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Abstract: An experiment was performed to test an alternative dredging strategy for the Westerschelde estuary. Clean sand dredged from the navigation channel was disposed seawards of an eroding intertidal flat in order to modify morphology and hydrodynamics, improving the multi-channel system with ecologically productive shallow water habitat. Five years of intensive monitoring revealed that part of the disposed sediment moved slowly towards the flat, increasing the very shallow subtidal and intertidal area, as planned. The sand in the impact zone became gradually finer after disposal, possibly due to reduced current velocities. Nevertheless, no changes in macrobenthic biomass, density, species richness and composition were detected in the subtidal zone, also demonstrating rapid macrobenthic recovery. In the intertidal zone, no ecological effects could be revealed superimposed on trends associated with long-term sediment fining. Thus, despite morphological success and absence of detected negative ecological impacts of the experiment, new beneficial habitat was not created. [ABSTRACT FROM AUTHOR]

Abstract: The structure of the benthic boundary layer over a bed of mussels (Mytilus edulis) was investigated in a large racetrack flume. Flow was observed to be modified both by the physical roughness of the mussel bed and by the momentum input of the exhalent jets of the mussels. Particularly when the mussels were closed, and filtering activity was reduced to a minimum, we observed an internal boundary layer, around 4 cm thick, within the log layer. This internal boundary layer was often masked when the mussels were filtering actively. The presence of an internal boundary layer indicates that the boundary layer is not only structured by friction drag, but that form drag due to roughness elements also plays an important role. Consequently, estimates of bed shear stress based on velocity or Reynolds stress measurements carried out more than a few cm above the bed may be inaccurate. Over inactive mussels the shear velocity in the internal boundary layer (the roughness sub-layer) is smaller and bed shear stress is consequently reduced. Filtration activity of the mussels increased the velocity gradient in the lower layer at low and intermediate velocities, but at higher flow rates velocity profiles were not affected. Clear effects of the exhalent jets on absolute levels of TKE could be measured at all ambient velocities, while the effect on the Reynolds stress was limited. Velocity normalised TKE and Reynolds stress also indicated that the effect of the siphonal currents was limited at high velocities. Our results indicate that mussel filtration activity may have an important effect on exchange processes at the sediment-water interface, but that the extent of the effect is highly dependent on the ambient flow conditions. [Copyright &y& Elsevier]

The increasing pressure on Earth's ecosystems due to climate change is becoming more and more evident and the impacts of climate change are especially visible on coral reefs. Understanding how climate change interacts with the physical environment of reefs to impact coral growth and reef development is critically important to predicting the persistence of reefs into the future. In this study, a biophysical model was developed including four environmental factors in a feedback loop with the coral's biology: (1) light; (2) hydrodynamics; (3) temperature; and (4) pH. The submodels are online coupled, i.e. regularly exchanging information and feedbacks while the model runs. This ensures computational efficiency despite the widely-ranged timescales. The composed biophysical model provides a significant step forward in understanding the processes that modulate the evolution of coral reefs, as it is the first construction of a model in which the hydrodynamics are included in the feedback loop. • A biophysical model framework for coral reef evolution is developed. • The model can be used to predict the coral response to the environment via process-based relations. • The model bridges the gap in timescales of processes from seconds to millennia. • Model predictions are within the accuracy of climate projections. • The model is an efficient tool for forecasting coral reef development to inform policy makers. [ABSTRACT FROM AUTHOR]

