We investigated the biomass and production rates of microorganisms associated with coastal upwelling and the anticyclonic Ulleung warm eddy (UWE) of the Ulleung basin in the East Sea. Shipboard observations revealed that depth-integrated phytoplankton biomass and production were higher in the moderately stratified ring of the UWE than in the vertically well-mixed eddy core or in the stratified region outside of the eddy in the Ulleung basin. Similarly, heterotrophic bacterial production was higher in the eddy ring than in the eddy core or outside of the eddy. Bacterial biomass in the highly productive eddy ring showed little difference compared to the bacterial biomass of the core site due to the impact of grazing by heterotrophic protozoa. Satellite imagery and diatom species composition data in conjunction with physico-chemical parameters demonstrated that winddriven coastal upwelling in the southeast of Korea was largely responsible for the phytoplankton bloom and enhanced bacterial production along the UWE. Overall, the results indicated that the UWE entrained highly productive upwelling coastal waters and delivered enhanced microbial biomass and production into the central Ulleung basin. The results further implied that the UWE and the subsequent effect on vertical particulate carbon flux may play a significant role in stimulating benthic respiration and in sequestering organic carbon produced by coastal upwelling down into the deep Ulleung basin.KEY WORDS: Bacterioplankton · Phytoplankton · Upwelling · Eddy · Ulleung basin · East SeaResale or republication not permitted without written consent of the publisher

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Organic matter oxidation represents a transfer of elements to inorganic nutrients that support biological productivity and food web processes. Therefore, quantiWcation of the controls of organic matter mineralization is crucial to understanding the carbon cycle and biogeochemical dynamics in coastal marine environments. We investigated the rates and pathways of anaerobic carbon (C) oxidation in an unvegetated mud Xat (UMF) and a vegetated mud Xat (VMF) of the Ganghwa intertidal zone of the macro-tidal Han River estuary, Yellow Sea. Analyses of geochemical constituents revealed relatively oxidized conditions and high reactive Fe(III) concentrations (40-100 mol cm ¡3 ) in the sediments. A pronounced depth stratiWcation in Fe(III) was observed at the VMF site likely due to the lower number of infaunal burrows along with dense root formation by the macrophytes, Suaeda japonica. Depth-integrated rates of anaerobic C mineralization as well as sulfate-and Fe(III) reduction at the VMF were consistently higher than those at the UMF, likely driven by the dense vegetation that supplied organic C substrates and electron acceptors to the rhizosphere. Sediment inventories revealed that solid Fe(III) was up to 17 times more abundant than pore water sulfate, and direct rate measurements showed that microbial Fe(III) reduction comprised an equal or larger percentage of C oxidation (36-66 %) in comparison to sulfate reduction (36-40 %) at both sites studied. Time-course experiments indicated that sulfate reduction rates were likely underestimated, especially in the VMF rhizosphere, due to the reoxidation of reduced S in the presence of high Fe(III). The high rates of C mineralization suggest that the Ganghwa intertidal mud Xats are a signiWcant sink against the external loading of organic compounds, and organic matter mineralization is enhanced by chemical exchange regulated by extreme tidal Xushing and macro-microorganisms interactions.

Abstract. Rates and pathways of benthic organic carbon (C org ) oxidation were investigated in surface sediments of the Ulleung Basin (UB) characterized by high C org contents (> 2.5 %, dry wt.) and very high contents of Mn oxides (> 200 µmol cm −3 ) and Fe oxides (up to 100 µmol cm −3 ). The combination of geochemical analyses and independently executed metabolic rate measurements revealed that Mn and Fe reduction were the dominant C org oxidation pathways in the center of the UB, comprising 45 and 20 % of total C org oxidation, respectively. By contrast, sulfate reduction was the dominant C org oxidation pathway, accounting for 50 % of total C org mineralization in sediments of the continental slope. The relative significance of each C org oxidation pathway matched the depth distribution of the respective electron acceptors. The relative importance of Mn reduction for C org oxidation displays saturation kinetics with respect to Mn oxide content with a low half-saturation value of 8.6 µmol cm −3 , which further implies that Mn reduction can be a dominant C org oxidation process even in sediments with lower MnO 2 content as known from several other locations. This is the first report of a high contribution of manganese reduction to C org oxidation in offshore sediments on the Asian margin. The high manganese oxide content in the surface sediment in the central UB was maintained by an extreme degree of recycling, with each Mn atom on average being reoxidized ∼ 3800 times before permanent burial. This is the highest degree of recycling so far reported for Mn-rich sediments, and it appears linked to the high benthic mineralization rates resulting from the high C org content that indicate the UB as a biogeochemical hotspot for turnover of organic matter and nutrient regeneration.

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