l
'/0111'11111nfCo(lsra/("olJs('r:
,: F:L'CC: Of'" :;,,'f'rn<
,liio/!
"I'sal(/,
(
5:,r:9~':O; 1C<i9.'d
1///!..
,
J II! Swe
/~N Jji
69
(
'/!'
Ecolo (rical effects of dumping Qfdredged sediments;
.
b..,
'.'
options for management
Essink, I(ar~l
Minisq)'
,
, of Transport, Public Works and Water Managemelit, Nationa~ Institute/or C9Qs:al,and
Marine Management! RIKZ, P. O. Box 207, NL-9750 AE. H.arel1.The N~therlands,
Fax +3150534(j772; E-mail k.essink@rik::..rws.m!TIvenw.11l
,
.
Dred5!edsedimentSrangefrom mud to slIt. The -dredged
material ~is partly .contaminated to suchan e~tent.that.
dumpingisa.I1owed only in contained sites, of which the
so-called ',S:ijJftf'r'ne.arfIObkbfHolfand with a capacity
of 120 x}06 m3 is. the biggest CVelling2,pers. corom.).
According to actual Dutch policy, only non- and low- .
~ated
dredii'ea-SedimIntS.7iii"'a;Howed"to-'15e-
Abstrat:t. Dredging' and dumping of dredged'seciimentsin
estuarine and coastal water$ may lead to increased turbidity and
. .enhanced sediinent d!~positionat dump sites, This mainlyaff~cts primary produc.tion by phytoplankton. p~rforman~e of
visual predators (e.g. fish, birds), and growthand. survival of
benthic organisms. This paper combines a compilation ofliteratur6fnformati6ri ahd results of additional experimental srudies
on. the effet:t,ofenhliDc:ed cohcentnitions of suspended i;Tlatter
(SPM) 'on groWth erf bivalve 'molhiscs,. and on .survival of
macro- and rrieiozooberitbQ~after dumpingof dredged sediments.
FurtD.ennore, it focuses on non~toxic dredged sediments onl)'.
. .'. Rel~e of. nutf,emsirom dredged sedimen~did. 'so far,
hardl)'1nfluence est1larin6phytoplanJ.,.'1onproductiop.1ncreased
-
lernS. Enhanced. SPM-concenci:;J.r,ions are unfavouroble
.
for yOung
';;>250 mglLEstull-
rin~ nematodes cranslfrViveburial by IOcm of dumped dredged
. ,sediment provided that its physical characteristicS are sirnilar
to those .ofthe original sediment. 'Sessile benthos organisms
such as mussels and oysters cancopewithsediment.depbSltion
9f only 1- 2 cm, Other macrozoonefithoscan survivesedi~
merit deposition of20 -30 cm.' Recov~ ofberltJ1osat a dump
site v.iilloccur if the iriterval
between successive d.umpings' is
..
...,sufficiently
I
'
16hg.
-, Options for management o.fdumplng of dredged sediments
are described. relatirig to different ,oc~tions'of dump' sites i'11
estUarine and; coastal waters. to different seasons,andlo the
.
.,
.'
actUaluse
(area andfrequency) ,of dump sites,
.
.
'.
. 1-
'"
..
h
"",,-.'
... -.,.
'''';"",''''<~',~~'~.''''
"""'
,...""-~,,
In 19Si5,
ca. 30 x 106 m3 ofdtedgedsedimentsweredflmped
in.
])utchesttlal'ine and coastal waters, of which ca. 56 %
originated from the Rotterdam harbour~, ca. 1.6% from
th~ WesterriScheldt, ahd 17% fronrtheWadden
Sea
;
Dumping
oLcontaminated
dredge
and Ems estUary ~
spoiJhas a variabk impact on the ecosystems,depending on the nature and amount of the contaminants (e.g.
Reisb 1980, 1981),
.
Durpping of non~conta.minatect sediments, however,
.mayalsohave adverse effects on the functioning of the
agua6c ecOsystem.Suspemsion offinesediments'may
influence nutrieritdynatnics in esruaries, affect growth
of filter~feed.ingorga.nismsandimpa:ir the performance
herring arid sITjelt. Gr0;vth of filter-feedIng bivalves ina~ be
fmpaired. espeCiaJly atSPM-concentrations
..
.,'
"'--(1u~~r{5~lbilill-i~,,~E~e..~EUa1jp~~'y'~:e~l:
thrbidity may affec\;dab ~s well as prey location by.sandwich
..
,
'
. ~.
of visual predators.
A.tdtimp sites, local b<:(ntho$ maybe
affected by burialanq by cha.nging sediment propeI1ies.
Dump sites in butch coastal waters range from 0.2. (e.g.
.' in Wadden Sea). to 30 km2 .(near HOQkof }foUand)~
For many years, the question ofwhere tq,etl,lmp
.
dredged
sediments
was mainly
y'
determine
of economitalnat\.1re'~'&J~a:
Keywords: Birds;Dredge spoil; Eelgrass: Ems EstUary; Fish;
Macrozoobenthe,s: Meiofauna; SPM; Wadden Sea.
~-~...s.hi£1.Rin.g the dumped'
sedimeh!s
'.
.
to,
~.~"~"I"~""'~"4'~\"\1!'~"'--'~
other related to the chan:te.of recirf:1.l1gJi9
,
""'-~'(,<'o"':""1i':'''*''''''~-'I'''fd''i.~,{'!i:'''1f~,.';-.,e.:;w.'1''''L.t';'~;;»l~Y;~J-'"":.~",",,y\"''';"'~~~~''''':
AbbreViation: .SPM ~ SUspeno.ed matter.
.
'-".-
Introduction
. Shipping in DutCh coastal waters' requires extensive
maintenance dredcim1:of navigatioria:\.routes and harbours
and.deepetJin&:of shippingchanriels. Ports situated in
estuaries (Wt:stetschelde; Rotterdam waterway Dutch
Wadden Sea.,Emsesruary) contribute with an annual tran~
. shipmentbf ca..400 inilIion tons. Of this total. the port of
Rotterd:iin takes ca. 290 million ton, or 77 % (F..Ottoet al.
1m.rep. 1993, Min.Verkeer en Waterstaat,mt. rep. 1996);
.
