Sediment with different densities of Corophium volutator (Pallas), ranging from 0 to 19 800 ind. m-', were incubated in laboratory microcosms, and rates of oxygen uptake, denitnfication and nitrate ammonification were determined from sediment-water fluxes. The measured processes were stimulated differently by C. volutator; oxygen uptake, denitrification of NO, from nitnfication within the sediment, and denitrification of NOT from the overlying water were enhanced 2-, 3-and 5-fold respectively in the presence of 19 800 ind. m-?. This differential stimulation was explained by the different characteristics of diffusional solute transport at the sediment-water interface and mass transfer of water into the burrows where O 2 and NO? was depleted. Denitrification rates were calculated by using the lSN isotope pairing technique. The applicability of the ' 5~ isotope pairing technique for measunng coupled nitrification-denitrification in bioturbated sedlment was confirmed in a test incubation with different levels of 1 5~0 < added to microcosms with 12 000 C. volutatorind. m-'

The impact of macrofauna on nitrogen and carbon mineralization was investigated in sediment of the shallow water Bering Sea Shelf. The main effort was focused on the probable role of macrofauna in the production of urea and the significance of urea turnover in the production of NH,+ Macrofaunal biomass was regulated by the quality and quantity of organic nitrogen available for degradation. This was illustrated by a low macrofaunal biomass in the sediment underlying the low productive Alaska Coastal water and a high macrofaunal blornass below the highly productive Bering Shelf/Anadyr water. A high macrofaunal biomass was correlated with high rates of urea gross production, high concentrations of urea and NH,+, and high sediment-water exchange rates of urea and NHdf. Based on a conceptual model of nitrogen mineralization in the Bering Shelf/Anadyr sediment, it was suggested that urea hydrolysis could b e responsible for u p to 80 % of the gross production of NH4+ The model intimated that a substantial part of the NH4+ produced (44 %) could have been cycled within the sediment.

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.

Fluxes of total CO2, 02, NO3-+NO2-, NH,', DON (dissolved organic nitrogen) and HSwere measured across the interface of a coastal bay sediment for 43 d. The seawater overlying the defaunated sediment cores was changed continuously. Two treatments were employed: oxygenated overlying water (OX-cores) and anoxic water (AN-cores). Fluxes were measured before and after addition of an organic substrate, and loss of POM (particulate organic matter) was measured at the end of the experiment. Loss of POC (particulate organic carbon) from the sediment was the same under aerobic and anaerobic conditions. However, the highest efflux of CO2 was measured in the OX-cores, and the highest efflux of DOC occurred in the AN-cores before and after addition of fresh substrate. Similarly, loss of PON (particulate organic nitrogen) from the sediment was the same in the 2 treatments, but the highest fluxes of NH,' and DON were measured In the AN-cores. Our conclusion is that degradation of POM is the same under both aerobic and anaerobic condltlons, but that mineralization of organic molecules is less effective under anaerobic condlhons. Before substrate addit~on there was an influx of NO3-in both OX-and AN-cores, and the estimated nitrification rate in the OX-cores was 1 to 2 mm01 m-' d-l The estimated denitrification rate contributed 3 to 6 '10 to total carbon respiration. After addition of the substrate, fluxes of all constituents increased. The estimated denitrification rate constituted < 3 '10 of total respiration. There was a large efflux of HS-in AN-cores During the experimental period, the losses of original POC and PON were calculated to be 2.6 and 4.0 %, respectively. Relative to the initial values, the pore water constituents increased with incubation, and the integrated AN/OX ratios for N H d f , DON and total CO2 were 1.2, 1.5 and 1.6, respectively, after 43 d.

For marine benthos communities, the assessment of a respiration budget encompassing the entire size range from microbes to mobile megafauna has seldom been attempted. An interdisciplinary field study in high Arctic waters (northwestern Barents Sea) in June/July 1991 provided the opportunity to concurrently est~rnate the oxygen uptake of the different benthic community fractions by a variety of approaches at water depths of 80 to 1010 m. The bulk respiration of micro-, meio-and small macrobenthos was assessed by sediment community oxygen consumption (SCOC) rates measured by shipboard sediment-water incubations of virtually undisturbed cores. The oxygen uptake of community portions not sampled adequately by corers (megabenthic in-and epifauna, including fish) was estimated by applying individual metabolic rates to density or biomass figures derived from seabed images, box corer samples or trawl catches. The respiration estimates of the various community fractions were subsequently compiled in synoptic models of the total benthic community oxygen consumption (BCOC) and its partitioning. In the study area, 2 benthic habitat types were distinguished, differing substantially in depth, sediment texture and, thus, benthic respiration pattern: (1) shallow shelf banks (<200 m) where the seabed is composed of coarse sediments and stones, and (2) deeper trenches or slopes (>200 m) characterized by fine sediments. On the banks, the patchiness of epibenthic brittle stars, which locally occurred in very high densities (up to 700 ind. m-'), controlled the benthic community respiration. On average, the megafauna was estimated to contribute about 25% to the median BCOC of about 90 pm01 0, m-' h-' (equivalent to an organic carbon mineralizat~on rate of 21 mg C m-? d-'). In the shelf trenches and on the slope, however, smaller endobenthic organisms predominated. SCOC, according to our estimates of meio-and macrofaunal respiration, was dominated by the oxygen uptake of microorganisms and accounted for about 85% of the median BCOC of about 140 ~lmol 0, m-' h-' (35 mg C n r 2 d-l). Our results suggest that current models of benthic community respiration should be amended, particularly for Arctic shelf b~otopes where abundant megafauna may represent an important pathway of the benth~c energy flow.