M. de Vries

TRADE‐OFFS RELATED TO ECOSYSTEM ENGINEERING: A CASE STUDY ON STIFFNESS OF EMERGING MACROPHYTES

Biologically mediated modifications of the abiotic environment, also called ecosystem engineering, can significantly affect a broad range of ecosystems. Nevertheless, remarkably little work has focused on the costs and benefits that ecosystem engineers obtain from traits that underlie their ecosystem engineering capacity. We addressed this topic by comparing two autogenic engineers, which vary in the degree in which they affect their abiotic environment via their physical structure. That is, we compared two plant species from the intertidal coastal zone (Spartina anglica and Zostera noltii), whose shoots are exposed to similar currents and waves, but differ in the extent that they modify their environment via reduction of hydrodynamic energy. Our results indicate that there can be trade-offs related to the traits that underlies autogenic ecosystem engineering capacity. Dissipation of hydrodynamic forces from waves was roughly a factor of three higher in vegetation with stiff leaves compared to those with flexible leaves. Drag was highest and most sensitive to hydrodynamic forces in stiff vegetation that does not bend with the flow. Thus, shoot stiffness determines both the capacity to reduce hydrodynamic energy (i.e., proxy for ecosystem engineering capacity) and the drag that needs to be resisted (i.e., proxy for associated costs). Our study underlines the importance of insight in the trade-offs involved in ecosystem engineering as a first step toward understanding the adaptive nature of ecosystem engineering.

Vegetation causes channel erosion in a tidal landscape

Vegetation is traditionally regarded to reduce the erosion of channels in both fl uvial and tidal landscapes. We present a coupled hydrodynamic, morphodynamic, and plant growth model that simulates plant colonization and channel formation on an initially bare, fl at substrate, and apply this model to a tidal landscape. The simulated landscape evolution is compared with aerial photos. Our results show that reduction of erosion by vegetation is only the local, on-site effect operating within static vegetation. Dynamic vegetation patches, which can expand or shrink, have a contrasting larger scale, off-site effect: they obstruct the fl ow, leading to flconcentration and channel erosion between laterally expanding vegetation patches. In contrast with traditional insights, our fi ndings imply that in tidal landscapes, which are colonized by denser vegetation, channels are formed with a higher channel drainage density. Hence this study demonstrates that feedbacks between vegetation, fl ow, and landform have an important control on landscape evolution.

Impact of vegetation on flow routing and sedimentation patterns: three-dimensional modeling for a tidal marsh

[1] A three-dimensional hydrodynamic and sediment transport model was used to study the relative impact of (1) vegetation, (2) micro-topography, and (3) water level fluctuations on the spatial flow and sedimentation patterns in a tidal marsh landscape during single inundation events. The model incorporates three-dimensional (3-D) effects of vegetation on the flow (drag and turbulence). After extensive calibration and validation against field data, the model showed that the 3-D vegetation structure is determinant for the flow and sedimentation patterns. As long as the water level is below the top of the vegetation, differences in flow resistance between vegetated and unvegetated areas result in faster flow routing over unvegetated areas, so that vegetated areas are flooded from unvegetated areas, with flow directions more or less perpendicular to the vegetation edge. At the vegetation edge, flow velocities are reduced and sediments are rapidly trapped. In contrast, in between vegetated areas, flow velocities are enhanced, resulting in reduced sedimentation or erosion. As the water level overtops the vegetation, the flow paths described above change to more large-scale sheet flow crossing both vegetated and unvegetated areas. As a result, sedimentation patterns are then spatially more homogeneous. Our results suggest that the presence of a vegetation cover is the key factor controlling the long-term geomorphic development of tidal marsh landforms, leading to the formation of (1) unvegetated tidal channels and (2) vegetated platforms with a levee-basin topography in between these channels.

