What is a Macrophyte Patch? Patch Identification in Aquatic Ecosystems and Guidelines for Consistent Delineation

Schoelynck J, Creëlle S, Buis K, De Mulder T, Emsens W, Hein T, Meire D, Meire P, Okruszko T, Preiner S, Roldan Gonzalez R, Silinski A, Temmerman S, Troch P, Van Oyen T, Verschoren V, Visser F, Wang C, Wolters J, Folkard A, in press. . Ecohydrology & Hydrobiology. DOI 10.1016/j.ecohyd.2017.10.00

Analyse diachronique d’une région côtière : la Mer des Wadden de la Frise du Nord (Allemagne)

peer reviewedLe but de cette étude est d’analyser l’évolution récente de la région côtière de la Frise du Nord sur la côte occidentale du Schleswig-Holstein, le Land le plus septentrional de l’Allemagne. A notre disposition, nous avions des séries de cartes topographiques et bathymétriques, de photos aériennes, ainsi que des images satellitaires SPOT obtenues dans le cadre du programme OASIS, qui ont rendu possible des approches variées, tant géomorphologiques que botaniques

Unravelling the controls of lateral expansion and elevation change of pioneer tidal marshes

Many overlapping mechanisms have been proposed to control horizontal seaward expansion of marshes and rates of elevation change that are associated with it. Key questions to resolve are: i) whether simple geomorphological conditions such as elevation are a reliable predictor of marsh expansion rates; ii) whether there are seasonal vegetation-induced effects on elevation change (both, increase and decrease of elevation); and iii) how steep the spatial gradient of elevation change is from the bare tidal flat into the vegetated marsh? These questions have been addressed with a two-scale study approach performed on two contrastingly wave-exposed marshes in the Scheldt Estuary (SW Netherlands and N Belgium) where <i>Scirpus maritimus</i> is the dominant pioneer species. On the one hand, we investigated the relations between large-scale, geomorphological parameters (elevation, slope) and clonal marsh expansion rates at both sites. On the other hand, we performed a small-scale monthly field monitoring over two years at the same two marshes where we investigated the relations between spatio-temporal variations in vertical elevation change and spatio-temporal variations in vegetation properties along cross-shore transects. We found that at the sheltered site, clonal expansion rates are almost twice as high as at the exposed site. Furthermore, expansion rates at the sheltered site related well to elevation. At the exposed site, this relation was less strong as wave exposure might cause a dominant disturbance. Moreover, we found clear seasonal elevation change patterns that closely followed the seasonal vegetation cycle, with prevailing increase in elevation in summer when above-ground biomass was maximal and decrease in elevation in winter when plant shoots had largely decayed. Especially at the exposed site, the presence of vegetation has a positive effect on increase in elevation within the marsh. Finally, our results show that clonal marsh expansion succeeded at elevations for which previous studies at the same locations showed that individual shoots could not establish, emphasising the importance of clonal integration for both survival and lateral expansion in disturbance-driven ecosystems

High-resolution bed level changes in relation to tidal and wave forcing on a narrow fringing macrotidal flat : bridging intra-tidal, daily and seasonal sediment dynamics

Sediment dynamics in tidal flats, ranging from daily to seasonal timescales, are particularly relevant as they control key ecological and geomorphic processes that ultimately contribute to the long-term evolution of coastal and estuarine landscapes. Yet, insights into bed level changes, including the full range of relevant timescales from intra-tidal to daily and seasonal scales, are currently limited due to a lack of efficient methods to record high-resolution (1 year). Accordingly, this contribution intends to improve our understanding on spatio-temporal patterns of long-term (>1 year) high-resolution (daily) bed level dynamics in tidal flats in relation to the dominant hydrodynamic driving forces, namely tides and waves. Specifically, this study was conducted along two 200 m long cross-shore transects on an intertidal flat located in the macrotidal (>5 m tidal range) Scheldt estuary, Belgium. Results showed that daily bed level changes at the low tidal flat (i.e. 4.10 m below mean high tide level) were dominated by tidal currents, with a strong fortnightly (or spring-neap) signature, whereas wave activity was of secondary importance. Conversely, bed level changes in the high tidal flat (i.e. 0.65 m below mean high tide level) were almost exclusively dominated by wave activity. Additionally, seasonal deposition–erosion cycles that superimposed on the daily bed level changes were associated with the seasonality of wind wave activity and benthic biology. Analysis of wave and current-induced bed shear stresses at the respective locations confirmed this spatial variability of tidal-dominated sediment dynamics at the low tidal flat versus wave-dominance at the high tidal flat, and comparison with local critical bed shear stresses for sediment motion also revealed differences in morphological impacts of the hydrodynamics between the two transects. These distinctive responses of bed level dynamics across the tidal flat can be partly explained by the spatially varying sediment properties across the tidal flat and may be further mediated, on a seasonal time scale, by the growth of the algal mat with its effect on stabilizing the sediment bed. In view of the large spatial and temporal variability of internal and external forcing revealed in this study, comprehensive and detailed field measurements are even more necessary to understand and predict long-term bed level dynamics and related ecological implications in tidal flats

Effects of contrasting wave conditions on scour and drag on pioneer tidal marsh plants

International audienceTidal marshes are increasingly valued for protecting shorelines against wave impact, but waves in turn may limit the initial establishment of tidal marsh pioneer plants. In estuaries, the shorelines typically experience a wide range of wave periods, varying from short period wind waves (usually of around 1–2 s in fair weather conditions) to long ship-generated waves, with secondary waves in the order of 2–7 s and primary waves with periods that can exceed 1 min. Waves are known to create sediment scour around, as well as to exert drag forces on obstacles such as seedlings and adults of establishing pioneer plant species. In intertidal systems, these two mechanisms have been identified as main causes for limiting potential colonization of bare tidal flats. In this paper, we want to assess to which extent common quantitative formulae for predicting local scour and drag forces on rigid cylindrical obstacles are valid for the estimation of scour and drag on slightly flexible plants with contrasting morphology, and hence applicable to predict plant establishment and survival under contrasting wave condi- tions. This has been tested in a full-scale wave flume experiment on two pioneer species (Scirpus maritimus and Scirpus tabernaemontani) and two life stages (seedlings and adults of S. maritimus) as well as on cylindrical reference sticks, which we have put under a range of wave periods (2–10 s), intended to mimic natural wind waves (short period waves) and ship-induced waves (artificial long period waves), at three water levels (5, 20, 35 cm). Our findings suggest that at very shallow water depths (5 cm) particular hydrodynamic conditions are created that lead to drag and scour that deviate from predictions. For higher water levels (20, 35 cm) scour can be well predicted for all wave conditions by an established formula for wave-induced scour around rigid cyl- inders. Drag forces can be relatively well predicted after introducing experimentally derived drag coefficients that are specific for the different plant morphologies. Best predictions were found for plants with a simple near- cylindrical morphology such as S. tabernaemontani, but are less accurate for plants of more complex structure such as S. maritimus, particularly for long period waves. In conclusion, our study offers valuable insights towards predicting/modelling the conditions under which seedlings and shoots of pioneer species can establish, and elu- cidates that long waves are more likely to counteract successful plant establishment than natural short waves