Abstract: The settlement stage of shellfish larvae is important in determining distribution patterns of adults. In order to reach the substratum, the larvae need to cross the benthic boundary layer near the sediment surface. Suspended larvae reach the substratum by a combination of passive sinking, turbulent advection and possibly active swimming. Subsequently the larvae need a certain amount of time in contact with the substratum to attach themselves to the bed. Flumes are commonly used experimental set-ups to study larval settlement in flowing water. Since turbulence plays an important role in the vertical transport of larvae and in the probability of resuspension, such experiments must take turbulence scaling into account. This study compares turbulence characteristics in the flume at the NIOO-CEME with field conditions encountered by intertidal bivalve larvae during their settlement stage. Turbulence in the flume was manipulated throughout the water column using a grid and in the benthic boundary layer by introducing roughness structures. Hydrodynamic parameters such as shear velocity (u ⁎), roughness height (z0), turbulence intensity (TI) and Reynolds stress were determined. We used the dimensionless Rouse number to scale the ratio between advective motion and random (turbulent) motion. Relative particle concentrations (Rouse distribution) near the bottom, as calculated theoretically for the flume under normal conditions and in the field, show that the flume is biased towards sinking. In the Oosterschelde, where flow is tidally driven, the ratio between advective motion and random (turbulent) motion for flow velocities around 0.35 m s− 1 is balanced. In this situation a small change in directional movement (e.g. by swimming) may have an effect on settlement success of larvae. The grid has a modifying effect on mixing throughout the water column, but close to the bottom turbulence intensities are similar to the situation without a grid. Turbulence introduced by bottom roughness has a different effect from that introduced by a grid. Turbulence introduced by bottom roughness has only minor effects on mixing higher up in the water column, but due to the wake-flow has dramatic effects on near-bed turbulence. We discuss the effects of turbulence generated at the bottom on larval settlement as a case study. Both grids and bottom roughness used to induce turbulence in the flume are valuable tools in scaling turbulence parameters for quantitative research on larval settlement. [Copyright &y& Elsevier]

Variations in abundance and diversity of estuarine benthic macrofauna are typically described along the salinity gradient. The influence of gradients in water depth, hydrodynamic energy and sediment properties are less well known. We studied how these variables influence the distribution of subtidal macrofauna in the polyhaline zone of a temperate estuary (Westerschelde, SW Netherlands). Macrofauna density, biomass and species richness, combined in a so-called ecological richness, decreased with current velocities and median grain-size and increased with organic carbon of the sediment, in total explaining 39% of the variation. The macrofauna community composition was less well explained by the three environmental variables (approx. 12–15% in total, with current velocity explaining approx. 8%). Salinity, water depth and distance to the intertidal zone had a very limited effect on both ecological richness and the macrofauna community. The proportion of (surface) deposit feeders (including opportunistic species), decreased relative to that of omnivores and carnivores with increasing current velocity and sediment grain-size. In parallel, the proportion of burrowing sessile benthic species decreased relative to that of mobile benthic species that are able to swim. Correspondingly, spatial variations in hydrodynamics yielded distinct hotspots and coldspots in ecological richness. The findings highlight the importance of local hydrodynamic conditions for estuarine restoration and conservation. The study provides a tool based on a hydrodynamic model to assess and predict ecological richness in estuaries. [ABSTRACT FROM AUTHOR]

The natural coastal hydrodynamics and morphology worldwide is altered by human interventions such as embankments, shipping and dredging, which may have consequences for ecosystem functionality. To ensure long-term ecological sustainability, requires capability to predict long-term large-scale ecological effects of altered hydromorphology. As empirical data sets at relevant scales are missing, there is need for integrating ecological modeling with physical modeling. This paper presents a case study showing the long-term, large-scale macrozoobenthic community response to two contrasting human alterations of the hydromorphological habitat: deepening of estuarine channels to enhance navigability (Westerschelde) vs. realization of a storm surge barrier to enhance coastal safety (Oosterschelde). A multidisciplinary integration of empirical data and modeling of estuarine morphology, hydrodynamics and benthic ecology was used to reconstruct the hydrological evolution and resulting long-term (50 years) large-scale ecological trends for both estuaries over the last. Our model indicated that hydrodynamic alterations following the deepening of the Westerschelde had negative implications for benthic life, while the realization of the Oosterschelde storm surge barriers had mixed and habitat-dependent responses, that also include unexpected improvement of environmental quality. Our analysis illustrates long-term trends in the natural community caused by opposing management strategies. The divergent human pressures on the Oosterschelde and Westerschelde are examples of what could happen in a near future for many global coastal ecosystems. The comparative analysis of the two basins is a valuable source of information to understand (and communicate) the future ecological consequences of human coastal development. [ABSTRACT FROM AUTHOR]