The effect of shoreface nourishments on the coastal ecosystem of The Netherlands

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The effect of shoreface nourishments on the
coastal ecosystem of The Netherlands

• Authors Martin Baptist Mardik Leopold

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Shoreface nourishments

• A shoreface nourishment supplies sand in between
  the outer breaker bars along the coast, or on the
  offshore side of the outer bar, at depths of
  about 5 to 8 metres below sea level.

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Shoreface nourishments
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Possible ecosystem effects

• Direct effects
• burial of benthic species.
• suffocation of nearby benthos.
• increased turbidity, possibly affecting primary
  production and/or the foraging success of
  predating fish and birds.
• disturbance by noise and ship maneuvering.
• Indirect effects
• habitat change the bed level, the penetrability,
  the organic matter, grain size and silt content
  change.

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Objectives of this study

• This study investigated the relationships for
• nourishments ? Spisula subtruncata ? sea ducks
• nourishments ? fish ? fish-eating predators
• Spisula subtruncata is a bivalve, living in sandy
  sediment and serving as an important food source
  to sea ducks, for instance the common scoter
  Melanitta nigra.

www.birdpix.nl
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Spisula research

• Since 1994, the number and distribution of
  Spisula is being monitored by IMARES.
• Bed samples are taken with a bottom dredge.
• Spisula is subdivided into a 0-year class and an
  adult class.

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Spisula numbers and nourishments over time

• By combining the temporal trend of Spisula
  numbers with nourished volumes along the Dutch
  coast one can find the relationship between
  Spisula presence and nourishments.

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Spisula rise and fall per region
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Nourishments

• The change in morphology directly after the
  nourishment is found to be very local, without
  widespread effects.
• The plume of suspended sediment will be
  transported in an alongshore direction and may
  stretch over several kilometers.
• The possible effects of nourishments on Spisula
  in cross-shore direction are probably very local.

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Spisula and nourishments in more detail

• With Inverse Distance Weighting the original
  point data were interpolated, and extrapolated to
  the beach line, to yield complete coverage of
  Spisula density (numbers per m2) for the entire
  Dutch coast.
• For each nourished coastal section we calculated
  the Spisula density
• at the exact location of the nourishment (but
  this may be based on extrapolated data).
• at an extended section down to the -20 m NAP
  depth contour (but this reaches out of the
  nourished range).
• The results for a number of sections are
  presented.

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Kijkduin Ter-Heijde 2001

• This 3.6 Mm3 nourishment was applied over a
  length of 4.9 km.

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Bergen aan Zee 2000

• This 1 Mm3 nourishment was located in an area
  with high densities of 0-year Spisula, which
  stretched further offshore as well.

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Groote Keeten Zwanewater 2003

• This 2.57 Mm3 nourishment was located in an area
  where high densities of Spisula have been found
  until 2001.

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Texel De Slufter 2004

• A 2.4 Mm3 nourishment was applied over a length
  of 2.8 km.

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Terschelling 1993

• This shoreface nourishment of 2 Mm3 was applied
  over a length of approximately 4 km in between
  two breaker bars at a depth of -5 to -8 m NAP, at
  a distance of 800-1400 m from the beach pole
  line.

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Conclusions

• High density Spisula-banks were found along the
  northern Dutch coast in the second half of the
  1990s.
• Since 2001, Spisula numbers are declining.
  Although shoreface nourishment volumes are
  increasing since 2001, we cannot state that there
  is a causal relationship.
• Other causes may have triggered the decline of
  Spisula along the Dutch coast.

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Discussion

• Adult Spisula has been fished away.
• The reproduction success of Spisula is highly
  variable. Factors that influence the reproduction
  success are
• the wind direction and force during the pelagic
  larval phase.
• presence, or rather absence, of dense banks of
  adult specimens in the neighbourhood.
• climate change, resulting in a shortage of food
  or in physiological stress.
• predation on spatfall by shrimp.
• competition with Ensis americanus.