Consequences of diatom mat erosion for carbohydrate concentrations and heterotrophic bacterial activities in intertidal sediments of the Ems-Dollard estuary

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Patches with a diatom mat were compared with adjacent eroded patches on an intertidal flat in the Ems-Dollard estuary. Distinction between patches was made on the basis of chlorophyll- a (chl- a) concentration and relief. Short-term variations and depth distribution were investigated for carbohydrates (water-soluble and water-insoluble carbohydrates), β-glucosidase activity and heterotrophic bacterial production and abundance. The major differences between mat patches and eroded patches were confined to the upper 6 mm. From 6 to 20 mm depth variables in mat patches and eroded patches were roughly comparable, with the exception of the bacterial abundance and β-glucosidase activity. Distinct increases in carbohydrates, bacterial production and abundance during tidal flat exposure were most pronounced in the mat surface layer, but were still detectable in the eroded patches for the different carbohydrate fractions. Concentrations of water-soluble carbohydrates were substantially higher in mats than in eroded patches. This concurred with a much higher bacterial production and abundance in mats than in eroded patches. The β-glucosidase activity in the mat surface was low compared to this activity in eroded patches. In diatom mats, β-glucosidase showed highest activities between 6 and 12 mm, while in eroded patches this maximum occurred at 0–2 mm. Patterns suggest inhibition of the β-glucosidase activity at high concentrations of water-soluble carbohydrate concentrations and a release of inhibition with declining carbohydrate concentrations. Interestingly, concentrations of water-insoluble carbohydrates (EDTA-extracted carbohydrates) did neither clearly differ between mat patches and eroded patches, nor changed substantially with depth. The observation that water-insoluble carbohydrate-to-chl- a ratios were higher outside the mat layer than in the mat, which has a larger stability than eroded patches, indicates that it is a poor index for sediment erodibility. The same holds true for water-soluble carbohydrate concentrations.