Relating spatial and temporal variability in sediment chlorophyll a and carbohydrate distribution with erodibility of a tidal flat

Abstract

Trends in the spatial distribution of chlorophyll a (chl a) and colloidal and total carbohydrates on the Molenplaat tidal flat in the Westerschelde estuary, Netherlands, reflected spatial differences in physical properties of the sediment. Results from a Spearman Rank Order Correlation indicated that many of the physical and biological measures covaried. Multiple regression analyses describing the relationship between colloidal carbohydrates and sediment properties resulted in several highly significant equations, although in all cases chl a was able to predict colloidal carbohydrate content. Relationships between sediment surface chl a and colloidal carbohydrate, and sediment erodibility (i.e., critical erosion threshold, Ucrit, and mass of sediment eroded at a velocity of 30 cm s-1) determined in annular flume experiments were examined. Overall sediment erodibility was lowest (i.e., high thresholds, low mass eroded) for the siltiest sediments in June 1996 when chl a and colloidal carbohydrates were high (56.9 mg gDW-1 and 320.6 mg gluc.equ. gDW-1, respectively), and greatest (i.e., low thresholds, high mass eroded) at the sandier sediments in September 1996, when chl a and colloidal carbohydrates were low (1.0 mg gDW-1 and 5.7 mg gluc.equ. gDW-1, respectively). When sediments were grouped according to relative silt content, the most significant relationships were found in muddy sand with a fine-grained fraction (<63 mm) of 25-50%. Thresholds of erosion increased, while mass of sediment eroded decreased, with increasing chl a and colloidal carbohydrate. A similar trend was observed for the sand-muddy sand (63 mm 10-25%). In the sand (63 mm 0-10%), there were no relationships for Ucrit, whereas mass eroded appeared to increase with increasing chl a and colloidal carbohydrate. The increased carbohydrate may stick sand grains together, altering the nature of erosion from rolling grains to clumps of resuspension.

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ORCID for Cathy H. Lucas: orcid.org/0000-0002-5929-7481

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