The influence of microphytobenthos on nutrient and oxygen fluxes in porewaters and shallow overlying waters, respectively, were examined for sediment-water systems of the Elbe Estuary, Northern Germany. The methods implemented were incubation methods with dark/light phases, in combination with microelectrode measurements. A mathematical model was developed for the detailed description of oxygen/nitrate profiles during periods when photosynthesis occured in the upper sediment layer and during dark periods. Sediment sectioning techniques were developed for the fine definition of porewater profiles. A High Pressure Liquid Chromatography method was adapted for pigment analysis in sediments.

Gross fluxes of oxygen of -30 to 170 mmol.m^-2.d^-1 were measured during algal production phases (net fluxes into the water: -9 to -160 mmol.m^-2.d^-1). The gross production and the net production rates within the sediment were 7.10^-6 mmol.cm^-3.s^-1 and 4.10^-7 mmol.cm^-3.s^-1 respectively. During the dark phases of the incubations fluxes into the sediment of 20 to 90 mmol.m^-2.d^-1 were measured. The corresponding oxygen consumption rates were about 4.10^-7 mmol.cm^-3.s^-1. The oxygen pentration depth was between 0.16 and 0.4 cm during periods of photosynthesis.

A two-layer mathematical model was used in all the experiments for the simulation of oxygen profiles in the sediments using the experimental results obtained as a basis. The data and profiles obtained using the model are in good agreement with the measured data when diffusion coefficients of 2 to 4.10^-5.cm2.s^-1 are used and the photic layer is designated a thickness of 0.08 - 0.09 cm.

Concentration measurements for nitrate-N in the water above the incubated sediments showed net fluxes of 0 to -105 mmol.m^-2.d^-1 out of the sediments during the illuminated incubation phases. During the dark phases net fluxes were directed from the water to the sediment. Porewater analyses clearly showed that the nitrate penetration depth was significantly deeper in those cores incubated with light than those incubated without. This effect is due to the inhibition of denitrification and/or the stimulation of nitrification. This is a direct result of greater oxygen sediment penetration as a result of benthic photosynthesis.

Effects resulting from algal nutrient uptake were too small with respect to mineralisation processes in the sediment to be observed.

The results from this work clearly show that all future considerations on the oxygen and nitrogen budgets of the tidal Elbe Estuary must include the effects which microphytobenthos can have on the transformation processes of these elements.