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.

No abstract

Seasonal variation of trimethylamme (TMA) concentrations in sediment porewater was detected at 3 different sites in a Danlsh estuary. A significant increase, from a pool size of 0 to 0.5 pM in fall and winter to 5 to 10 FM during spring, was observed in 2 fauna-rich, sandy sediments. An organlcnch silt ulth few animals showed low TMA concentrations throughout the season Depth profiles of TMA also correlated with the distribution of burrowing infauna. Only fauna-nch sedlments showed detectable efflux of TMA to the water phase. With few an~mals, the flux was negligible due to efficient TMA consumption at the oxic sedunent-water interface. Animals are proposed to influence production and emission of TMA by osmoregulatlon and physical activity in the sediment.

Net release of soluble reactive phosphorus from mass developments of bryozoans, grown on artificial substrata in Mikolajskie Lake, Poland, was measured in depth profiles from 0 to 4 m. Release rates up to 94 pg P dm-2 h-l were recorded. P uptake by algae in the bryozoan community was a minor component of the P balance. The P exchange was several times higher in this community than typical values for other periphyton communities in the same lake. It is concluded that well developed bryozoan communities, when present, can contribute considerably to the recycling of phosphorus in the littoral of smaller lakes.