Hydro-ecological relations in the Delta Waters

fn tidal water systems sedimentation and erosian of sediment continue
above mean low waterlevel (KLW) and even mean high waterlevel (HXW).
The lowest parts of the tidal zone have no vegetation except algae and
some water plants (Zostera spec.). These tidal flats are m important
feeding habitat for birds (Meire, et al. 1989). Some decimetres below
MEW the ffrst smi-terrestrial vegetation starts to grow (Salicornia
spec., Spartina spec.). This vegetation is usually inundated Cwice k
day. %?hen accretion contintles to above MEW, vegetation develops to a
variecy of plant communities. Geomarphology and vegeration of these
higher salt marshes are much more difgerentiated than on the mud flats
and the lower salt marshes.
The above-mentioned pattern of succession (see also Fig. 1) represents
the situation it! the sdine tidal are= of the South-West Netherlands.
In the fresh and brackish tidal water systems this pattern is not
essentially different, but there is a difference in vegetation types of
the marshes (reed, rushes and willars).
The constru~tian of the Delta Works reaulted ia an ewrmous chanse of
the estnariae environment. In most areas a more or less fixed
wacerlevel w s realized somewhat aound NAP (butcb Ordnanoe Level). The
consequence was that the lmr parts of the tidal land were pennwently
inundated, while the higher parts were hardly iufluenced by the
flooding water. Large parts of the latter could develop as (semi-)
natural ecosystms. Thfs development and ite future perspectives for
nature conservatLon are the subjects of this paper.
2 ECOLllGICBI. DYNdMtCS I N VARIOUS WATERS
The processes influencing the development of animal and plant
communities cm be ordered in an hierarchical model (Fig. 2, derived
from Bakker, et al. 1981). In this model the outer compartments
dominate the inner ones. Though there is some influence in the opposite
direction, this is of minor importance becsuse of its lower