Морской биологический журнал (May 2024)
Assessment of carbon stock in the Zostera marina Linnaeus, 1753 ecosystem on sandy sediments of the Srednyaya Bight (Peter the Great Bay, the Sea of Japan) based on field observations
Abstract
Coastal seagrass ecosystems, particularly Zostera marina Linnaeus, 1753 ones, are capable of accumulating organic carbon by fixing carbon dioxide via photosynthesis. Seagrass biomass is considered as a short-term carbon storage, and underlying bottom sediments, as a long-term one. The research on organic matter accumulation by seagrass ecosystems is mostly carried out in areas with stable sedimentation. For such ecosystems, the importance of seagrass areas within the concept of blue carbon was shown. However, for the seas of temperate latitudes, coastal waters with unstable sedimentation and prevalence of sandy sediments are common, and the scale of carbon storage in seagrass ecosystems is not obvious. In this work, biomass and carbon stock in Z. marina leaves and roots, as well as Corg concentration and carbon stock in the upper layers of bottom sediments (0.25-m and 1-m thick), were determined for typical habitats in the semi-open Srednyaya Bight (Peter the Great Bay, the Sea of Japan), where sandy sediments prevail. Z. marina roots were characterized by 3–20 times lower biomass than its leaves. This difference increased from April to July in accordance with seasonality. Carbon concentrations in the seagrass leaves and roots were similar (33.3 and 31.3% dry weight, respectively). In the habitats with a projective coverage of 50–80%, carbon stock in Z. marina tissues was (96.8 ± 37.4) g C·m−2; with 100% coverage, the value increased to 253 g C·m−2. Corg concentration in bottom sediments of the Srednyaya Bight ranged within 0.04–0.46% and correlated with content of silt fractions. Under dense Z. marina coverage, Corg content and the fraction of silt particles in sediments were higher than under sparse ones. The vertical distribution of Corg concentration within the upper 15–35-cm layer did not reveal a downward trend in the cores. The main factor controlling Corg content was the particle-size distribution of sediments, which suggests a weak expression of reduction diagenesis and the effect of wave mixing of the upper layer of sandy sediments. Data on the bulk density and Corg concentration in sediments allowed to calculate carbon stock for the layers of 0.25 and 1 m. The quota of organic carbon in the seagrass tissues did not exceed a third of its amount in the upper layer (0.25 m) of underlying sandy sediments. When extrapolated to a 1-m thick layer, the quota of bottom sediments to Corg pool exceeds 90%. Organic carbon enrichment of sandy sediments under the seagrass beds compared to sands of similar particle size beyond the seagrass beds indicates a significant role of Z. marina in carbon storage, even in the habitats with the lack of stable and intensive sedimentation. The major factor controlling carbon stock in Z. marina ecosystems is Corg content in underlying bottom sediments which depends primarily on their particle-size distribution. In this case, the range of variation in carbon stock in the upper layer is an order of magnitude or more. Maps of the seagrass distribution in April and July 2021 were built. The absolute values of carbon stock were calculated, both accumulated in Z. marina biomass and deposited in the seagrass-covered sediments. The area of potential Z. marina distribution in the Srednyaya Bight was modelled using the MaxEnt 3.4.4 program. According to the results, areas with a predicted probability exceeding 0.5 for the seagrass occurrence occupy about a third of the total area of the bight; out of them, the area with a probability of Z. marina occurrence exceeding 0.75 accounts for 11.83 hectares. In fact, the seagrass meadows occupied > 70% of the area with a predicted probability of the species occurrence exceeding 0.5. As shown, the assessment of the contribution of seagrass ecosystems to the storage of carbon accumulating in the coastal zone requires differentiation of water areas by sedimentation regimes and types of bottom sediments. Moreover, the creation of databases with data on Corg concentration and stock per unit area is needed. Information on the areas of ecosystem distribution obtained by direct mapping and remote sensing is of high significance as well.
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