The deposition of a Skeletonerna bloom onto sediment collected in a coastal bay was simulated and the effects on 02, N and P fluxes were recorded. A comparison of calculated net production and observed net efflux of inorganic nitrogen (IN = ammonium + nitrate) for the period prior to the addition of algae, indicated that two-thirds of the produced ammonium was nitrified and subsequently denitrified. Algal additions induced a dramatic and persistent rise in oxygen consumption (1.6 X ) and release of IN (4.5 X ) . DON (dissolved organic nitrogen, 2 X), and Pod3-(3 X). Of the added algal N (76 mm01 N m-2), 33 % was recycled as NH,' and DON during the 19 d of incubation and, according to Pod3-measurements, the same was true for the algal P, giving a half-life of the algal biornass of ca 4 wk. Comparison of calculated net production and observed net efflux of IN indicated a considerably decreased denitrification/mineralisation ratio due to algal additions. Extrapolating to the size of Laholm Bay, SE Kattegat (sampling site), suggests that the settlement of algae (realistic amount used here) would make the sediment the most important nutrient source of this bay for a few weeks period. DON was a significant part of the released, non-gaseous nitrogen, both before (46 %) and after (27 %) the addition of algae. In cores subjected to anaerobic conditions, the concentrations of DON decreased considerably, suggesting that the production of DON was inhibited during such conditions and that much of the DON in this sediment was easily utilized.

Influence of sublittoral microphytobenthos on the flux of oxygen and inorganic nutrients ( N , P, Si) at the sediment-water interface was studied using undisturbed cores of sandy and muddy sediment incubated in a laboratory continuous-flow system, either in darkness or with a 16/8 h L/D cycle at in situ light level during summer. Sediment was collected In July at 15 m depth in a non-tidal, stratified bay in SE Kattegat. To test whether the higher content of inorganic nutrients below the halocline, compared with surface waters, could stimulate microphytobenthic growth, 2 levels of nutrient concentrations were used. Diel variations were found In L/D cores, but not in darkened cores, for oxygen, dissolved inorganic nitrogen and phosphorus content in the water overlying the sediment. The flux of NH4+, NO3-and ~0~~-out of the sediment decreased during light periods and occasionally a net uptake was recorded. Light-induced O2 production, and correlations between A fluxes (differences between day and night fluxes of 0, and nutrients), chlorophyll a content and algal cell numbers in the sediment, indicate that the decreased outflux of IN and POA3-was mediated by photosynthetic organisms. Diel variations were not studied for silicon, but a significantly lower outflux, or even an uptake, of Si(OH), from L/D cores supports this conclusion. This suggests that diatoms play a major role in the nutrient flux between sediment and water. Also, the differences in pore-water nutrient gradients between LID and dark cores point to the importance of sediment-associated organisms. Daily (24 h) net fluxes of nutrients were primarily out of the sediment, but the magnitude depended on both light conditions and sediment type. Daily net outflux was significantly lower in U D cores than in darkened cores for all nutrients except NO3-in muddy cores and NO2-in sandy cores. Net uptake in L/D cores was recorded for Si(OH)4 and NO3-In sandy sediment. Outflux of nutrients was significantly higher from muddy sediments in comparison w~t h sandy sediments (except NO3-), especially in permanent darkness. No significant effect of nutrient enrichment on the abundance of sublittoral benthic microalgae could be shown. Results suggest that m~crophytobenthos can influence sediment-water exchange of inorganic nutrients even at sublittoral depths, and when measuring nutrient flux in permanently darkened cores from depths around 15 m in the Kattegat, summer flux rates will be overestimated by a factor varying between 2 and 6, depending on sediment type.

Nitrification and dissimilatory reduction of nitrate to ammonium were measured concomitantly with nitrogen release from marine coastal sediment samples from 2 fjord sill stations. Dissimilatory ammonium production (DAP) and nitrification were measured using core injections of "NO; and H1"CO;, respectively. DAP was detected in all segments of the cores by tracing "NH: evolved from the added "NO;. 15NH: recovery increased with increasing core depth, ranging from 1.6 to 10 % for Stn H and from 0.3 to 2.9 % for Stn L. Nitrification activity at Stn L was in the order of 10 nmol cm-3 h-' in the upper 2 cm but was not demonstrated in deeper strata. Consequently, DAP was most pronounced in the upper few centimeters although it was anticipated that more than 97 % of the nitrate produced was denitnfied. Mean fluxes of ammonlum out of the sediment were 24 and 12 pm01 m-2 h-' for Stns H and L, respectively, and corresponding nitrate fluxes were 2 and -24 pm01 m-2 h-'.The sum of ammonium and nitrate release in the individual cores did not reach the rates expected from their oxygen consumption rates, which implies that inorganic nitrogen was lost, probably due to coupled nitrification and denitrification. This estimated loss was about half of the obtained nitrification rate at Stn L. Furthermore, the estimated loss was larger in cores with a rich macrofauna and especially with high numbers of Arnphiura spp. (brittlestars). It is suggested that these animals stimulate both nitrification and dissimilatory nitrate reduction.

Simultaneous measurements of nitrification in the Baltic Sea were made at 10to 30-m intervals in the months of June and November by three isotope techniques: ['5N]nitrate dilution, N-serve sensitive ["4C]bicarbonate incorporation, and ['5N]ammonium oxidation to nitrite and nitrate. Nitrification rates of 1 to 280 nmol liter-' day-' were recorded, and each method showed that the highest rates of nitrification occurred below the halocline. Even in the presence of ammonium, dark incubations of mixed layer (above ca. 50 m) waters never yielded nitrification rates exceeding 45 nmol liter-' day-'. The rates measured by the ammonium oxidation method were two-to sevenfold greater than those obtained by '4C incorporation or 15N dilution. The merits of each technique are discussed, and it is suggested that the [15N]ammonium oxidation method should be used in conjunction with the [14C]bicarbonate incorporation method. Reported measurements of nitrification rates in the sea are few, undoubtedly due to the lack of sensitive methods. Previously, the only isotope used in such measurements in the open sea was ['5N]ammonium, by Hattori and Wada (9) and Miyazaki et al. (15, 16) in the West Pacific Ocean and Sagami Bay. Olson (17) observed the Southern California coastal waters, the central North Pacific gyre, and the Scotia Sea. The latter study showed that the addition of [15N]ammonium (1 ,uM) did not stimulate nitrification, and the reported rates were between 1 and 10 nmol liter-' day-' in the euphotic zone and between 2 and 50 nmol liter-' day-'

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