Stability of river bifurcations in ID morphodynamic models

Based on model-technical as well as physical considerations a nodal-point relation at bifurcations is proposed for one-dimensional (ID) network morphodynamic models: the ratio between the sediment transports into the downstream branches is proportional to a power of the discharge ratio. The influence of the nodal-point relation on the behaviour of the morphodynamic model is analyzed theoretically. The exponent in the nodal-point relation appears to be crucial for the stability of the bifurcation in the model. For large values of the exponent, the bifurcation is stable, i.e. the downstream branches remain open. For small values of the exponent, the bifurcation is unstable: only one of the branches tends to remain open. The exponent also has a strong influence on the morphological time scales of the network. The conclusions from the analysis have been verified by numerical simulations using a package for one-dimensional network modelling.

Flow hydrodynamics on a mudflat and in salt marsh vegetation: identifying general relationships for habitat characterisations

We present an overview of a large collaborative field campaign, in which we collected a long-term (months) high-resolution (4 Hz measurement frequency) hydrodynamic data set for several locations at the mudflat–salt marsh ecosystem and linked this to data on sediment transport and to a biological description of the organisms on the mudflat and the marsh. In this paper, part of this database has been used to identify general relationships that can be used for making hydrodynamic characterisations of mudflat–salt marsh ecosystems. We observed a clear linear relation between tidal amplitude and the maximum current velocity, both at the mudflat as well as within the marsh vegetation. Velocities in the vegetation were however a magnitude lower than those on the mudflat. This relationship offers promising possibilities for making hydrodynamic habitat characterisations and for validating hydrodynamic models.

Flow interaction with dynamic vegetation patches: Implications for biogeomorphic evolution of a tidal landscape

Feedback between vegetation growth, water flow, and landform is important for the biogeomorphic evolution of many landscapes, such as tidal marshes, alluvial rivers, and hillslopes. While experimental studies often focus on flow reduction within static homogeneous vegetation, we concentrate on flow acceleration around and between dynamically growing vegetation patches that colonize an initially bare landscape, with specific application to Spartina anglica, a pioneer of intertidal flats. Spartina patches were placed in a large-scale flow facility of 16 × 26 m, simulating the growth of two vegetation patches by increasing the patch diameter (D = 1–3 m) and decreasing the interpatch distance (d = 2.3–0 m). We quantified that the amount of flow acceleration next to vegetation patches, and the distance from the patch where maximum flow acceleration occurs, increases with increasing patch size. In between the patches, the accelerated flow pattern started to interact as soon as D/d ≥ 0.43–0.67. As the patches grew further, the flow acceleration increased until D/d ≥ 6.67–10, from which the flow acceleration between the patches was suppressed, and the two patches started to act as one. These findings are in accordance with theory on flow around and between nonpermeable structures; however, the threshold D/d values found here for permeable vegetation patches are higher than those for nonpermeable structures. The reported flow interactions with dynamic vegetation patches will be essential to further understanding of the larger-scale biogeomorphic evolution of landscapes formed by flowing water, such as tidal flats, floodplain rivers, and hillslopes.

Comparing ecosystem engineering efficiency of two plant species with contrasting growth strategies

Many ecosystems are greatly affected by ecosystem engineering, such as coastal salt marshes, where macrophytes trap sediment by reducing hydrodynamic energy. Nevertheless, little is known about the costs and benefits that are imposed on engineering species by the traits that underlie their ecosystem engineering capacity. We addressed this topic by comparing ecosystem engineering efficiency defined as the benefit-cost ratio per unit of biomass investment for two species from the intertidal habitat: the stiff grass Spartina anglica and the flexible grass Puccinellia maritima. These species were selected for their ability to modify their habitat by trapping large quantities of sediment despite their contrasting growth form. On a biomass basis, dissipation of hydrodynamic energy from waves (a proxy for benefits associated with ecosystem engineering capability as it relates to the sediment trapping capability) was strikingly similar for both salt marsh species, indicating that both species are equally effective in modifying their habitat. The drag forces per unit biomass (a proxy for costs associated with ecosystem engineering ability as it relates to the requirements on tissue construction and shoot anchoring to prevent breaking and/or washing away) were slightly higher in the species with flexible shoots. As a result, stiff Spartina vegetation had slightly higher ecosystem engineering efficiency, due to lower engineering costs rather than to a higher engineering effect. Thus, Spartina is a slightly more efficient rather than a more effective ecosystem engineer. Ecosystem engineering efficiency was found to be a species-specific characteristic, independent of vegetation density and relatively constant in space. Analyzing ecosystem engineering by quantifying trade-offs offers a useful way toward developing a better understanding of different engineering strategies.

