B. Walles1*, S. van Donk1, V. Escaravage1, T. Ysebaert1, P.L.M. de Vet2,3, J.J. van der Werf2, F. Aarts4,T. Bouma5, J. van Belzen5, K. Buis6, C. Schipper6, E. van Zanten6
1 Wageningen Marine Research; 2 Deltares; 3 Delft University of Technology; 4 Deltamilieu projecten; 5 NIOZ; 6Rijkswaterstaat
Estuaries are world-wide under pressure due to anthropogenic activities and climate change. Many face system-wide erosion resulting in loss of intertidal areas, leading to negative ecological consequences. Intertidal areas provide important foraging grounds for shorebirds along flyways. Loss in one estuary can affect populations on a global scale. It is therefore important to mitigate locally. The Eastern Scheldt estuary is a drowning estuary in the southwest of the Netherlands. In 2019, 1.13 million m3 of sediment was nourished on the Roggenplaat (1440 ha), covering 15% (211 ha) of the tidal shoal with a 30-80cm thick layer of sediment, to preserve foraging habitat for the coming 25 years.
The nourishment buried locally most of bird's prey (benthic macrofauna). Through recolonization prey abundance recovers over time. However, it is unknow how much time this takes. The Roggenplaat nourishment consist of seven elements (Figure 1), each element is unique regarding their location, elevation and thickness of the sediment layer. Differences between these elements enables us to study how nourishment design affects recolonization speed of the benthic macrofauna. These insights could steer optimisation of future nourishment designs and inform coastal managers about the potential of this mitigation measure to other estuaries worldwide.
Figure 1: Areal pictures before (left) and after (right) nourishment placement (image courtesy: satelietdataportal.nl).
Since 2015 an extensive long-term eco-morphological monitoring programme is executed on the Roggenplaat. Benthic macrofauna was sampled before and after the nourishment to study changes at tidal shoal scale (113 sampling stations) and nourishment scale (120 sampling stations). Total bird numbers foraging on the tidal shoal where counted 8 times a year. During bird counts, spatial and temporal data were collected enabling us to analyse spatial heatmaps of the birds.
Recolonization of benthic macrofauna differed per nourishment element and showed a trend with thickness of the sediment layer. Two years after nourishment placement, worm biomass was restored to the pre-nourishment situation on the thinnest nourishments. Shellfish biomass increased significantly between the first and second year after nourishment placement. Total bird numbers foraging on the tidal flat did not change before and after the nourishment. An increase in habitat use occurred in areas which were not nourished, whereas nourished areas were avoided even two years after construction, despite the increased biomass of prey species. These result indicates that the carrying capacity of the Roggenplaat seems to be sufficient to buffer for a temporal decline in foraging area. This short-term decline is necessary to mitigate future recession. In the long run, nourished areas are expected to be fully recolonized and become valuable again due to their increased emersion time. From this we learned that, in order to compensate for the temporary loss of carrying capacity due to nourishment, undisturbed areas needs to be of sufficient quality and quantity to support birds visiting the tidal shoal.