Hydro-ecological relations in the Delta Waters

hierarchical position. Tn the egtuarine situation the water regime is
one of the met essential factors detemising processes in soil.
gegstation and fa-. The relation between the average salt
concentation and the distrlbotion of some vegetation types is a sood
example wf this dominance (Big. 3).
Ae saon ae great changes occur in the outer cmpartntents of the made1
/by human interference of other), this has impertaut consequences for
the hierarchically lowar parts. Cholllges in the lower camgartments have
Lesa cons+quenees on the system a5 a whule. Figure 4 (derived from
Reftlnk, 1979) gzves a good exwle nf such a situation. In the wet
autnmn of 1961 severe waterlogged conditions occurred on a higher salt
marsh in the Grevdingen estaary. This was the primclry cau6e for a
die-bmk of the vevetation of Halimione pnztulacoides on that WSh. In
the long run there was, however, no chenge of the saltmarsh ecssyatem
as a whole and after sonre years the orfgmal vegetation could recover.
The erosion of a tidal flat or a salt marsh and the developmt of new
onea elsewhere in the same tidal system are an exmple of the same
e).clic dynaunica on a larger scale of space and tb.
The con$truction af the DelCa Works resulted in quite different types
of ecological dynamics. The water syetm a5 a whole changed t~tally and
resulted in a cmplere new and irrevexsihle development of water and
lad (Beeftink. 1987). After the fidehtx~g of rhe dmh many of the
estuafkre plants and animals died and nerv species had to celonize the
disturbed ecobystems. Id Che beginning the daminaring species in the
vegetation changed from year tcr pear. Later on changes took place much
more gradually. There were also marked differexices between habitats.
The vegetation of the lowar salt mrshes died totally sithin two or
three years. The higher salt marshes were well adapted 60 less frequent
inundations and vegetation changed gradually after the disappearance of
the tides. In tha reed marshes and willow woedlands of the brackish and
fresh estuaries the same pattern occurred. Theae vegetattons actually
did not die, btrt durtng some S to l@ years after damming their species
composition and structure changed very mufh.