Mechanisms of sediment carbon sequestration in seagrass meadows and its responses to eutrophication

Abstract

Seagrass meadows are recognized as important and productive coastal ecosystems. They are globally-significant hotspots for sediment organic carbon (SOC) sequestration, storing about 4.2–8.4 Pg, and could up to 19.9 Pg SOC—an amount equivalent to 10-times that stored in the earth’s terrestrial soils. Sediment organic carbon (SOC) storage stocks in seagrasss meadows are affected by SOC sources, composition and transformations. Unfortunately, seagrass meadows are declining globally at a rate of 5% per year mainly due to eutrophication, which could influence the supply (source), composition, and transformation of seagrass SOC. We systematically reviewed the literature of seagrass SOC sources, composition, transformation, and storage, as well as summarized their response to eutrophication. Seagrasses develop organic-rich sediment composed of both autochthonous and allochthonous organic carbon. Seagrass, epiphyte, macroalgae, and suspended particulate organic matter (SPOM) trapped from the water column can be common sources of sediment organic carbon in seagrass meadows. The main sources of SOC, however, are retained within seagrass and SPOM. According to a worldwide database of δ 13C, the average contribution of seagrass to SOC is approximately 50%. The global average SOC content is 1.5%, with higher SOC concentrations have always been observed in the sediment of seagrass meadows compared to bare sediments. Additionally, there are significantly different of SOC contents between temperate regions and tropics. The SOC in seagrass meadows are composed of both labile organic carbon and recalcitrant organic carbon. Dissolved organic carbon and microbial biomass carbon are vital fractions of labile organic carbon, which are often served as an important indicator of change and future trends in SOC. Recalcitrant organic carbon is largely composed of belowground seagrass detritus. Sediment microorganisms determine the balance between SOC storage and remineralization processes within seagrass meadows, and high microbial activity and SOC transformation efficiencies are often observed in seagrass meadows. This is mainly due to excretion of amino acids, easily degradable sugars and oxygen, and suitable niches creation by the seagrass rhizosphere. However, the SOC transformation efficiencies decrease with increasing the sediment depth, which result in the high SOC sequestration potential. The main reasons for high SOC sequestration capacity of seagrasses are due to: high primary productivity, strong SPOM trapping capacity from the water column, and low decomposition rates. Although the sediment within seagrass meadows have high carbon sequestration capacity, carbon sequestration by seagrasses is spatially variable. The average SOC stock beneath seagrass meadows in the regions of East Asia, Southeast Asia and Australia are only quarter of the global average values. Eutrophication in coastal areas can trigger the overgrowth of algae, most commonly in the form of epiphytes and macroalgae within seagrass meadows. This cause algal material, which is more labile, to dominate SOC sources in seagrass meaodows, consequently elevating microbial activities/biomass and accelerating SOC breakdown. Meanwhile, the nutrient-induced loss of seagrass biomass can decrease the capacity of organic carbon trapped from seawater. Therefore, changes of SOC sources, composition and transformation induced by nutrient loading can ultimately weaken SOC storage potential. As is often the case, there are exceptions in nature, and some studies have found no impact of nutrient loading on seagrass SOC content. Possibly there is a non-linear and hysteretic response of seagrass SOC to eutrophication, such that nutrient thresholds must be reached before responses can be detected. For future research, we recommend: (1) conducting studies of SOC storage stock at global scales; (2) strengthening the research of the effects of environment changes on storage mechanisms; (3) developing carbon sink amplification technologies; and (4) exploiting new methods for recalcitrant organic carbon measurements. The research of carbon sequestration in seagrass meadows should be strengthened in China, which could provide scientific perspectives for climate negotiations and CO2 trading with other countries in the future.