An estimation of the effects of Ensis directus on the transport and burial of silt in the near-shore Dutch coastal zone of the North Sea
Graphical abstract
Introduction
Presently the American razor clam Ensis directus is the most abundant bivalve species along the Dutch coast. This species which originates from the US east coast has rapidly spread along north western European coasts since it was first observed in the German Bight in 1979 (Von Cosel et al., 1982, Armonies, 2001, Severijns, 2002), and from where it migrated to other areas (Arias and Anadon, 2012, Severijns, 2002, Dauvin et al., 2007). E. directus prefers to live in dynamic sedimentary conditions mostly in mobile sands where it can rapidly retract itself deep into the sediment (Drew, 1907, Trueman, 1967). Annual surveys by Goudswaard et al. (2013) showed that this species is presently dominating the macrobenthic biomass in the Dutch coastal zone and that its local standing stock is substantially higher than estimates of total bivalve biomass prior to its appearance. There is some debate whether this newcomer outcompeted native species. Dannheim & Rumohr (2012) supposed that that was not the case because its preferred habitat of mobile sands with high current speeds (Dekker & Beukema, 2012) has never been fully occupied by native bivalves. Severijns (2002) furthermore argues that it is very unlikely that E. directus has outcompeted other Ensis species, since historical records suggest that latter were not very common along our coast. The other species were still found along the beaches even 10 years after E. directus had invaded Belgian waters.
Since E. directus invaded European waters it has become an ecologically important species in coastal waters. Fish and birds have started feeding on it (Tulp et al., 2010, Cadee, 2000, Wolf and Meininger, 2004) and densities have become so high that a commercial Ensis fishery has developed. Dense beds might support a higher diversity of associated macrofauna in response to associated changes in the silt and organic content (Armonies & Reise, 1999). Near the island of Sylt they found an increase of the silt and organic content in a dense E. directus bed which they ascribed to the local production of fine faecal material. Though winter storms removed this fine fraction from the surface, it was retained in deeper sediment layers at their Sylt station. Given the massive numbers of E. directus along the entire Dutch coast (Goudswaard et al., 2013) a similar entrainment of silt could possibly be a significant term in the budget of alongshore tidal transport of silt and other SPM (RIKZ, 2002). At the same time this stretch of coast is subject to erosion requiring continuous shoreface and beach nourishments for maintenance. In the period 2013–2016 roughly 28 × 106 m3 sediment is supplied on the North-Holland coast consisting mainly of sand with a small admixture of silt, which adds to the turbid coastal “river” running north. If E. directus actively influences the burial of silt its dense coastal population might have a significant influence on the transport budget of this fine sediment fraction.
Quantitative data on the rate of silt entrainment by E. directus are lacking. This study aims to assess this rate and explores Ensis' relationship with sediment grainsize on a wider scale along the coast of North-Holland. We experimentally tested whether living E. directus is capable of changing the sedimentary characteristics of a sediment core under in-situ conditions in this shallow, dynamic coastal zone, and compared the sedimentary impact of alive individuals with that of empty shells and bare sand. The experiments were conducted in conjunction with a survey on spatial distributions of this species and associated sedimentary conditions in the coastal zone of North-Holland.
Section snippets
Coastal distribution Ensis directus
In June 2011 a synoptic sampling survey was conducted covering a large part of the coastal zone of North Holland. Median grain size is 222 μm with 5.1% silt. The area is characterized by a marked seasonal cycle in bottom water temperature and primary production. At the end of summer the highest temperature reaches 18 °C. The salinity varies between 26 PSU in winter-spring and 32 PSU in summer. Maximum current speeds vary over the neap-spring cycle between 70 and 120 cm s− 1. The most near shore area
Sedimentary distribution
The average median grainsize for all stations and transects in the boxcore survey was 222 μm (± 65 sd) with 5.13% (± 4.3) silt (< 63 μm). The two sampled sediment layers (0–5 cm and 5–10 cm) did not differ in median grain size (t-test: n = 103, p = 0.59) nor in the percentage of silt (t-test: n = 103, p = 0.41). The median grain size tended to increase in northward direction but only the most northern transect differed significantly from the other transects (Tukey HSD, p < 0.05). The percentages silt showed the
Discussion
During our synoptic survey we found lowest densities of E. directus at the stations furthest offshore. The average shell length was almost double the size of those found close to the shore. The offshore animals are mainly larger and older (ring counts, Cardoso et al., 2013) suggesting that survival is better here than in the shallow nearshore area. The low BMI (0.78) of the offshore animals, however, suggests that conditions for (tissue) growth are sub optimal compared to the shallow nearshore
Acknowledgments
This work has been made possible by financial support of a grant of Ecoshape (project nos. NTW3.1–NTW2.5) within the framework of BWN and NIOZ itself. The practical work could not have been done without the enormous input of the crews of RV “Pelagia” and RV “Navicula”. We thank Carola van der Hout for her help on board. This work preceded a two years project funded by the la Mer foundation.
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