Distributions, stoichiometric patterns and cross-shelf exports of dissolved organic matter in the East China Sea

Abstract

This paper sets out to elucidate distributions, stoichiometric patterns and cross-shelf exports of dissolved organic matter (DOM) in the East China Sea (ECS). Surface distributions of dissolved organic carbon (DOC) in the ECS varied spatially, ranging from 85 to 120 μM in the China Coastal Water (CCW), from 75 to 85 μM in the Kuroshio Water (KW), and from 60 to 70 μM in the upwelling water. DOC concentrations in most regions of the shelf mixed water (SMW) were between 72 and 85 μM. Temporal variations were insignificant with the exception of the CCW, where the concentration was greater in summer and autumn than in spring and winter. Spatial patterns of dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) were less variable in the spring season, ranging from 6 to 9 μM for DON and 0.15 to 0.25 μM for DOP. The elemental ratios of DOC, DON and DOP are much greater than the Redfield ratio, ranging from 8.9 to 15.3 for C/N, from 19 to 83.6 for N/P and from 200 to 853 for C/P. Such ratios also showed a general increase with depth. However, the slopes of linear regression of DOM pairs become smaller and were about 8.4 for ΔDOC/ΔDON, 19 for ΔDON/ΔDOP and 129 for ΔDOC/ΔDOP, suggesting the possible ratios derived from the recently produced fraction. Vertical increases of ratios in DOM pairs may suggest that the recycling of DOP and DON is more rapid than that of DOC to overcome the biological overconsumption of DIC to DIN and DIP (ΔNDIC:ΔNDIN:ΔNDIP=1:14.3:129), as the ratio of ΔPOC/ΔPON (6.74) approaches the Redfield ratio in the euphotic zone. Degradation rates of DOC and DOP are variable with water types and are generally greater for the CCW than for the KW, suggesting the different lability of DOM in various waters. The residence times of bulk DOC, DON, and DOP in the shelf are about 1.10, 0.98, and 0.92 yr, respectively, which are close to the ranges of the mean residence time of shelf water reported previously, suggesting that there is little to no export off the shelf. Finally, total inputs are closely balanced by total outputs for bulk DOC, DON, and DOP in the ECS shelf, if temporal variations of DON and DOP are negligible. However, net exports across the shelf are highly probable if only labile and semi-labile DOC (3.8±0.6×1012 g C yr−1), DON (482±160×109 g N yr−1), and DOP (37.5±18.5×109 g P yr−1) are involved in the budget calculation. The ECS shelf is likely to be a small source of degradable DOM for oceanic waters.