.
".
.'
"
.'.
""">oor.
.-."'.
.'
'"
,
..'..
.
'.
'.
.
..
.
.~.,
1
,
~
""',;'
"M,~~giw.*ti~o,~thJ6agii1aL~~6:,$i.t~. Since 199°'9.".,.""
" ~nmenfuJ effeci§~have'becomemor<:( arid more impor~ "
tant"'.m'Ihe"'PQlicyregarding
dumping- of 'dredged
sediments (E Hallie Int.rep.1990). As. a:con.sequence,
in the late 19EOs a research project'BAGI-IW AD' was
i,nitiated by the NatiOnal InstitutefotCoasta1 and Ma.riPe Manageme~t in, cooperation .withotheC'RijkS.waterstaat'Directorates.This
proje.c:tfocused on nonal)d Ibw-contaITlinateddred~espoil;itsaims
were (1) to
make available r~leya!1ti,nformation, (2) to obta:inaddi~
tional informati~n on the.eftectS bf increhsed sedimentation at dump sites and of enhar;ced suspended matter
/
./
I
/
I
---
.,
.
...
70
L'sink. K. .
(SP\1)-C(1n~cntrations due to dredging. and dumping
opcTaiions.and (3) to provide options for management.
Some preliminary and selected results have already
. been published (Essinket a1. 1992: Essink 1996).
The main effects of dtimpin~ of dredged sediments
relate to ( 1) burial of benthos at dump site~; (2) reduc~
tion of primary production clue to increased rurbidity:
(3) impaired functioning of filter feeding organis:msdue
to enhanced SPM-concen'trations.
Effects of nutrient
inputs
frdJ.
~~
fI"IcJaZt~
Estuaries. play an itnpoJtant role in the' ~f
nitrogen, phosphorus and siJicium.. In the Ems esruary
and the Dutch Wadden Sel, P rather than N is the
limiting elem.ent detennining annual phytoplankton production (DeGroodt & de Jonge 1990; M.l'y1.Engelkes
Int. rep.1989;deJonge 1990). Therefore, atttntion waS
focused on release ofP frorn dred~ed sediment at dump-'
ing and on'thebehaviour of suspiI1ded particle bound
pposphorus during estuarine t~ansport.
The estuaril1ephosphorus
cycle
The 'esruarine P-cyc1e is shown in Fig. L Qrganic
rnatterls not only being produced inside the esruary (e.g.
by phytop1ankton) bur may also be imported. Through'
mineralizaticm in the eSttlarY, dissolved organic andinorganic Pis formed, which in turn are transported to the sea
(Postma 1954; ,de Jonge & P,?stma 1974 J. In the estuary
itself, dissolved P is not evenly distributed as a result of
geochemical, physical and h:ological processes; here; the
behaviourof suspendedpaniculate matter is imporrant .
Calcite. which is formed at sea and is a componentof
the SPM, is transported up the e~ruaiy. Duririgthis
transport phosphareadsor'Js' tocalcit~. In the upper:
e~tuary, however, part of lhecalcite dissolves due to
lower ambient pH, rdeasing the phosphate. bthercomponents 'of SPM, e.g. clay minerals ~nd iron 6xide&;
alsoc;~n adsorb phosphate, e~pecially in the upper
estuary. In this WllY, transport of SPM. may playa
significant role in the estuarine P-cycl~ (de Jonge &
Villerius 1989; Postma 1981).
Experi~ents showed that calcite plays a minor role .
in the estuarine phosphorus cycle, Iron(oxy)hydrox;ides
provedtQ be moreimponant by bindingca, 85% of
Particulate inorganic P (M.M. Engelkes Irit. rep. 1989;
qe Jonge & Engelkes 1993; Rehm 1985).'BesidesFe;
Al plays a:role in estuarine phosphatedynarnics (van
Beusekorn & de Jonge 1994).
Release of phosphate from dredged $edimen~s
Release of phosphate from dredged sediments at dumping was srudiedexperilnentally(M.M.
Engelkes Int. rep.
I
19~ij). Original harbour sediment from the 'Eem:-ha\'cn'
.(
Em~estUary)
was uSed, containing
20
- 40mmol
PO 4
per gram dry'sediment. Aliquots were added to distiBed
water, artificial sea-water of 25 psu, artifici.al sea-water
of 30 pSl1and fi1[rated water from the Ems estuary. For
each 'water type three..treatments were prepared with'
addition of 0, 1 and 2 roMl?O... respectively.
Release of pho~phate from harbour sediment WaS
lowest in highly saline water, (contra Carritt & Goodgal
1954) and. does not lead to the recomrnendationfor
dumping in a specific estuarine salinity zone in ordetto
achieve a limitation of phosphorus release. Presence of
PO.. in the water reduced the release of p. from the
sediment. As there will always be phosphate in estuarine waters, itmakes sense to recommend drecj.gingand
dumping operations not to' take place in spring when
phosphate concentrations in the water are lowpue to the
sprihgbloom of phytoplankton. A better period would
be autumn or wimer, because then phosphate concentrations are high. Simulation of.the effect of a 30-day dump"
in~operation 60000 ton QrYweight harbour sediment in
the Ems estl,lary resulted in an increase. .of phosphaJe
'concentrations. of 0.3, roM at mo§t (M.M. Engelkes. int
rep. 1991). Such an increase in autumn wou1dnot have a
significant effect on phytoplankton growth.
Discussion and Conc1~lsion$
}'fhe tole of Fe-; Mg-~n Al~hYdroxicte:sin phosphate
dyn~mics duriilg estuarine transport nee~s'to be further
unniveJled before relevant options fOr man?,gemenrc,an
be fomulated regarding the location of dumping of
dredged sediments in the estuarine gradient. Although a
large proportion of nutrients is contained in the.sediments
of the Ems estuary (van Beusekom &de JOnge 1998).
releasebf phosphate from dumped harbour sediments is
~1~~~t~dJ,nG1;~,Qi~.Jl\a~~82,
hr fact, the effect of increaseCl ~urbidity due to
~
dumping is likely to surpass. and thus counteract, the
effect of increased phosphate concentrations (DeGroodt
& de Jonge 1990).. Increase of nutrient concentrations,
'SEA
RIVER
ESTUARY
~
rI
:~~:
!
