A seasonal study of NH: and NO; fluxes and concentrations at the sediment-water interface was carried out at a 15 m deep station in Aarhus Bight, Denmark. In winter, NHZ and NO; were released from the sediment at comparable rates (0.20 to 0.40 mm01 N m-2 dC1). A phytoplankton bloom developed rapidly in early spring. Immediately after mass sedimentation of diatoms, the sediment transiently released NH: at a high rate (up to 1.5 mm01 N m-'d-') and a dramatic change to a NO; uptake was observed (flux ca-0.80 mm01 N m-2 d-l). Subsequently both the NH.; release and NO; uptake decreased (summer fluxes ca 0.35 and-0.15 mm01 N m-' d-', respectively). From late summer, NO? was again released from the sediment (ca 0.30 mm01 N m-' d-') and a second, weaker maximum of NH: release (ca 0.70 mm01 N m-' d-l) was observed in fall. Seasonal variation of NHJ and NO: concentrations at the sediment surface (upper 2 mm) and in the bottom water agreed well with observed flux patterns. The high NH; release and NO; uptake immediately after spring bloom sedimentation indicated rapid increases of mineralization and denitrification. Sediment nitrification seemed to be inhibited, however, probably because the O2 penetration depth was reduced after sedimentation. Uptake of bottom water NO: rather than nitrification therefore seemcd to support the denitrification maximum. Even when calculated for a whole year, about 50 % of the NO< consumed during denitrification was supplied from the bottom water. Annual nitrogen budgets also indicated that denitrification accounted for 25 O/O of the total inorganic n~trogen release from the sediment. generally considered to limit primary coastal waters (Ryther & Dunstan 1971), subordinate role of phosphorus has recently been discussed (Hecky & Kilham 1988, Howarth 1988). Of major importance in controlling the concentrations of inorganic nitrogen species in shallow waters is their rate of exchange across the sedimentwater interface (i.e. benthic fluxes).

Concentration profiles of NH., and NO? in pore water and particulate matter were determined at high spatial resolution (mm bcale) in surface sediment from a coastal bay area (Aarhus Bight, Denmark) at 15 m depth during an annual cycle. Pore water pools of NH: and NO; were always considerably lower than particulate pools in the surface sediment. Particulate NH: and NO; were apparently intracellular pools in deposited microalgae and were extracted after freezing sedlment samples in liquid NZ (-196 "C). Pore water NH: and most of the adsorbed (KCI-extractable) NH: were also extracted by the freezing technique, and an estimate of the intracellular NH: pool was obtained by difference. In the absence of an adsorbed NOS pool, intracellular NO? was determined by subtraction of the pore water pool from the liquid NZ-extractable pool. Highest concentrations of intracellular NH: and NO; were always observed in the upper 2 mm of sediment, declining sharply with depth. A distinct seasonal maximum for both pools, ca 200 nmol cm-3 at 0 to 2 mm depth, appeared after sedimentation of a phytoplankton bloom in early spring, and should be compared to a minimum of only 25 nmol or less in fall and winter. The freeze-extraction technique is proposed for a reliable estimate of intracellular NH: and NO; pools in surface sediments r~c h In microalgae, and may thus be used as an indicator of sedimentation of phytoplankton blooms. The signiflcance of intracellular pools for sediment nitrogen cycling is discussed.

Rates of sediment O2 uptake, CO2 production, sulfate reduction and acetate turnover were examined during January 1992 in the Ao Nam Bor mangrove, Phuket, Thailand. The impact of air exposure on O2 uptake was most pronounced in the intensely bioturbated high-intertidal zone (6.4 times higher than during water cover), and decreased to almost zero in the low-intertidal zone. This indcates a gradual increased area of sediment-air contact zones with tidal elevation due to changes in surface topography. Based on an average water cover for January, the diurnal O2 uptake -and thus total decay of deposited detritus -was 4 to 5 and 8 times faster in the high-intertidal compared to the mid-and low-intertidal zones, respectively. Sulfate reduction rates were generally low. The depthintegrated (0 to 30 cm) sulfate reduction was highest in the mid-intertidal zone, and supported 85 % of the estimated daily CO2 release. In the high-and low-intertidal zones, sulfate reduction supported 11 and 92%, respectively, of daily CO2 release. Rates of acetate uptake were also higher in the midthan in the low-intertidal zone (no data from high-intertidal) However, the depth-integrated acetate uptake was consistently about 2.6 times the rates of CO, release and 5 to 6 times the 0 to 11 cm integrated sulfate reduction, which suggests that pool sizes of acetate and thus uptake rates may have been overestimated. In conclusion, while benthic respiration in the mid-and low-intertidal zones of the Ao Nam Bor mangrove was dominated by sulfate reduction with acetate as carbon source. 'suboxic' conditions related to bioturbation in the active high-intertidal sediment made respiration by other electron acceptors than SO.,-more important.