Assessment of carbon stock in the Zostera marina ecosystem on sandy sediments of Srednyaya Bight (Peter the Great Bay, Sea of Japan) based on field observations

Abstract

Seagrass Zostera marina, are able to store organic carbon by fixing carbon dioxide during photosynthesis. Seagrass biomass is considered as a short-term, and underlying bottom sediments as a long-term carbon storage. Most of the research on C_org accumulation by seagrass ecosystems relates to areas with sustained sedimentation. The importance of seagrass areas within the concept of "blue carbon" has been shown for such ecosystems. However, in the temperate regions, coastal areas with unstable sedimentation and a predominance of sandy sediments are common, and the scale of carbon storage in these seagrass ecosystems is not obvious. In this work, the biomass and carbon stock in the leaves and roots of the seagrass Z. marina, as well as the concentration and the carbon stock in the upper layers of bottom sediments (0.25 and 1 m thick) were determined for typical habitats in the semi-open Srednyaya Bight (Peter the Great Bay, Sea of Japan), where sandy sediments dominate. The roots of Z. marina have 3-20 times less biomass than the leaves. This difference increased from April to July in accordance with seasonality. In the habitats with a projective coverage 50-80% the carbon stock in the tissues of zostera was 96.8±37.4 g C.m^-2; and increased to 253 g C. m^-2 with 100% coverage. The concentration of C_org in the bottom sediments of the Srednyay Bight ranged from 0.04 to 0.46% and correlated with content of silt fractions. Under dense coverage of Z. marina, the concentration of C_org and the share of silt particles in sediments was higher than under sparse ones. The vertical distribution of the concentration of C_org within the upper 15-35 cm did not reveal a downward trend in the cores. Data on the bulk density and concentration of C_org in sediments allow to calculate the carbon stock for the 0.25 and 1 m layers. The quota of organic carbon contained in seagrass tissues did not exceed a third of its amount contained in the upper layer (0.25 cm) of the underlying sand deposits. When extrapolated to a 1 m thick layer, the quota of bottom sediments to the organic carbon pool exceeds 90%. Organic carbon enrichment of sand sediments under seagrass beds compared to sands of similar grain size outside the seagrass beds indicates a significant role of Z. marina in carbon storage, even in the habitats with the absence of sustained sedimentation. The major factor controlling the carbon stock in ecosystems with Z. marina is the concentration of C_org in the underlying bottom sediments. The areas of potential distribution of Z. marina in the Srednaya Bight were modelled using the Maxent 3.4.4 program. Maps of the distribution of seagrass in April and July 2021 were constructed, and the results showed that areas with a predicted probability > 0.5 of Z. marina presence occupy 32.47 hectares (about a third of the total area of the bight). In fact, seagrass occupied more than 70% of this area. The differences between the areas occupied by the Z. marina community in 2013, 2014 and 2021 were within the error range of mapping. At the same time, not all suitable habitats are permanently occupied, the local disappearance of seagrass confirm the possibility of restoring fields after periodic destruction by storms. It is shown that assessment of the contribution of seagrass ecosystems to the carbon storage needs differentiation of the areas by sedimentation regimes and types of bottom sediments. The creation of databases with information on the concentration and stock of C_org per unit area, as well as information on the areas of ecosystem distribution obtained by direct mapping and remote sensing are also necessary.

Keywords: blue carbon, Zostera marina, concentration of Corg in sandy bottom sediments, remote sensing, Sea of Japan, Peter the Great Bay