Spatial and Temporal Wader Distribution Related to the Environment on an Intertidal Shoal in the Oosterschelde

Abstract

Intertidal areas in the Oosterschelde estuary decreases due to a disturbed sediment-balance caused by construction of a storm surge barrier and two compartmentalization dams. The intertidal area provide valuable foraging ground for the benthivore wader populations of the East-Atlantic Flyway. In 2019 seven nourishments were constructed at the Roggenplaat intertidal shoal to mitigate loss of foraging area and preserving wader populations. We study abundance of waders at the Roggenplaat prior to nourishment construction in a spatial and temporal manner and its relation with environmental factors to be able understand natural distribution of the waders and to evaluate the selected nourishment locations regarding interference with natural wader distribution. Relation between wader abundance and environmental factors like exposure time, food availability, and sediment composition increases understanding of the impact of drivers behind natural patterns in wader abundance.

This study uses intertidal shoal covering bird counts that are obtained by counting from a boat that makes three two-hour tours (period 1, 2 and 3) around the Roggenplaat on 25 counting days. Bird counts were executed from one hour after high tide up to one hour after low tide. All waders that are protected by Nature2000 regulations for the Oosterschelde are studied. Environmental data of the Roggenplaat were obtained by combining tide gauge and LIDAR measurements for exposure time and collecting field samples at 113 stations for benthic infauna abundance and sediment composition. Spatial hotspot maps of foraging wader density were created for each period to find most important foraging ground of each species. Hotspots of foraging behavior were related to spatial differences in environmental conditions by comparing mapped data.

The results show that significant temporal differences are present within a tidal cycle since less birds are counted from one hour after high tide up to three hours before low tide (period 1) than from three hours before low tide up to one hour after low tide (period 2 and 3). Spatial density hotspots and the periodic movement over the intertidal shoal of each species are divergent. Moreover, environmental factors show different relations with each species. Significant higher densities in areas with short exposure time are found for Dunlin, Bar-tailed godwit, Grey Plover, Sanderling and Common Redshank during low tide. Furthermore, there is an overlap between Oystercatcher, Red Knot, and Sanderling distribution and peak biomass or density of their preferred prey, which was not observed for the remaining wader species. Also sediment composition distribution shows a relation with wader hotspots since Sanderling mainly forage in area with larger d50 sediment grain size, while other species generally ignore this area and use foraging ground with smaller d50 sediment grain size and higher mud fraction.

Wader behavior differs in a temporal manner since tidal dynamics prevent waders from foraging in low lying intertidal area during ebbing tide. The remaining differences in spatial distribution of wader species are primarily caused by the relation of each species with environmental factors. No obvious relation between benthic infauna biomass or density peaks and wader density has been found for several species. Their distribution is however affected by food availability in another way. Tidefollowing behavior increases prey accessibility, which has been observed for Dunlin, Sanderling and Common Redshank. Distribution of Bar-tailed godwit and Grey Plover shows a relation with polychaeta dominated area despite low total biomass. This indicates the relation between wader distribution and benthic infauna goes further than a direct relation with peak density and biomass.

Considerable parts of the nourishments at the Roggenplaat overlay valuable foraging ground of most of the studied wader species, which changes the local environmental conditions. It is therefore recommended that selection criteria for suitable nourishment locations should include avoidance of foraging wader hotspots in the future. The impact of these nourishments on the value of foraging ground for wader populations can be defined by studying spatial wader abundance at the Roggenplaat also for the modified situation after construction of nourishments. The outcome should be compared to the natural wader distribution as perceived by our study to identify changes in behavior. Understanding of wader distribution in a spatial and temporal manner and incorporate this knowledge in future nourishment design approach will increase success of human interventions to preserve the deteriorating intertidal area and its valuable foraging ground.