Abstract The effects of buried decaying macroalgae on meiobenthos were examined in intertidal sandy sediments of the Wadden Sea of Lower Saxony. In situ experiments confirmed that one of the principal causes of the formation of reduced surface sediments or ‘black spots’ on the tidal flats is the increasing occurrence and subsequent decomposition of filamentous green algae ( Enteromorpha spp.) buried in the sediment. Five to fifteen days after algal material had been buried, the sediment surface turned black. The impact of these black spots on meiobenthos was dramatic: the changed chemical conditions in the sediment resulted in long and drastic reductions in meiofaunal abundance and number of taxa. A multi-dimensional scaling (MDS) analysis of data on meiobenthic abundances revealed that samples from black-spot areas were clearly separated from those of control and reference areas. Re-oxidized black spots showed recolonization by meiofaunal animals, with numbers of individuals and taxa similar to those of oxidized surface sediments. The use of abundances of members of higher meiobenthic taxa to monitor changes in the sediment's chemistry, especially those caused by biomass overload, is discussed.