Tjeerd J. Bouma

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.

Positive Feedbacks in Seagrass Ecosystems: Implications for Success in Conservation and Restoration

A bstractSeagrasses are threatened by human activity in many locations around the world. Their decline is often characterized by sudden ecosystem collapse from a vegetated to a bare state. In the 1930s, such a dramatic event happened in the Dutch Wadden Sea. Before the shift, large seagrass beds (Zostera marina) were present in this area. After the construction of a large dam and an incidence of the “wasting disease” in the early 1930s, these meadows became virtually extinct and never recovered despite restoration attempts. We investigated whether this shift could be explained as a critical transition between alternative stable states, and whether the lack of recovery could be due to the high resilience of the new turbid state. We analyzed the depth distribution of the historical meadows, a long-term dataset of key factors determining turbidity and a minimal model based on these data. Results demonstrate that recovery was impossible because turbidity related to suspended sediment was too high, probably because turbidity was no longer reduced by seagrass itself. Model simulations on the positive feedback suggest indeed the robust occurrence of alternative stable states and a high resilience of the current turbid state. As positive feedbacks are common in seagrasses, our findings may explain both the worldwide observed collapses and the low success rate of restoration attempts of seagrass habitats. Therefore, appreciation of ecosystem resilience may be crucial in seagrass ecosystem management.

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.

Are all intertidal wetlands naturally created equal? Bottlenecks, thresholds and knowledge gaps to mangrove and saltmarsh ecosystems

Intertidal wetlands such as saltmarshes and mangroves provide numerous important ecological functions, though they are in rapid and global decline. To better conserve and restore these wetland ecosystems, we need an understanding of the fundamental natural bottlenecks and thresholds to their establishment and long‐term ecological maintenance. Despite inhabiting similar intertidal positions, the biological traits of these systems differ markedly in structure, phenology, life history, phylogeny and dispersal, suggesting large differences in biophysical interactions. By providing the first systematic comparison between saltmarshes and mangroves, we unravel how the interplay between species‐specific life‐history traits, biophysical interactions and biogeomorphological feedback processes determine where, when and what wetland can establish, the thresholds to long‐term ecosystem stability, and constraints to genetic connectivity between intertidal wetland populations at the landscape level. To understand these process interactions, research into the constraints to wetland development, and biological adaptations to overcome these critical bottlenecks and thresholds requires a truly interdisciplinary approach.

Plant resistance to mechanical stress: evidence of an avoidance–tolerance trade-off

External mechanical forces resulting from the pressure exerted by wind or water movement are a major stress factor for plants and may cause regular disturbances in many ecosystems. A plants ability to resist these forces relies either on minimizing the forces encountered by the plant (avoidance strategy), or on maximizing its resistance to breakage (tolerance strategy). We investigated plant resistance strategies using aquatic vegetation as a model, and examined whether avoidance and tolerance are negatively correlated. We tested the avoidance-tolerance correlation across 28 species using a phylogenetically corrected analysis, after construction of a molecular phylogeny for the species considered. Different species demonstrated contrasting avoidance and tolerance and we demonstrated a significant negative relationship between the two strategies, which suggests an avoidance-tolerance trade-off. Negative relationships may result from costs that each strategy incurs or from constraints imposed by physical laws on plant tissues. The existence of such a trade-off has important ecological and evolutionary consequences. It would lead to constraints on the evolution and variation of both strategies, possibly limiting their evolution and may constrain many morphological, anatomical and architectural traits that underlie avoidance and tolerance.

Ecosystem engineering and biodiversity in coastal sediments: posing hypotheses

Coastal sediments in sheltered temperate locations are strongly modified by ecosystem engineering species such as marsh plants, seagrass, and algae as well as by epibenthic and endobenthic invertebrates. These ecosystem engineers are shaping the coastal sea and landscape, control particulate and dissolved material fluxes between the land and sea, and between the benthos and the passing water or air. Above all, habitat engineering exerts facilitating and inhibiting effects on biodiversity. Despite a strongly growing interest in the functional role of ecosystem engineering over the recent years, compared to food web analyses, the conceptual understanding of engineering-mediated species interactions is still in its infancy. In the present paper, we provide a concise overview on current insights and propose two hypotheses on the general mechanisms by which ecosystem engineering may affect biodiversity in coastal sediments. We hypothesise that autogenic and allogenic ecosystem engineers have inverse effects on epibenthic and endobenthic biodiversity in coastal sediments. The primarily autogenic structures of the epibenthos achieve high diversity at the expense of endobenthos, whilst allogenic sediment reworking by infauna may facilitate other infauna and inhibits epibenthos. On a larger scale, these antagonistic processes generate patchiness and habitat diversity. Due to such interaction, anthropogenic influences can strongly modify the engineering community by removing autogenic ecosystem engineers through coastal engineering or bottom trawling. Another source of anthropogenic influences comes from introducing invasive engineers, from which the impact is often hard to predict. We hypothesise that the local biodiversity effects of invasive ecosystem engineers will depend on the engineering strength of the invasive species, with engineering strength defined as the number of habitats it can invade and the extent of modification. At a larger scale of an entire shore, biodiversity need not be decreased by invasive engineers and may even increase. On a global scale, invasive engineers may cause shore biota to converge, especially visually due to the presence of epibenthic structures.