=rr
POM-P
~
po.-p
.
AI;a:
:.
L-.J
.
\
Fig..I. SchematiCpresentation of the estuarine phosphorus
AI;ae
~
cycle. POM = Particulate organic matter. HorizontaJarrows
indicate transpOrt: vertical arrows indicate t:rp.nsformationprocesses (after de Jonge 1990).
- Ecological
!f(G.ct.so.;~,hmil'il1g
Tabid. Management options for the 1ocation l~fdumping of
dredged ".~diments in ,vie".. of the effect on l1utrient dynamics
-
Oprjon-for'dumping
,.Effect.
eelgra.sd)ed.
.
_:' . ,,' .
-
."
I
.
ho~ever,may.lead to 'increased ~o\Vth'ofepiphytic algae
':::onsea-grass leaves (Nevetausk~1987). A thick epiphyte
cover restricts gas exchange andp]1otosynthesis of seagr.a$sesCF;hilLipset'at 1978; Shepherd et aI. 1989).
In Table 1 some options for dumping of dredged
.-
.7
i
..
\
-.-
primary productien ef 2;n'estuaryorof a tidal basin in
whi<;:hthe dumping operation is Carried out. Only in
ca1jes whel'e lecal dredging at the seaward. end of .an
estuary er tidal basin leads to.increased SPM-concentrations ina la~ger pait of the watetbody (cf. de Jonge
1983), a significant decrease in biomass and produ,ctivityof plfYtOplanktqn; zooplankton ,and filter-feeding
.benthos can be expectecLIn a ~imulatien experiment for
the Ems estuary; DeGroodt & de Jonge 0990) showed'
that a decrease in primary pred\:lction weuld be larger in
the inner than'
in the outer half of the estuary.
.
: -.Possible incre:lSc of epiphyte growth
,- .and deterioration of eelgrass
ReleaSe of nuuierlts,.little chance,
.,
. .'of, incre;\sed phytoplankton oloorn
. ReJease of J:luuiems. no chance
ofincreased phytopl~nkton bloom
.
Burialofnutrients
foHewed by
later rele:lSe from sediments,
Near moUth of eStUary ,
or in coastal watet,
,
'Inupper
estuary or
",'
near tidril watershed,
sedi/1.1('Ti.tS -
So, dredging and'Jumping
epeT~ilion~ are likely to.
affect phyteplankton pr(~l1uctivi!y by increasing the \\':3.-'
tel' turbidity; however, this will be rather local and
restricted in time, and there fere have little effect en total
. , andnUtri-..:nt-dependentbiota.
- "'N~;1t
(If drr:<;'c'd
,
'I
.
.
Ejff}.cts on eelgrass
sediments are 'g'Sted with regard to. the effectS of associm;ed.I1utrient re1ease on phytOplanktenand
'eelgrass.
v;",;';
.'
...
"'.
:Effec:ts of increasedtilrbidit);
. .. ..'
,&>.-..
!
":::, Dredging and dumping .bperatienswi1! cause local
.
and'temp'eral resuspension .of sediments, in turn causing'
increasedtUrbidity~ Dredging at specific sites,' where
removal of sand bars leads. 10.a .change.in the .erosionsedimenthtion equilibrium in estuaries, may even cause
increaSe ef :SPM-cencentratioris in a large.partofthe
estuary for a much longer period of time (de Jonge 1983),
IncreaSed ttirbidi~.n:ray impair fl1nc;:tioriingof erganisms
Ejfects on visual predators
-
visual predators (e;g. :5.sh and fish~eating birds). .
spectralcornppsition and 'pelarisatien pattern of the
dependent onthe'peI'iod of tidal emergence for phote-
light (J.M. Bavece into rep..1988).
,
.
The degree tb Which visual predators are hindered by
increased turbidity.is deterrninedby mim'y facters- Some
synthesis, due" to thl: na'turai turbidity:of the aqu3,tic
system, Thet6taLannual primary~reduction at an intertidal ilat.is l~gely depe.ndenton it~eleYationwhich
. determines ,its.expo~~ure .to ihcidint radiation (Colijn
, -1'982~Colijn &de.Jbnge 1984).. Consequently, increased,
waterturqic4ty due tb-dredgingor.dumping qperations
will not havdmy significaI).t effect on thepreductivity
ef micro-algae. inhabitinginterticla1ilars.
of these are related to thepredatOritself,
, juvenile.and
'i:d'hy!opl,atUctonEroq~cJ.iQnj~.-djrec9x
~n
between
Wadden Sea and outer Ems (Cadee& Hegeman 1974).
a different.
adult specirnens.
Other,factQrs relate to. the
prey, e.g. prey. size and enhanced escape chances in
.mereturbid water. The net effeCt:Qf increa:sed turbidity
.
will thus be the resllltantef various influences, bbthbn
Ih.rhtJ;2enetI<iuonintothe water column, whicfrin:'i~rn
, "lt
sttomdY .impeded'-bLhign'~N~s.Old
'~~c1a5:TC:a~& Hegern.an'1974;Ma~ueis~PO~t;a 1974)
show thatanriual primary' productien in~themere wrbid
. Outer .Ems estuary was ca..50 %' of that in tl1eweStern
.Dutch Wadden. .Sea..-without ,significant .differences in
.' algaL$tanding stock, m,eas.ured ?$.chlorophyJla,
e.g.
li'ghtthreshold, different resolutions of the, fISh eye in
,
.
Fish'
. Light plays animportant role'i~ firiding,recognizing
and capturing prey. An iricrease in watert1Jrbidity due to
increfsed 'SPM-concentratio.ns~aynegativdy
i~f1uc
.ence ~'perfonnanceofavlSUal
predatOr,nOt enly by
,. . the decrease in light imensity bpJ also by changes in the
Mi-crophy7obenrhosand phytoplankton'
,.' MicrephytobenthosofintertidaHlats
ih areas such
..as estU13.n.es
and the Wadden.Sea; is almost cempletely
'.
.
ditionsf9f growth of this and other macrciphytespecies.
such as phytoplankton, micrbphytepenthos,'eelgrass and
,
.
