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Although the seagrass Zostera marina L. (Z. marina) is expected to significantly contribute to environmental carbon dynamics, the residual potential of organic carbon (C org ) in this plant during its decomposition process in seawater remains insufficiently understood. In this study, the factors regulating this potential were examined by evaluating Z. marina decomposition in seawater and conducting a multiple regression analysis on data from the experiment as well as the original plant organic matter composition. The residual ratio during the decomposition experiment (R DEC ) of carbon in Z. marina for 111 days ranged from 0.30 to 0.84 (n = 16), and the variation resulted from differences in the sampling season of this plant. Regression analysis was conducted to explain the R DEC of carbon using the R DEC of nitrogen, concentrations of carbon, nitrogen, HCl-extractable C org and nitrogen, contributions of carbohydrate (C CAR ) of total and non-structural fractions and lipid (C LIP ) to carbon concentration. The R DEC of carbon was most adequately fitted using a multiple regression including three parameters, carbon concentration, C LIP, and C CAR of total carbohydrate with a significant determination coefficient. The former two parameters (carbon concentration and C LIP ) have negative coefficients, indicating that these parameters correspond to bacterially available C org in Z. marina in the regression analysis. The latter parameter (C CAR of total carbohydrate) shows a positive coefficient that indicates recalcitrant C org in the regression. Parameters related to nitrogen were not included in the regression formula, although this element is reportedly an important parameter regulating the environmental decomposition rate of C org . Finally, we suggested that total carbon, carbohydrate, and lipid are especially important factors to regulate the environmental residual potential of Z. marina C org among organic components of this plant., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier Ltd. All rights reserved.)