Samenvatting

The natural value of Dutch sandy beaches has been declining for decades. These relatively unstable, low nutritious, wave-washed environments lack the primary producers that represent the base of the food web. Thus, the sandy beach food web relies almost entirely on nutrient influx of other systems. Beside phytoplankton that supply nutrients to the lower beach suspension feeders, organic beach wreck deposits provide one of the two major foundations of food supply for the successive trophic levels in sandy beach ecosystems. The combination of the unpleasant odours caused by the natural decomposition and the plastic and other human litter poses a problem for tourism and beach-users. To rid tourists and beach-users from unpleasantries, beach management often resolves to “clean” the beach from organic wreck and plastic. This practice, however, represents a problem for the natural value of Dutch beaches, as one of the major food sources enhancing the biodiversity and abundance of life along the coastline is compromised. One of the key species groups that inhabit sandy beach ecosystems in the Netherlands are beach-nesting birds. Ringed Plovers (Charadrius hiaticula) are vulnerable migratory beach-nesting birds that, without interference, may be unable to successfully breed in the Netherlands. The province of Zeeland has a specific responsibility to protect Ringed Plover, as around 30 percent of the total Dutch population breed in the area. To facilitate presence of breeding pairs with nests, and ultimately increase breeding success, it is necessary to adjust beach management to complement essential habitat requirements. While the breeding success, presence and migration patterns of Ringed Plover in the Netherlands are well-studied topics, there seems to be relatively little known about the importance and usage of food resources. For the identification of important foraging habitats, it is essential to gain insight on what important food sources on sandy beaches in the province of Zeeland are, what types of prey are available, and in what microhabitat or zones disturbance may limit foraging time or opportunities.

This study investigated foraging activity and disturbance of present breeding pairs of Ringed Plover on the beach of Oranjezon and the Veerse Gatdam / Banjaardstrand, the Netherlands. The main objectives are: (I) To determine the relative importance of key foraging habitats, (II) To identify major disturbance factors associated with the habitats, (III) To find correlations between the foraging activity and disturbance on the foraging habitats with e.g. time-based parameters. Three key foraging habitats were investigated in this study: the intertidal area, fresh wash-up (depositions by prior high tide) and the winter flood-mark (older depositions of spring or storm tides). Foraging activity, presence of potential disturbance factors and disturbances was monitored in defined plots, during roughly 300 5-minute blocks, from a stationary lookout during a 5-week period (March – May 2020). Prey types were sampled in by estimating abundance in 0.5m2 quadrants on the tidal deposition habitats, and counting polychaete holes in quadrants, and sediment samples of 50cm top layer on the intertidal area.

Results showed that foraging activity was highest on the intertidal area, but also evident on both tidal deposition habitats. General presence was highest on the winter flood-mark, indicating it is primarily used for other purposes than foraging. Foraging on the winter flood-mark may be a more opportunistic and sporadic feeding behaviour, as Ringed Plover may pick-up prey when they notice it nearby. Prey types were abundant on both tidal deposition habitats, with the highest abundance of Diptera, Amphipoda and Coleoptera. High abundance of Amphipoda were seen during night-time. Although the sample size was small, results indicate that foraging activity during the night was high on the fresh wash-up. These results therefore indicate that Ringed Plover may utilize the high Amphipoda activity during the night. It is stressed that the importance of both tidal deposition habitats may be of greater importance to chicks than results of this study have shown, as the proportion of insects is higher in the diet of chicks. Prey type abundance was extremely low on the intertidal area. Possibly the sampling method missed small Polychaetes (mesh size 0.5mm), or coarse sand and a dynamic intertidal area resulted in low densities. Ringed Plover were observed foraging on the banks of tideways present on the beach, where washed-up prey, or small Polychaetes may accumulate. Moreover, no significant relation was found between foraging activity and tidal phase on the tidal deposition habitats, but there seemed to be a slight preference for foraging on the intertidal area during a ebbing tide, which could be explained by the high macrozoobenthic activity during a retrieving tide. However, foraging activity was also evident during a rising tide. It is suggested that tideways increase the foraging time on the intertidal area and may also offer different prey types not traditionally associated with the intertidal area e.g. Diptera. Ringed Plover seemed to prefer foraging during the evening, which was also the period most disturbances occurred. Disturbance was induced most frequently by pedestrians, dogs on a leash and stationary beach-visitors. Disturbances were notably low (28 cases out of roughly 300 records). Disturbances were not significantly higher during higher tides, which was surprising since the beach is narrower during high tide, and RP may not have as much opportunity to avoid potential disturbance factors. This study took place during the worldwide corona-virus pandemic under extensive measures taken by the government and, as a result, (foreign) tourist and other big crowds were completely absent from the study area. Recreational intensity was likely too low to significantly dictate foraging activity and habitat use in the study area.
It is suggested that halting cleaning the beach of organic wash-up may improve breeding success and regulating access around tidal deposition habitats near or in breeding territories may increase foraging opportunities and ultimately increase breeding success. It is suggested that limiting or halting cleaning the beach of tidal depositions near present breeding territories or potential breeding habitat will increase food availability, and likely for chicks in particular. It is advised to leave both tidal depositions, the fresh wash-up and winter flood-mark, where occurring naturally.