Theoccu!Tence and growth of eelgrass Zostera marina is highly depyndent pn the transparency of the water,
as Was shown for the Dutch W adden Se~by Giesen et at
. (1990). De longe, &. de Jeng (1992) .showedaclear
,reliitionship between water column transparency and'the
maximal depth at which sublittOral stand,s of Zmariria
are found. Decrease inlightpenetrationinto
the water
rdue to. dredging apddumpingmay
.therefore. iI11pair con-
i
the predatOr: and on its prey.
In Dutch coastal waters" the Visual pre daters herring
(Clupea harengus) and sprat (Sprczttu.s sprattUs) avoid
turbid waters. After 196P, adult dab (Limanda lima/ilia) a
thencJ.omihahtflat fish in the Dutch Wadden Sea declined;
-young plaic~ ,(Pleilronectes platessa) is now dominant.
The d~creaseis related to an increase in uitbidiry in the
.western part of this sea (deJ()nge et a1. 1993).
.
:;2
Es,\.ink,
Birds
Fish~eat~ngbirds, e.g. cOnTlorants(Phalacroeora.x;
K.
smelt (Osmer(IS:C!JCr/aI111S). H~rring OCC\lrs in the Dollard
exclusively
in wi!irer (A~on. 1985), but during the greater
part of the year .in the western Dutch Wadden Sea
earbi). use their eyes in chasing and capturing their.prey
(Fonds 1978). Spr:lt is present in both areas almost the
under water. For this species, which also forages in,the
Wadden Sea, it is not knovv11.whether turbidity of the
yeafround. During summer. smelt is less abundant than
water affects its foraging st)ccess; but 'the sandwkh tern
in other seasons due to reduced tolerance of hiS!her
(STernasandvicensis)' needs clear water in order to locate
temperatures (Fbnds 1978).Afiorderto
minimize effects
its prey - mainlycorisisungofyoungherrillg,spratarid
011h~rring: and smelt,dredglng\Voutd1rave'tCJt[ke£l~G~
sand-eel (Ammodytes sp.). The increase in turbidity in the , "<:;irr"summeF."Tl1rS"m.eans.-that,'no.commo,Fl'optiotfdan'
be
Durch coastaL zOne since toe 1960s is considered as a
~dVlse(rforb6tlfI?11ytopl'aftRtofi:'antrfisi{'Drecrglngana-:'
."
=-aum!:W'lrra1iromnmaY1'fjeifh~a"tompro3mrse:-~
.possible cause of reduced breeding success of sandwIch
,e::::-A arge'~rorar£y;su.ch'asm~y
arise
tern breeding in the Wadden Sea area. This seems to be
caused by the greater distance from the breeding colony
in specific dredging operatioI1s'(see de longe 1983), is
likely to affect demersal- as well as pelagic fish in
adults have to cQverin obtainipg prey forth~iryoung CA.
. Brenninkrneijer &12. Stienen Int rep. 1992;,B. ten Brink
esruariesand coastal waters. If dredging and dumping
& F. Colijn lIlt. rep. 1990}.,
' ,
act1vities in the ~o~stal 40ne and outer deltas of the
biving duc!.<$such as the eider (So7nateria mollissi/71a)
WaddeD.Sea during spring would increase turbidity, this
take their food (Mytilus edulis, Cerast9demzG edule,
could negatively influence the deve19pment of the popuHitiori of the, sandwich tern in The'Netherlands.
,
Car-eimcs171a.enas)
fropn tbe bottOm ofthe Wadden Sea. It
The above~mentioned resulfsand considerations.sugis notknov,'n whet\1ervisual ioca:tionofptey is i~volved
geSt sevenii optibns for dredging and dumping,' mainly
(C.Swen!}en'per~. comm.) In,common scoter,(Melaniita
relatihgto.thelQtatio21and timeof the year (Table 2).
nigra) which dives down to more than 20 m to catch
fOQd(mainly Spisulaspp.) (M,F. Leopold Int. r~p. 1996),
.
visual
clues. cannot playa :5ignifici'!-ntrole.
-,
Effects , ofenha.FlcedSPM~concentrations
.'
~
Discussion andConclusion.s
,
"
'
,
I
(
Red~c~io~.oJ g~~~Jct9n"
,'~~
J,2~~as"producfion
mcreaSed turbzd~~~ze9
"
bydredg-
.2~g=iJI.,!:?J,;!W<~Qr;JY.iRter~~~,.'
,
'n~proaW:QvlfY,
~~tj.,~~~rn~t:lt~
Rf filter.
yders
arelow. The negative effect of increased turbid- '
""'-Try..,as..,1l"'Te'su1f
of dreclgingand durnpin.g in spring Or
summer, will be dependent Oh(l) the ~xtent bf.the area
affe(;ted,. (2)'the,exteht to which lightpenet~ation in the
water is reduced: a!1d (3) the duration of increased
turbidity conditions. These aspects will bedetennined
'by local c;onditions.
Increased turbidity would make the Wadden Sea and
Ems estUary less attractive to fis1:llike herring, sprat and
(,
Table ,2. Manageinenr options for the location and time of
'dumping of,dr~dg~clsediments in,¥iew of the effect 'cm,water"
and light ,dependentbiota.
,
~,~
~
Optio'n'Jordumping'
..
.
,..'.".
"
Near eelgrass beds
".
Bffeclon waterturbiditY
"
andbiota,,'
..'
Deterioration
.
,
,
\'
of eelgrass
Outer e~tUary,Wa,dden Sea Some damage to herring. sprat and smelt
Near mouth of estUaty'
- Rtdut:ed fo'raging for san,9wich tern
Decrease in phytoplankton production
In upper est\1aI)' or
near tidal watershed
In spring/swnmer
In winter
~rrn autumn
.
Sorr\.edecrease inphytoplankwn
production
Decrt;1Se of phytoplankton production
Negative effect o,n visual predators
'No decrease of phytopla~kio~ production
Negativeef!ect on herring in upper estuary
No decreas.e ofphytopianktOn production,
Nb m:£,!tiveeffect on visual predators:
During dredging' and .dumping,large amounts of
sediment are brought into suspension; increased cbncehtriltionsmay interfere with rood intake of filter-feedihg
benthos (bivalves) aI)d c9pepock Also, functi6ning of
gills pffishrriay be impaifeddueto clogging.
