Mineralization was studied withln the upper 2 m of sediments from the Belt Sea, Kattegat, and Skagerrak at 15 to 200 m water depth. Radiotracer measurements of sulfate reduction rates were related to porewater chemistry (SO,'-, HC03-, P O~~-, NH4+, H2S, and CH,), to solid-phase chemistry (C, S , N, and Fe), and to bacterial distributions. Sulfate penetrated 0.9 to > 3 . 5 m into the sediment. Sulfate reduction rates decreased > 100-fold from m a m a of 6 to 74 nmol cm-3 d-' at the surface to between 0.1 and 1 nmol cm-3 d-' at 1 to 2 m depth. Between 8 and 88 O/O of the total, depth-integrated sulfate reduction took place within the uppermost 0 to 15 cm of the sediment. Maxlma of sulfate reduction or bacterial densibes at the sulfate-methane transition indicated a zone of anaerobic methane oxidation 0.8 to > 2.5 m below the surface. The fraction of the iron pool, which was bound in pyrite, was 17 to 42 %, even in the presence of > 1 mM H2S. Only 4 to 32 % of the H2S produced from sulfate reduction was permanently buried as FeS2 while the rest was reoxidized. Sediment accumulation rates determined from 'I0pb age determinations were 0.3 to 6.2 mm yr-'. The total deposition of organlc carbon, determined from the sum of organic C mineralized by aerobic and anaerobic respiration plus net burial of organic C, was 16.7 to 52.3 mm01 m-' d-'. This was equivalent to about 50 % of the primary productivity in the water column. The net burial rates of organic C were 1.5 to 26 mm01 m-' d-' corresponding to 9 to 50 ' % of the deposited organic C. The bunal of pyritic sulfur corresponded to 9 to 37 O/u of the reducing equivalentes buried as organic C.

Characteristics of marine heatwaves (MHWs) in the East Asian Marginal Seas (EAMS) were investigated using the daily Optimum Interpolation Sea Surface Temperature for 37 years (1982–2018), focusing on seasonal changes and regional differences. The summer MHWs occur 54% more frequently (2.7 events/decade) in a relatively wide area than in other seasons. The strong (up to 3.7 °C), long-lasting (up to 38 days/event) winter MHWs are concentrated along the subpolar front (SPF) in the East/Japan Sea (EJS) where the MHWs are 20% longer (2.2 days/event) than in the Yellow and East China Seas (YECS). The summer MHWs are primarily driven by increased shortwave radiation associated with reduced cloud cover and latent cooling from the weakened wind over the western flank of developing subtropical highs. Driving mechanisms of the winter MHWs differ by region. The YECS MHWs occur mainly due to the atmospheric processes associated with weakening continental highs while the EJS MHWs are largely driven by the northward shift of the SPF. Although large-scale atmospheric processes primarily drive the summer MHWs occurring in a wide area in the EAMS, our findings suggest that ocean processes can be a major contributor to intensified MHW generation in limited areas especially in winter.

Walleye pollock Gadus chalcogrammus (hereafter, pollock) was one of the predominant fish species in Korean waters during the 1970s and 1980s, but catches decreased rapidly beginning in the late 1980s and the fishery collapsed in the 2000s. The biological characteristics of fish species tend to depend on their biomass and environmental conditions. To investigate the changes in the biological characteristics of pollock (such as their size frequency, condition factor, and maturation) associated with biomass levels as well as the possible causes of the stock collapse as a result of environmental changes, we used data collected by the Korean National Institute of Fisheries Science (NIFS) during high (1976)(1977)(1978)(1979)(1980)(1981)(1982)(1983)(1984)(1985) and low (1991-2000) periods of pollock biomass. Density-dependent effects on the pollock population were evident in Korean waters: pollock were larger and heavier at length during the low-biomass period. The gonadosomatic index also indicated that the peak spawning season was broad (from October to February) during the high-biomass period and narrower (December and January) during the low-biomass period. The population abundance and recruitment success of Korean pollock appear to have been affected by the warming of the ocean surface from the early 1990s. Thus, we suggest that the biomass fluctuations associated with environmental changes during their early life stages were responsible for the observed changes in the biological properties of pollock.

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