Distribution of extracellular carbohydrates in three intertidal mudflats in Western Europe

In this study, the spatial distribution of two operationally defined extracellular carbohydrate fractions (water- and EDTA-extractable carbohydrates) were examined in three intertidal mudflats in Western Europe (Dollard, the Netherlands; Marennes, France; Humber, UK). The three mudflats were sampled along cross-shore transects and sediment cores were sliced to a depth of 5 cm. In these mudflats, diatoms were the dominant component of the microphytobenthos. Carbohydrate content showed little variation with depth, but it varied along transects within each mudflat. Carbohydrate contents were also significantly different between the mudflats, and the carbohydrate contents of the stations within a mudflat grouped together resulting in separate clusters. This was also observed when the Marennes mudflat was investigated on a temporal scale. These results suggest that processes that act on the scale of whole mudflats determine the variations in extracellular carbohydrate contents. In the surface 0.5 cm of the sediment, water-extractable carbohydrates showed a correlation with both chlorophyll a content and median grain size, while EDTA-extractable carbohydrates were only correlated with median grain size. Incubation experiments also showed the importance of microphytobenthos as a source of extracellular carbohydrate, especially when subjected to the light. Analyses of the monosaccharide distribution of the carbohydrate fractions revealed that the carbohydrate composition was largely similar between the areas investigated. Structurally, the carbohydrates found in these sediments seem to represent a biorefractory part of the freshly produced carbohydrates that remained after rapid degradation of the more labile component [KEYWORDS: Extracellular carbohydrate; Microphytobenthos; Grain size distribution; Tidal flats; Monosaccharide distribution; Marennes-Oléron Bay; Ems-Dollard estuary; Humber estuary

Destabilization of cohesive intertidal sediments by infauna

Bioturbation activity was reduced in four plots on an intertidal mudflat in the Humber estuary (UK) during 4 days, by spraying the sediment with an insecticide, namely vydate. Macrofaunal, especially Nereis diversicolor and oligochaeta, and meiofaunal densities decreased, while the diatom biomass did not change. This resulted in a 300% increase in sediment stability, caused by a reduction in bioturbation and grazing pressure and a decrease in the water content [KEYWORDS: sediment stability, benthos, erosion, intertidal flats, Humber]

The influence of Corophium volutator abundance on resuspension

Two experiments were performed to test the hypothesis that Corophium volutator affects the turbidity of water in estuaries through active resuspension of sediment. One experiment was done in a flume system under different flow velocities, and one in aquaria. A diatom film developed at the sediment surface in both experiments before Corophium was added. This diatom film was supposed to have a stabilising effect on the sediment. In both experiments, the concentration of suspended solids in the water column increased with the density of Corophium individuals. No effect of flow velocity on suspended solids concentration was found. This indicates that, in our flume experiment, active resuspension by Corophium was more important than physical resuspension, at least at low flow velocity (<20 cm s(-1)) and in the presence of a diatom him. The critical erosion threshold decreased with increasing Corophium density in the aquarium experiment, indicating that indirect effects of Corophium grazing may become more important at high levels of bottom shear stress. The implications of our findings for suspended solids concentration in estuarine systems are discussed. [KEYWORDS: Corophium; resuspension; turbidity; sediment stability Intertidal sediments; burrowing invertebrates; southeastern england; wadden sea; stability; bioturbation; behavior strength; diatoms; mudflat]

The influence of <i>Corophium volutator</i> abundance on resuspension

Two experiments were performed to test the hypothesis that Corophium volutator affects the turbidity of water in estuaries through active resuspension of sediment. One experiment was done in a flume system under different flow velocities, and one in aquaria. A diatom film developed at the sediment surface in both experiments before Corophium was added. This diatom film was supposed to have a stabilising effect on the sediment. In both experiments, the concentration of suspended solids in the water column increased with the density of Corophium individuals. No effect of flow velocity on suspended solids concentration was found. This indicates that, in our flume experiment, active resuspension by Corophium was more important than physical resuspension, at least at low flow velocity (-1) and in the presence of a diatom film. The critical erosion threshold decreased with increasing Corophium density in the aquarium experiment, indicating that indirect effects of Corophium grazing may become more important at high levels of bottom shear stress. The implications of our findings for suspended solids concentration in estuarine systems are discussed

Interplay between biology and sedimentology in a mudflat (Biezelingse Ham, Westerschelde, The Netherlands)

The aim of this research was to investigate the importance of biological processes on the sediment characteristics and the morphology of a mudflat in the Westerschelde (The Netherlands). For this purpose, a transect in the Biezelingse Ham mudflat was sampled on a monthly basis. In spring, the muddy part of the mudflat was dominated by a biofilm of microphytobenthos that altered the morphology of the mudflat and resulted in a two- fold increase in sediment stability. The biofilm also bound fine-grained sediment that was deposited. From June onwards, wind generated waves dominated the conditions at the mudflat which resulted in the disappearance of diatom biofilms, and caused a gradual erosion of the mudflat. During this period, meio- and macrofauna densities increased from which it was concluded that the hydrodynamic forces did not have a big impact on these communities. Spatial variations in sediment characteristics, morphology and biology were observed between stations 1 and 2 on the one hand and station 3 on the other. This resulted in different responses to the changing conditions on the mudflat. In general, the results from this field study indicate that sedimentology and biology, interact in a complex manner with the hydrodynamic regime both on a temporal as well as on a spatial scale. [KEYWORDS: estuary; mudflat morphology; extracellular polymeric substances; sediment characteristics; microphytobenthos; hydrodynamics Intertidal cohesive sediments; epipelic diatoms; biogenic stabilization; corophium volutator; benthic diatoms; microphytobenthos; stability; erodibility; sandflat; bacteria]

Spatial patterns of benthic diatoms, carbohydrates and mud on a tidal flat in the Ems-Dollard estuary

The chlorophyll a content and two operational fractions of carbohydrate (water extractable and EDTA extractable) were measured every three months during one year along transects on a tidal flat in the Ems-Dollard estuary (The Netherlands). Chlorophyll a was used as an indicator of microphytobenthos biomass, which was composed predominantly of epipelic diatoms. Both carbohydrate fractions correlated significantly with chlorophyll a. EDTA extractable carbohydrates were more resistant towards degradation than the water extractable fraction. During most of the year, concentrations of chlorophyll a and carbohydrates were low, but in June, high concentrations of up to 90 mug chlorophyll a/g sediment were found in a narrow zone running parallel to the channel. Maximum concentrations of water extractable carbohydrates and EDTA extractable carbohydrates ranged between 800-1200 and 600-800 mug/g sediment, respectively. The mud content was high (+/- 90%) at the margin of the tidal flat. This was not limited to the growth season of the diatoms, but was observed throughout the year. This indicated that the high mud content at the mudflat margin was mainly caused by hydrodynamic factors, and not by biostabilization. In June, exceptionally high diatom densities were found in sediment with a high mud content. There was only minor evidence that biostabilization by epipelic diatoms lead to a further increase in the mud content of the sediment. [KEYWORDS: carbohydrates; chlorophyll a; Ems-Dollard estuary; estuarine sedimentation; intertidal environment; phytobenthos Dutch wadden sea; chlorophyll-a; intertidal sediments; epipelic diatoms; dynamics; microphytobenthos; biomass; stabilization; resuspension; populations]