Recovery from black spots: results of a loading experiment in the Wadden Sea

In field experiments to study the response of intertidal flats to the burial of organic carbon, we loaded southern North Sea sandy sediments with varying amounts of starch. The loaded areas temporarily turned black due to sulphate-reducing degradation of the organic burden. The appearance and disappearance of black sediment surfaces were monitored over a period of 12 months by means of pore-water and particulate-matter analysis; organic-matter degradation was traced by δ13C (TOC). Biogeochemical data showed the duration and severity of `black-spot diseaseʹ to depend on the amount of organic carbon buried. Results met expectations in a qualitative manner; several causes of the absence of linear correlations between the initial organic-carbon load and DOC or sulphate concentrations are discussed. The critical experimental load that caused black spots ranged between 0.4 and 0.8 kg m−2 Corg for a single organic-carbon entry. Given a situation of annually repeated entries, we estimate the critical load to be less than 0.4 kg m−2 a−1 Corg. Depth profiles indicate OM-caused enrichment in sulphur and pyritisation below 20 cm depth. This study emphasises the conditions of recovery.