Filter-jeedingbhfalves
.
Filter-feeding bivalves uri1izethedigestible part 'of
~he,SPM,' such as phyroplanktQn, bacteria and decom~
pbsable ,detritus. fg!.ii1crease ,of !he.~E!ilP~!:.~~lli
anorga~ic particles (sedimell2l,du,e,t<24re~g,i~gor dU$:th
~~Ci~~-
~~
Widdows.et at (1979) prese.nted a practlcat
. summaryofthe proCess offoodintakeand~igestion
arid
its reiationship,with SPM-concentratibn (Fig. 2). The,
eventual1y assimilated energy will be depen'dent onthe
cO~tentof utilizable components in SPM. This ,was
corro])orated by Essink &Bos (1985) who traQ-slocated
bivalves (Mytilus edulis, Cerastoderina eduie, Mya, :
armaria) and found that growthV{as strongly correlated
with'Jhechlorophyllaand.organic matter content of
SPM. Asimilarindication of growthre!gulation by food'
"quality of SPMwas found in MaeomCi balthica' (Laane
et at 1987), a bivalve which is a deposit-as well as a
filter-feeder (Kamermans 1994).
,
The filtration by bivalves of the ambient particle
suspension ovel' the gills can beTegulat~d in relation to:
p~rticle cQncentrati~n (see Fig. 2). The qu;ility of food
.:..2
.
;
,
..'~
,""
~
.
,,
,
, (, ,
_.~,
..!.,-,. . ~-_.
~ ~ _,a..if~..-r.~~'Jr,,<~';'.
.
. ..
-'",
. .~~
I
!;-ci.1/o:,,<,jc,'a
I (ljects
'
°
I
{~J (~U,'l[>lng°
.
(~(,-::o~}do~..f
'I'"
°d.:;
.'{ents
~
73
/':{'
partic1es .;. detetm,inedby the de~ree of mixtUre of silt
and phytoplankton (Prin:? & Smaal1989) - is important,
A secondpossibilitYSgfegulate food intake is adaptationof the size of ~J~ and labialpalps. Partitks
...
con('cnm.ltion \\'as 17' I}. higher than near. the surface.
SPl\ Fcon~~ntrations >250 mgll Qccurred morefrequ~ntly
retained on 'thegills' get :;ortedby the labialpalps. Non- \
utilizable particles are diverted from the mouth and ex-'
.'
.
'pelled aspseudofaeces.Theissen
(1982) found that blue'
mussels, (Mytilusedulis) have smaller: gills and larger
palps in theDariishWadden.Seathanin
Balticwatep.
These differences relate to, the higher SPM~concyntta-.
;tiohs'iri, the Wadden Sea. :This was confirmed by data
obtained 'on mU$selsfrom the Dutch Wado.en Sea and
NorthS~a(Fig.3) and on mussels from a SPM-gradient
in'the. Bnstol Channel:'S evern ,estUary from 5 - 500 mgl1
(Mettam 1993). In the North Sea (0 ~ 20 mg/ 1SPM),
relative gill size is high in order to be able to'catch eDough.
particles, and relative piilp size is low because little
sorong h2,sto be:o.one. 1n theWadden Sea(SPM-'conceDtrat10nS.40
".400 mgll), gills .arerelatively small (ample
.
'panicle sUPE~Y),but palps are relatively large because.
muchsoftinghistb
bedone,due to admixtUre of silt and
o.etritUs:Between 0 i3.rid~Omgll of SPM the relative size
.
Of gills and .I?alps is. :Linearlxrelated toinean SPMconcentration (S.Brett & A. Wa:ssenaarInt.rep. 1990).'
Indeed, rec:iproc:altranslocationexperimeDtswithMytilus
eduliscarried out between Wadden Sea 'ana North Sea
showld a dear adaptationofthe size ofgi11s~mdpalps of
I
.
,-,.--"'.'
'._-R
'-"
(1979). F~)f 3-cm large musselsmaxirrial
filtration~ate
was found at SPM-concentrations of ca. 125mgl1. At 225
mg!1ofSPM, filtration,rate had decreased toea. 30 % and
at 250 mgl1.to~ero. So, reduced, growth of near-'bottom
f
rilUssels was most likely caw~ed by more frequently
curringhigh SPM:'concentrations,
not withstanding
to S~M~concentrations in the Wadden Sea above ca. 50
mgl1 (see above). The. observations of faster growth of
Mytilus edulis in more tUrbid enviroriments (Hawkins et
at 1996) refer to SPM-concentratiorls of ca. 90mg/1 as
comparedJo 1Omg/1; l!l this range of 8PM, thebivalv;e is
able tQ make a profitableuseofthe
higher organic matter
.
concentrations at higher SPM-concentrations, being in
agreement with. the first part of the optiml,lm'c:ur-ye
rehiti6nship presented by Widdows et a1. 097 9).
..
r
. .
.
.
.'
.
.
0
is dependent also on .
of zooplankton in tbeErrisestuary
.
SPM-concenrrationS
thatmight
interfere with food selec~
tion,by copepods (Baretta & Ruardy 1988). Findingsby
Cherv,in (1978)" that net growth efficiency of Acartia
tpnsa is inversely related to. the ptoportion of detritUs in
tqe1rf06d, indicate that growth of copepods can be af~
fected by dumping of dredged sediments with a high
organic matter content. One may wonder whetber these
findings canbe transferredtoth'e Emsesruary and Wadden
Sea --Chervin's obserVations are from the.Hudson ,river
estUary and New 'York J?ight, at SPM-coDcentrationsof
only a few mg!1.Res\llts. 6f labQratory.experiments' sug-
",.-..-
t'senonl
rate
fIItratiDn
I
-.---
. .
dmo3.h;"
~
(I
--~----.......
}'
.-'---'--
0
300
'.
-'.(..-~-'
- '"."-_""'.'-,
..
o,,~
.oJ~
-'. - -~"'-'"
-. -.-------------
.
Zooplankton
lnestuar-ine environments,' salinity and temperatUre
largely determine distribution andseasonal.succession
of
zooplahkton. Itis notc1ear to what e;.>teritthedistribution
hO'
"
~on
-.-".,
.:
',')00
rng.dm~]'
pseudcfaece$.
protU~bon
.'-,-
h
-"~.- -.- .
