Do benthic diatoms influence erosion thresholds of coastal subtidal sediments?

This study reports diatom cell abundance, concentrations of water-soluble and water-insoluble carbohydrates, and concentrations of chlorophyll—a equivalents as indicators of microbial exudates and biomass in the uppermost sediment layer of two coastal sites (19 m water depth: fine sand; 25 m water depth: mud) in Mecklenburg Bight, south-western Baltic Sea. Correlation analysis with experimentally determined erosion thresholds measured in five different periods of a year showed that the sediment water content and the content of water-soluble carbohydrates may indicate erosion thresholds for both sediment types. Erosion thresholds were significantly negatively correlated with sediment water content at the mud site. At both sites, colloidal carbohydrates, as indicators for diatom EPS, showed significantly negative correlations with sediment water content and no correlation with diatom biomass. Therefore, following current interpretation, we argue that micro-biostabilsation effects of benthic microalgae play a minor role in the investigation area. Light is likely to be insufficient for phototrophic growth on the seafloor of the investigated sites most of the year, as indicated by in situ measurements and calculated values of photosynthetically active radiation. We consider lateral transport of benthic diatoms from shallower to deeper areas of Mecklenburg Bight and subsequent to be the mechanism influencing diatom abundance. Exhaustive biofilms are absent at these depths and mass erosion is not likely to be affected by benthic diatoms.