shells and release CO2 to the atmosphere. Whether this process is the sink or source of atmospheric CO2 is controversial. Biological deposition by oysters can transport large volumes of organic carbon to the oyster reef sediment, the organic carbon accumulation rate can reach 30~270 g C/(m2·a), which is equivalent to the carbon sink rate of blue carbon ecosystems. Meanwhile, juvenile oysters have higher biological deposition rates than older oysters. The physical sedimentation in oyster reefs is also an important process of carbon deposition, the complex physical structure of an oyster reef can slow water flow, attenuate wave energy, and facilitate the deposition of particulate organic carbon. The influence of physical sedimentation by oyster reefs reaches far beyond the boundary of the oyster reef, the area with a carbon accumulation rate higher than 100 g C/(m2·a) surrounding the reef can be over twice the size of the oyster reef. Water velocity is a key factor affecting the resuspension of oyster reef sediments. A study of an oyster reef in an estuarine intertidal zone found that most uptake of particulate material by the oyster reef took place at velocities below 15 cm/s, and the release of particulates mainly occurred at velocities above 15 cm/s. It is more conducive to achieving long-term burial of sedimentary carbon in oyster reefs with low water velocities. The highest CO2 emissions from oyster reefs may come from the oysters themselves. Therefore, oyster respiration should be one of the main sources of CO2 from an oyster reef ecosystem. An evaluation of oyster reef carbon source-sink function needs to comprehensively consider multiple and complex biological processes. The oyster reef carbon sink functions do not only include the ability of the oyster reef habitat to bury carbon, but also their ability to improve the primary and secondary productivity of other organisms. Oyster reefs can promote the growth of macro-algae or salt marsh plants in the reef area by improving water transparency, stabilizing water flow, weakening wave erosion, and accelerating the biogeochemical cycle. Oyster reefs can also improve the productivity of fish, crustaceans, cephalopods, shellfish, and other organisms in the oyster reef ecosystem. In general, oyster reefs and macroalgae, salt marsh plants, and marine animals can jointly improve the carbon sink capacity of a coastal ecosystem. If the oyster reef is not damaged, the organic carbon buried by the reef can be preserved for a long time. The serious destruction of oyster reefs by human activities causes large volumes of organic carbon buried in oyster reefs to be released, which easily decomposes and returns to the atmosphere. The estimated global carbon emission caused by the destruction of shellfish reefs is approximately 400 million Mg, which destroys the carbon sink and storage functions of oyster reefs. Therefore, protecting the existing oyster reefs and preventing them from being damaged is important for reducing global atmospheric carbon emissions. At present, the formation of oyster reef carbon sinks have not been completely clarified, and a unified evaluation method of oyster reef carbon sink functions have not been established. There is no clear conclusion whether the global oyster reef is a sink or source of CO2. To clarify the carbon sink function of oyster reefs, we suggest research of oyster reef carbon sinks should be given priority in the future, including: 1. the effects of oyster calcification on carbon exchange between seawater and the atmosphere at different time scales; 2. the dynamic carbon budget of oysters in oyster reefs; 3. the carbon metabolism beneath the taphonomically active zone of oyster reefs; 4. the synergistic carbon sequestration effects between the oyster reef and macroalgae, salt marsh, phytoplankton, and marine animals; 5. the effects of global climate change on the carbon cycle of oyster reefs; and 6. the construction of carbon sink evaluation technology for assessing oyster reefs. These researches will determine the formation mechanisms of oyster reef carbon sinks, build oyster reef carbon sink evaluation technology, and establish oyster reef protection and restoration technology to improve the carbon sink capacity of oyster reefs.