0
Fig. 2. Schematit presentation
of filtr3.tiotJ.and
"
- ---~'-ingestion
Qf suspendedmatter-(se'st~m)
and of
fupctipn of suspended Inatter concenu.a"tior;-(aftcir
Widdows -et1rl:"1979).
'
O':lL
~-'.
pseudofaeces production by Mytilus etl/11isas
a
oc~
the
adaptation of the mussds' filtrati on and sorting apparatUs
.
transplanted mussels to the SPM-regime~bf their new
iocations (Essink etal.1989). Translocation within the
Wadden, Sea .showed that'mussels only adapted their
pa1psize; gill size remained basi~al1y unchangeil., be,cause of the highly variable SPM-cbncentrationsCEssink
et a1: 1989). The energetic costs of these adaptations,
whichrnay result in reduced growth, are not known..
.. 'Indications of energetic: c(Ystsof dealing v,rith eb~
hanoed SPM-cbncentrations were obtained by exposing
-. Mytilus edulis of 3' cm sklelllength,to three levels in the
'water column in a tidal channel of theE~s estUary (R.
Essink et aLlnt rep. 1990). Near ~e bottom, m~~mSPM-
(2 - 3x) near the1Jottom than' at mid-depth Or near the
surfa~e. Growth ofthe soft partS under near-bottom conditions'was r~duced,though not significantly, This is .in
agreement with experimental results from Wijidows et al.
sefton
uptake:ratt
.
-
)CiO
mg;dm'3
'
..
'
.
, '--ingested
, .
. .
,
l
t.etts,
P~U d O
.
a3!
0
"
.~
g.""
.
.
"
300'
scstOn.mg.dm~3
.
..
sertor'l-
,
74
a
Essink, K.,
<2-1
I ..
38-
Table 3, Management options for the 1ocationand time of
dumpipgof dredged sediinems in vj~w of ,me.e{fett,.,(),Qsusmatterconten.~r2:~ions
and $u$pension,feeding"--&tmih
"~-,. ,pended
.
~ .;:,; ,iZ:"~:':'"''
.~~ .':"~.',.::..."'=.
ReI. Gill 'Size
'
0-<'
'0
l
26
\,
.
Oplion for dumpinj:
Eff~ct
N~ar mouth of ~sluary
Possibknegative
~ffecl onzoopJankton
Negative ~ffect on growth of filt~r fe~ding
b~nthos if increase of SPM > :!O'70
'.
221
16
';""-
'
~
l~
,
'
in upper estuary or
near tidal watershed
,
,00
260
.460-
300'
500
WIn
"
No negative
effect
on zooplankton
h
Less n~gative effect.on filter feeding benthos
No, ne!iarlve effect, on (ilt~din2
~~""..:.,-,"
:.~~~,~~..~,:_~~
autumn/wimer
benthos
j
b
B
ReI. Palp Size
'' '
."
.
r.o"'.
.
.~'
- .'.
-
"
'
,'"
:
I
.
-"",--",
,
,-,"",',
,
"
"".',,',
,.,',,
...
"""
.,'-
..'.'.
0'
Enhancement of mean SPM-concentrations in estuaries with 10- 20% win not cause. problemS'for growth of
the blue lIlussel, but if dredging or dumping leads to an
inc~eas'ein the,size-specific filtration rate limiting SPM.
concentrations, negative effectS on mussel gro'wtb, is
expect~d: Results for Myrilus may be eitrapolated tp
other filter feeding bivalves (Essink- & 13.os1985;
M!25hlenberg.&.R.iisg'ard
1978,1979; Prins & Smaal1989);
With respect to the' effi~cts ofincreaser:J,' SPM-concentratior1s some !?ptionsfor du~pingare possible, most
of which relating to the location of dumping (Table 3).'
."
'
,00
.20:)
300
Fig. 3. Rel~tionship betWeen mean suspended matter concen.
tration (SPM:m&l) and a; Relative ,gill size and b.reJative
paJpsizepr MyriltlSedulis fr6m DutchcoastaJ waters, N6rth
Sea. Bali:icSea. Limfjord,Kattegat.SPM
data :forJ)utch
Waters and North Sea refer to ApriJ- August; data for Danish
waters front Theisen( 1982) "(from S.E. Britt &A. Wassenaar
Int. rep. 1990).
,
Mects
of enhaIl:j~
=i:~~ta,~~Of
'.
.
to~()pe v:it10e£9?i,
. ~t du~p site.s, l~cal be~~!l9..§.has
,Bon ofsedllIlentS whIch ~,m,mM-y_c;~~~t;r.Q!1gli <J!1]~19:.:...
qf bentl1o~J~,b..ri,Q~S°;Y~J~q .b¥..,dr.edg~~
,~~~!1
Sed~mell t s 1 S st ro~glY.d.ependent.O? t~
iC ess of
gest fu~tfood intake, by cbpepodssuch
as' Acania tohsCl
and Eury1emora affiliis ism:ga:tive1y influenced at SPM.
concentrati~ns > 250 rngll (Sherk et al.. 1975).
'
sedunems.and~~~;~~?
.
Fish,
According,to a literatur<;:survey by J.M,Baveco (Int.
rep. 1988), enhanc;ed SPM.concentrati6nscan cause subopfunalfundion.ing of gil1shy clogging, causing decreased
gas exchange. The sensitivity offish to this varies,with .
species and life stage, nature of suspended particles.
temperature and dissolved oxygen cDntel1tof the water.
It is assumed that pelagic fish are mor;:sel1sitive to high
SPM-c;oncentrations than demersal fiSh. Plaice survived
laboratory exposure to SPM better than some pelagic
species (Newton 1973 -cited by Moore 1977). Generally,juveroles are more sensitive than adults. J.M. Baveco
.(Int rep. 1988) does not re:port significant fish mortality
near dredgin,g operati ons. Neither was increased morta17
ity, demonstrated iI! caged fish near dredging locations.
.
.
"..<.-
,',
.
MicrophytC!benrhos,
!e~~~~~~~~ .
the
.
