Mariculture has rapidly grown worldwide, which might greatly influence the biogeochemical cycle of organic carbon in coastal seas. In this study, several geochemical parameters, including grain size composition, sedimentary total organic carbon (TOC), total nitrogen (TN), stable carbon (δ 13C) and nitrogen (δ 15N) isotopic compositions, were analyzed for surface sediments collected from different mariculture zones of Sanggou Bay and in different seasons. We investigated the composition and distribution of organic matter in surface sediments and further evaluated the contribution of mariculture activities to TOC sources. The TOC and TN contents (mass percentage) in the bay were in the range of 0.14% to 1.45% and 0.03% to 0.20%, respectively. The spatial distribution indicated that sedimentary TOC and TN contents in shellfish monoculture and shellfish-kelp polyculture zones were higher than in other mariculture zones, which might be related to grain size composition and mariculture organisms. Seasonal variations of TOC contents were observed in different mariculture zones. The TOC/TN atomic ratio (C/N), δ 13C and δ 15N were in the ranges of 5.97 to 10.97, −21.76‰ to −13.14‰ and 2.13‰ to 8.08‰, respectively, implying that sedimentary organic matter in Sanggou Bay was the mixture of marine phytoplankton, terrestrial and maricultural sources. A simple mixing model based on δ 13C was applied and the results indicated that the relative contributions of organic carbon sources in Sanggou Bay followed the order kelp (36.6%) > marine phytoplankton (28.7%) > shellfish bio-deposition (23.8%) > terrestrial input (10.9%). Surface sediments in Sanggou Bay were dominated by mariculture-derived organic carbon, which on average accounted for 60.4% of TOC.