Variation in sediment stability and relation to indicators of microbial abundance in the Okura Estuary, New Zealand

Secretions of mucus by benthic microbes potentially bind estuarine intertidal sediment, thus affecting stability by raising the erosion threshold. Existing models for predicting onset of erosion—such as the Shields diagram—have been built from laboratory studies of abiotic sediments, hence their accuracy when applied to natural sediment may be limited. In this study, variability in critical shear velocity (u*crit) of natural intertidal sediments is correlated with indicators of microbial abundance, and based on those correlations we develop a predictor for the erosion threshold of natural sediments. Sediment cores, collected over 9 months from four sites of contrasting grain size (77–185 lm) in the Okura Estuary (Auckland, New Zealand), were eroded in a laboratory flume to determine u*crit. Critical shear velocity for initiation of motion ranged from a minimum of 0.52 cm s 1 at the coarse-grained site to a maximum of 1.45 cm s 1 at the fine-grained site, values of which are up to three times those measured for equivalent abiotic sediment. The increase in u*crit with decreasing grain size was correlated with indicators of microalgal biomass (pigment) and mucilage content (carbohydrate) in the surface 2mm of sediment. Abiotic measures of hcrit (non-dimensional u*crit) were adjusted via a stability factor, which was expressed as a function of sediment pheopigment content to estimate the erosion threshold in natural sediments in a better way. This method takes into account the temporal variations in sediment stability that occur irrespective of grain size.