.
macro-algae
a'td eelgrass
Microphytobenthos, mainly livingonintenidatSand~
and mudflats, will only pe confronted with deposition
of'dredged sediments after restispension .and secpndary
deposition in the vicinity of a dump site. Bpipelic;tiiatoms among the.microphytobenthos are well adapted ,to
sediment reworking by Waves and currents.. With help of
,
their raphe they can move through the sediment at rates of
-
)
.
.
..<!.! 1',.,.
,
Discussion and ConClusions'
No boundary 'value for SPM is k.nownabove which
gil1, functioning in fish is seriously impaired. If the
results of Sherkin et al. (1975) hciid for field c()nditiol!ls, .
copepods wilJ .meet unfav'ourab1e conditions ina large
part of the Enb e-;ruary, i.e. where the natUra1SPM-.
concentrati.on is ,< 250 mg 1.
.
overlying water (R. IJ erk Int. rep. 1988).
.
.'
.
.
-,I.
"
.
.
0.2 25 mm/s, depending on the species. Maximal crawling "velocity is abo\lt twice as h~gh (Harper 1977).
In an area of the Dutch Wadden Seawith 85 A renipola
marina :;md500 HiteromastL£Sfiliformis individuals per.
rn2Cadee (1976, 1979)Jound thatsedimel1t x:eworking
by thesepolychaetes amounts to c:a.30 and 4 crn/yr,
respectiv:e-1y.There isno il1dicationfQr a..reduced standing StQckof microphytobentho$ in assemblages, with
Arenicolaand HeteromastL£s. Diatoms can sUrVive.sedi-.
ment burial for 20-80 days. even when the sediment is
anoxjc(A.dmiraal&Peletier~979).
No data werefound regar~ingthearrlOunt of sediment
deposition fata.l to intenidal1y occurring macrbalgae,
.
..
,
.
.
I
- Ecological
"'-
effeCts ,f'dumpil1g
,
~<e'l'i"l<8ol""-""..."-"".,,:_:,:,~~,.,,..~,,"",,,,~~~~.",~'I.~
,
;:j-~-~I,r
',-<tl
,-<JIJ.InIM'
ThlCkI)ess
WIll be
an ~-rector
at sites where large-amounts of
dredged sediments are dumped in a short period of time
('incidental deposition'). Sedimyntation rate is'relevant,'
in situations 'of intermitte~t dumping spread over a
.
:-.longer peribd dftime.,
In instances ofincidentaldepositjon, sessile species
such as oys.ter{Ostrea sp.), mus~el (Mytiluseduli<s) and
'mud anemone (Sagartia ,sp.) have a low tolerance of
sediment cover. For motile species, tolera
,.
strongly per spec~
eCles a"fat~l depth'
,
,
~-,'
,
,was,
"
',,'
,;,'
-""".,'
denved.
mdicating
u'"
the
maximal
'
sediment
thick"
ness a species can overcome by crawling to the sediment surface (Fig. A). atal depth is m.. J;R an!.Y caseS
.
~
,
"
"1M
,"
-~J\f~-\
,
",
,
",
~!N,'I1!,t)Jj.
,
,
,
'
'
"\I1:l"_'~
,
,
-
"
many species juveniles do survive sediment deposition
les~ than adults; with the exception' of Mya, where the
adults are not able to .move upward after deposition of
layers of mud Or sand. Benthic animals that arecovered 'py a layer of sediment experien(;e anoxic condi.
tions and increased sulphide concentrations. Species
"inhabiting muddy sedimentS, such as Mya have a natJral
tq!etance of such conditions; However, the brown shrimp
(Crangon crallgcm) is very sensitiveCI'heede 1973).
, Frequent dumping of dredgedsediments with short
intervals has an effect similar to increased sedimentation during the whole period cifdumping. Even before
fatal burial depth is reached, benthic animals can be
affected, espeCially when sedimentation rate is higher
than the rate at which the animals can maintain their
position relative to' the sediment-water interface in
the newly deposited sediment. Fig..S shows anapproximation of the maximal tolerance 9fvarious
benthic species to continuing ~edimentation of d'umped
fine sand and mud. In general, tolerancers greatedor
sedimentation 6ffine sand than for mud. Deposition of
sediment Ol1,lTlusselbeds may also hinder sett}ing ahd
sur'vivalofmus~ellarvae
(Bender & Jensen 1992) in
'
,
!i(',ti0ms can withstand depo-
sition . by, sand better' than by mud. Furthermore, in
Macrpzoobenthos
,
"In the extensive literatUre search by R. Bijkerk (int.
-rep~ 1988) on effects of enhanced sedimentatioJ;l, atten-'
'tion wa:sfocused on factors such-as (l)~ss
of the
sediment deposited, (2) characteristics of the sediment,
"
-,
species inhabitingsanJy
'
",
75
differeI1t f9.,Lo9.~R\;t~iti.qrL.pr))lud.,.y~. ~and. .In general,
.
such as Ulva anJEnreromorplui.
Stability of intertidal sediments plays an important
'role in the'estllblishm[~nt and 'maintenance of beds of
Zostera marina and Z. noltii (Philipp art et al. 1992).No
data were found in the literature on the sensitivity of
eelgrass to deposition of sedirrfenL
-(:5).sedimentatio~~..!IJ£!J1~.
-
of dredged sediments
.
",
"
OItPC*ftIOt\
~
,,",,",(...1
OePlflCc:n't}
70
t20
"
POLYCHAET'ES
'r.iou.uscs
('n18ntdal) ,
60
tOO
50
80
. "I,
40
60
'30
,
AO
20
20
to
1;';>"
~
"
'./
.i"
,
l'
""~,
".""
I
.~
,/
~
.t
.
~,.".-,
w-
-'-""
W',-,-'.-,,-,.
,
--
70
>
OTHER
GO
SPECIES
1""-'1
:f
,so
£0
""[
301
Fig. 4, Fatal depth (cm).for macrozoobenthos at,.
depoSition 12~liIiiid ~(far1<COlumns1oJ
.-,
:1
.
.J
r
0
L.
:1
.
J .'
,
sand (white column). After R. Bijkerk
(lnt. Rep.,
,,'
1988).
if../:
oeoo:ation
---
leY
~
.