Flow paths of water and sediment in a tidal marsh: Relations with marsh developmental stage and tidal inundation height

This study provides new insights in the relative role of tidal creeks and the marsh edge in supplying water and sediments to and from tidal marshes for a wide range of tidal inundation cycles with different high water levels and for marsh zones of different developmental stage. Net import or export of water and its constituents (sediments, nutrients, pollutants) to or from tidal marshes has been traditionally estimated based on discharge measurements through a tidal creek. Complementary to this traditional calculation of water and sediment balances based on creek fluxes, we present novel methods to calculate water balances based on digital elevation modeling and sediment balances based on spatial modeling of surface sedimentation measurements. In contrast with spatial interpolation, the presented approach of spatial modeling accounts for the spatial scales at which sedimentation rates vary within tidal marshes. This study shows that for an old, high marsh platform, dissected by a well-developed creek network with adjoining levees and basins, flow paths are different for tidal inundation cycles with different high water levels: during shallow inundation cycles (high water level <0.2 m above the creek banks) almost all water is supplied via the creek system, while during higher inundation cycles (high water level >0.2 m) the percentage of water directly supplied via the marsh edge increases with increasing high water level. This flow pattern is in accordance with the observed decrease in sedimentation rates with increasing distance from creeks and from the marsh edge. On a young, low marsh, characterized by a gently seaward sloping topography, material exchange does not take place predominantly via creeks but the marsh is progressively flooded starting from the marsh edge. As a consequence, the spatial sedimentation pattern is most related to elevation differences and distance from the marsh edge. Our results imply that the traditional measurement of tidal creek fluxes may lead in many cases to incorrect estimations of net sediment or nutrient budgets.

Under niche construction: an operational bridge between ecology, evolution, and ecosystem science

All living organisms modify their biotic and abiotic environment. Niche construction theory posits that organism-mediated modifications to the environment can change selection pressures and influence the evolutionary trajectories of natural populations. While there is broad support for this proposition in general, there is considerable uncertainty about how niche construction is related to other similar concepts in ecology and evolution. Comparative studies dealing with certain aspects of niche construction are increasingly common, but there is a troubling lack of experimental tests of the core concepts of niche construction theory. Here, we propose an operational framework to evaluate comparative and experimental evidence of the evolutionary consequences of niche construction, and suggest how such research can improve our understanding of ecological and evolutionary dynamics in ecosystems. We advocate for a shift toward explicit experimental tests of how organism-mediated environmental change can influence the selection pressures underlying evolutionary responses, as well as targeted field-based comparative research to identify the mode of evolution by niche construction and assess its importance in natural populations.

Low-Canopy Seagrass Beds Still Provide Important Coastal Protection Services

One of the most frequently quoted ecosystem services of seagrass meadows is their value for coastal protection. Many studies emphasize the role of above-ground shoots in attenuating waves, enhancing sedimentation and preventing erosion. This raises the question if short-leaved, low density (grazed) seagrass meadows with most of their biomass in belowground tissues can also stabilize sediments. We examined this by combining manipulative field experiments and wave measurements along a typical tropical reef flat where green turtles intensively graze upon the seagrass canopy. We experimentally manipulated wave energy and grazing intensity along a transect perpendicular to the beach, and compared sediment bed level change between vegetated and experimentally created bare plots at three distances from the beach. Our experiments showed that i) even the short-leaved, low-biomass and heavily-grazed seagrass vegetation reduced wave-induced sediment erosion up to threefold, and ii) that erosion was a function of location along the vegetated reef flat. Where other studies stress the importance of the seagrass canopy for shoreline protection, our study on open, low-biomass and heavily grazed seagrass beds strongly suggests that belowground biomass also has a major effect on the immobilization of sediment. These results imply that, compared to shallow unvegetated nearshore reef flats, the presence of a short, low-biomass seagrass meadow maintains a higher bed level, attenuating waves before reaching the beach and hence lowering beach erosion rates. We propose that the sole use of aboveground biomass as a proxy for valuing coastal protection services should be reconsidered.

Aquatic interfaces: a hydrodynamic and ecological perspective

ABSTRACT Ecologically-appropriate management of natural and constructed surface water bodies has become increasingly important given the growing anthropogenic pressures, statutory regulations, and climate-change impacts on environmental quality. The development of management strategies requires that a number of knowledge gaps be addressed through interdisciplinary research efforts particularly focusing on the water-biota and water-sediment interfaces where most critical biophysical processes occur. This paper discusses the current state of affairs in this field and highlights potential paths to resolve critical issues, such as hydrodynamically-driven mass transport processes at interfaces and associated responses of organisms through the development of traits. The roles of experimental methods, theoretical modelling, statistical tools, and conceptual upscaling methods in future research are discussed from both engineering and ecological perspectives. The aim is to attract the attention of experienced and emerging hydraulic and environmental researchers to this research area, which is likely to bring new and exciting discoveries at the discipline borders.