Seagrass beds are considered to be substantial sinks of “blue carbon”. However, differentiation in the carbon sink capacities of seagrass beds in different regions with distinct nutrient conditions remains unclear. In this study, sediment carbon stocks, seagrass biomass, and microbial community structures and potential functions of seagrass beds in eutrophic seawater adjacent to Hainan Island and oligotrophic seawater around the Xisha Islands were compared. Our results showed that sediment mineralizable organic carbon and dry bulk density were substantially higher on Hainan Island than on the Xisha Islands (t-test, p < 0.05), while sediment carbon stocks and the total organic carbon were comparable between the two regions (p > 0.05). Similarly, seagrass biomass was much higher on Hainan Island (p < 0.05). Sediment carbon stocks positively correlated with sediment nitrogen and negatively correlated to sediment pH and grain size (p < 0.05). Bacterial diversities were similar in the two regions, while fungi were more diverse on Hainan Island (p < 0.05). Proteobacteria, Desulfobacterota, Ascomycota and Basidiomycota could account for degrading organic carbon on Hainan Island. Proteobacteria and Bacteroidota may contribute primarily to carbon loss in the seagrass beds of the Xisha Islands. This study strengthens our understanding of the effects of human activities on carbon sequestration in seagrass bed ecosystems.
This study quantified the absorption ability of the seagrass Zostera japonica and the macroalgae Ulva pertusa for dissolved inorganic nitrogen (DIN) (ammonium and nitrate) and dissolved organic nitrogen (DON) (urea and glycine) under different light conditions. The plants were cultured in filtered seawater (31‰, 25°C) for 2 weeks under three light levels. Macroalgae and the above- and belowground parts of seagrasses were separately placed into four different manmade seawater solutions with DIN (ammonium and nitrate) and DON (urea and glycine) stable isotopes for 1 h. The results showed that macroalgae had higher absorption rates for ammonium and nitrate after higher light (14.67 ± 2.50 and 1.29 ± 0.16 mg−1 dry weight (DW) h−1) than after lower light (4.52 ± 0.95 and 0.18 ± 0.12 mg−1 DW h−1) treatments. Compared to the belowground seagrass portions that had previously been grown in high and low light conditions, the seagrass leaves assimilated ammonium more quickly. Z. japonica preferred glycine to nitrate and urea after the high- and low-light treatments; that is, the absorption rates of the belowground seagrass parts for glycine were 14.71 ± 1.85 and 6.38 ± 0.52 mg−1 DW h−1 after the high- and low-light treatments, respectively, which were higher than those of ammonium, nitrate, and urea. The absorption rates of algae were lower than those for ammonium previously grown under medium- and low-light treatments. These results indicate that light reduction can impact the assimilation of DIN by Z. japonica and U. pertusa, and both have the ability to directly assimilate DON. This study provides information that could help reduce the negative effects of eutrophication on macroalgae and seagrasses in order to protect seagrass meadows.
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