Cadmium and copper accumulation in the common mussel Mytilus edulis in the Western Scheldt estuary: a model approach

The Western Scheldt of the Dutch Delta area is severely contaminated with trace metals. Accumulation models of trace metals in the mussel Mytilus edulis are required to predict the biological efficiency of reductions in the metal and organic matter load. Two models are constructed: a black-box model and a physiologically structured model. The black-box model predicts metal accumulation in mussels from uptake and elimination parameters. The physiological model attempts to improve predictions by taking into account the kinetics of individual uptake and elimination routes. These in turn, are taken as depending upon two more general physiological processes, the ventilation rate and the metabolic rate. Metal uptake via food and water are expressed as relative fractions. Metal input is differentiated into particulate adsorbed, and dissolved species.The reliability of the two models is evaluated by comparing predicted concentrations for mussels with measurements. Model predictions for copper deviate less than 100% from measured concentrations, but neither model appears to predict cadmium concentration with sufficient accuracy since deviations of more than 100% occured. The introduction of physiological refinements did not improve performance. Food mediated contributions for cadmium and copper to total body burden had been overestimated in the model by a factor of 100 when compared to literature values. The physiological model did predict that the ratio of food mediated contribution to total body burden is probably different for cadmium and copper and decreases with increasing salinity for both. As yet there are no measurements available to confirm such predictions.We conclude that additional laboratory experiments should be done for a better understanding of why there is poor agreement between the few field observations and the simulations. In these experiments mussels grown under different environmental condition can be tested for their accumulation capacity of trace metals. More field observations are needed.

Conflict resolution support for air traffic control based on solution spaces: Design and implementation

In earlier research, it has been shown that air traffic controllers can benefit from support tools when performing their conflict detection and resolution task. The support tools used in this research vary from alerting the controller when a conflict is detected to automatically generating a conflict resolution. To take advantage of the controllerspsila knowledge and experience the support tools should show the solution space rather than only one computed solution. The solution space shows the controller options to create a conflict avoidance maneuver or displays the information the computed solution is based on. A case study has been performed to identify the information currently used to construct an avoidance maneuver. The study showed that controllers determine the avoidance maneuver by estimating the current and future air traffic situation using extrapolated tracks and experience regarding the location of busy areas. Besides future aircraft positions, the solution space comprises all data that has an influence on the avoidance maneuver. Information in the solution space can be based on prediction with an uncertainty distribution. It is important to show the controller the reliability of the predictions and consequently must be included in the solution space. The research described in this paper aims to develop the data structure needed to integrate the different constraints including the uncertainty distribution. Subsequently, the implementation of the concept in a first prototype is discussed.

Integrated conflict resolution for efficient autonomous hazard avoidance

During a mission, unmanned aerial vehicles (UAVs) are exposed to several types of hazards. These hazards can be divided into two main categories: the static part and the dynamic part of the environment. Databases with the known elements in the static environment and the available information on the dynamic environment form the basis for planning a conflict-free route. New information on the dynamic part of the environment may become available during the mission, thus, possible conflicts with the dynamic part of the environment must be detected and avoided in flight. Since performing an avoidance maneuver may lead to a new conflict with another part of the environment, it is necessary to integrate constraints from the available conflict-detection functions when determining an avoidance maneuver. This paper aims to analyze different approaches to developing an integrated conflict-resolution function. Subsequently, the implementation of the concept in a simulation environment is discussed.