The investigation was carried out in a shallow water area in the northern Wadden Sea where the euphotic zone reaches down to the bottom. The nutrient flux was measured monthly during 1980 in three communities (an Arenicola-flat, a seagrass-bed and a Nereis-Corophium-belt) in connection with measurements of community metabolism. The nutrient fluxes of NH₄ ⁺, NO₃ ⁻, NO₂ ⁻, Si(OH)₄ were determined in light and dark bell jars in relation to temperature, O₂ consumption and concentrations of nutrients in the ambient water. Experimentally increased abundance of macrofauna (Arenicola marina) enhanced the regeneration of NH₄ ⁺, NO₃ ⁻, NO₂ ⁻. The plants took up NH₄ ⁺ more intensely in the light than in the dark. A community dominated by Zostera noltii showed the highest rate of NH₄ ⁺ uptake (−355 µmol NH₄ ⁺ · m⁻² · h⁻¹). NO₃ ⁻ was mainly taken up by the communities (up to −354 µmol NO₃ ⁻ · m⁻² · h⁻¹). Si(OH)₄ was released as well as taken up by all the three communities. Highest rates of Si(OH)₄ release were measured in the Arenicola-flat (up to + 1493 µmol Si(OH)₄· m⁻² · h⁻¹). This flux was correlated with the O₂ consumption, mainly by macrofauna. The system of the three sand flat communities imports nitrate and nitrite and exports ammonia and silicate on an annual basis.

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