Temporal variations of δ13C and δ15N in organic particles collected by a sediment trap at a time-series station off the Tokyo Bay

Tokyo Bay is one of the most eutrophicated embayments in Japan with a high primary productivity compared to the open oceans. A large quantity of dissolved and particulate material is exported out of the bay, but the source and pathways of this particulate material are not clear. In this study, a sediment trap was deployed at the mouth of the bay from 1995 to 2002, with time resolution of one week, in order to monitor the variability in quantity and composition of sinking particles flowing out of the inner bay to the open ocean. In addition, monthly variations in composition of sediments and suspended particles in the surface layer of the inner and outer parts of the bay were measured from 2000 to 2001 to differentiate the biogeochemical processes associated with particle transformation and transport. The mass flux of trapped particles shows an irregular variability with an average of 20 g m−3 day−1, which is about 25 times higher than in the adjacent Sagami Bay. In contrast, isotope ratios and contents of organic carbon and nitrogen show seasonal variations. The δ13Corg and δ15N values of trapped particles are heavier (δ13Corg: −21.0‰, δ15N: 6.7‰) in summer than in winter (δ13Corg: −22.0‰, δ15N: 5.7‰). These isotopic values are slightly lighter than those of the sediments, which have little seasonal variability (δ13Corg: −19.8‰, δ15N: 7.0‰). The suspended particles in the surface water have the largest seasonality at the inner bay (δ13Corg: −16.1‰ in summer to −23.1‰ in winter, δ15N: 11.9‰ in summer to −0.7‰ in winter) and at the bay mouth (δ13Corg: −19.3‰ in summer to −23.8‰ in winter, δ15N: 8.2‰ in summer to 2.3‰ in winter). Organic contents in trapped particles (C: 4.0%, N: 0.46%) are quite similar to those of sediments (C: 3.5%, N: 0.48%) in winter but a discrepancy is observed during other periods with an increase in both C and N contents (C: 5.0%, N: 0.57%) in trapped particles. ong-term high frequency survey suggests that the sinking particles collected at the bay mouth consist of two components: the resuspended sediments associated with reworked organic matter and the organic matter freshly derived from primary production in the bay surface. A closer analysis of the temporal variability in the organic components and the microscopic observation of sinking particles indicate that the trapped particles are primarily produced in the inner bay and, then, transported to the deep water of the trap site within a few weeks. This rapid transport could be attained only when: (1) resuspension of the surface sediment occurs in the entire bay, and (2) the fresh particles, sinking from the surface water, mix rapidly with the resuspended sediments and convey the temporal signal in the fresh particles to the trapped materials. It is not possible to discover the existence of such mechanism without the long-term high-frequency observation.