Environmental Engineering Reference
In-Depth Information
is limited by one of these nutrients rather than by light
(Colijn & Cadée 2003), an increase in organic input
would lead to an increase in primary production
(Heip 1995). One might expect that an increase in
primary production by bacteria and algae would
automatically lead to an increase in the intertidal stocks
of benthic invertebrates that feed on these primary pro-
ducers. However, despite there being some temporal
correlations between benthic biomass and nutrient
levels (Tubbs 1977, van Impe 1985, Beukema &
Cadée 1986, Beukema
et al.
2002), the evidence for a
firm causal relationship between the two is lacking
(Essink
et al.
1998). At the negative side, near the point
of discharge of enriched organic material the upper
layer of intertidal flats may become anoxic, and in
such stinking black-coloured areas benthic species
usually do not do well. However, further away from
the point of discharge, organic enrichment may be the
cause of particularly high stocks of some species.
In the Dollard estuary on the border between the
Netherlands and Germany, ragworms (
Nereis diversi-
color
) were particularly abundant near the point
where the effluents were discharged. In years when
the discharge was high, ragworm densities were high,
as was the predation pressure by waterbirds feeding
on these worms (Essink & Esselink 1998). Typically,
however, only some organisms benefit from organic
discharges. In the Dollard example decreasing eutro-
phication was associated with an increase in the
densities of the mudshrimp, the amphipod
Corophium
volutator
, and the avian predators associated with it.
13.4 Changes in salt marshes
13.4.1 Exploitation
The first colonists in north-west Europe settled on the
highest parts of the salt marsh, on levees along
watercourses in the 7th century
BC
. Farmsteads were
initially built on the marsh bed. In response to
increased risk of flooding during the next ingression,
people started to build their dwellings on artificial
mounds. The number of mounds along the north
coast of the Netherlands suggests intensive exploita-
tion of the salt marshes (Fig. 13.3). When salt marshes
extended seaward, new settlements were built on
the younger marshes until the entire coastline was
Groningen
Leeuwarden
artificial mound
open water (until AD 1000)
salt-marsh ridge (AD 700-1000)
salt marsh
fen
clay on peat
sand
Fig. 13.3
Landscape types and the occurrence of artificial mounds along the north coast of the Netherlands about
AD
1000. Rijksdienst voor Oudheidkundig Bodemonderzoek, personal communication.