,~
"oeo11\(cml
MOL.1.USCS,
r--iiiciaental
oF"
'-
~
o.ptn (em}
~--'
. ~.:{'
,/
1'-
"
" I;'~-"
,
P
:!'
c!'
,
~
.J-<:- 1 ',',1
'#
. ~-..
,:
t?:.
~,'"
,.,,,,,- "
.,t!
s" .
,
.§'
n
~
F.
,;>,'
~I>J
,
'
I
i
/
',f.
~§
,,;".,. l
<
/'
r.
i'
,l~
~
Ess ill.~tK.
76
thi/ W:I) pre\'eiHingnatural
succession
of mussel
commuhities.
bed
.
thicker than ca. 0.3 n1. For instance, the po15'chaete
Nephrys hombe rg ii \.\ia~'very sensiti ve to sediment depo- .'
sition of3dmand
mnre (KJeefet al. 1992); this was
more sensitive than f9und in t.heJjrerature (see Fig. 4). A
.similar conclusion wa$draw~ with respect to Mar;oma
IN).6j~1~3;'~L.!~£1R.~~~~.~E~.s
mortality
among
balthica. (;;;:1 yr) and :Scoloplo8 armiger. MacoTn(l de.,g~~c.ro~22~~B~S)s.was .lower and $ere','~as:~Ji@fer'toJerance of low oxygen' conditions; Th~ percentageQ( ilnim:]ls
creased rnuchmqre thapexpected Jromihe liter:rtt1r~
. esc:lpihg:!t~m9.Liriftl...QY-£fcJYfEIl,g;:iiEY;;az~lEtQ.l,!gb.,tl:1~dewith a fatal depth of 0.5 m (Fig. 4), while Scoloploshad
was
always
lower
aL2swer
Qosjte~t
s~qim~mL.E:~~~vert
compleielydisappeared.
The almosLcompletedisap,
pe:arance ofBathypor~ia sp. is in agreement with infor~
'.Jemperarores.Thes~ re$ul~: 'afCte1afid toseas'(ffiaI differ-'
mation fOlmd in the literature (s.eeFig, 4).' Juv~nile
~nces in 'metaboli~ activity,ofth'ese'pbikilotl1effriitarumals.
'."'FI~ftr'obsetV'iti~~;';er:emadean:"criimpsit~ntIie
Macol'[Labalthica (Qcgroup), .on the other qand, had
doubled its density in the dump site. This apparent
Ems estuary .where ca. 544 000 rn3 or sedimem!rorn the
pos~tive reaction of Mar;oma sp~t to the dumping of
Eemshayen harbour (70.58~ 2°J..U1l)w~re dumped in
dredged sediments is infactnegauve
because in. the
December 1989. and ca., 850 000 m3 iri October 1990
(H.L. Kleef et at Int. r~p; 199~). After the first dumping
reference area directly around the dump site Macoma
campalgnof ca.$.weeks, ~~decrease in speciesridmess
spa~ increased more than' si:dold from ),efore to after
dumping. TheJ::mer increase is the result or nonnal
and abundance of major speci'es was ob.served at the
migration of Macoma spat from high intertidal. flats to
dump/site. This decrease was most clear in those paJ;1:s0f'
deeper l6tations in winter (Beukema & de'VIas1989;
;:he dump site ",:,h~re the sediment layer deposited Was.
Essink
& Beukema 1'991). The dumping of dredged
,
. cmlmonth
sediments inOttober 1990 did nQtl1avethe saTrreeffect.
20.
R. Bijkerk(Int.rep.
19B8) compared results obtained
:n higher and lower temperatures (cf. summer and win-.
.
.
,
.
f.
'.
'.
.'"
"
0\1 Mac01:n.a spat because thenltw?s
still too
the normal ~inter migr:ation..
,
.
During spring .andsummer
of 1990. the
.fauna at the dump site re.covere,dbotpregarding
richness and abundance ofNephfys !lombe"gi~
FlOesand
1S
10
early for
b~nthic'
species
(Fig. 6).
Me iojaun.a
.
The sqrvival of nematodesil1, s.ediment cQres from
the Ems estuary inteJ1jdal.flats was measured in experimental dum.ping treatm.ents (1
- 10 cm
deposition)
with'
various sediment types (K. R.omeyn & J. Leiseboer Int.
rep~ 1989)..4 layer of up t6 10 cm thick did not cause
<:mImonth
40
Mud.
+
Nephtys
\
Species
N
'1
50
30
20
t
':~
40
6
30
4
20-
2
foJ
"-I
10
ci
[:
,g-
~.
-#.
",Q
6'
0
,,p
';10
[:~
~~
,:,~..
,,.<i. .R'otJ' ti i§i . ' ~.
.
,,<:, . .","
~'
§
ii.<!<i. ~"
~
ti<:' <:'
,,<"
"'~ "tif?'
'".:
,<;:ti,
.
Fig. S,Maximal tolerance of macrozoobenthos to coritinuing
sedimentation (rate-in cm.~er month) of dumped fine sand and
mud. For different speci~s me;isured with different methods
(after R,.BijkerJ.;:lnt. rep.l988).'
nov
8.9.
jan
90
Q'
noy
89
jan
90
i3.ug
90
Fig. 6. Mean number df species per sampling st::l.tionand mean
a):nmdance of Nephryshombergii before dredge spoil dumping
. (Nov.l989)'~.k!;l
after ding
Ja .
~~.Q ~e~ghS-.,
I].QJlYb.$...o.f...r.eJ;..Q.~e
ug.
1990)
(rrof\l.
H.L
Kleef et a1. Int.
..~
rep. 1992).
~
- EcqlhgiCaf
;.J'
effccts Ofi!:l17JfJin:.;pf,.!r,',ly,cc!
,.,'dimel1is
- '
"."
any negatiYe,eff,.~t..Wi)hil1 1 -2 weeks,time, nem~todes
migrated \.lpward through the newly deposited sedimen~
.- arid survived"ilL1eas.i: 'lihen. both.,theoriginal.and
the
,.. deposited..$ediment was fme..sand. However, in Case of
:'
deposition of mud on sa.nd; upwardmj.gration ofnema. "t6des was limited to a few CIflonly.
.(,ption;
CTable.-4-4I~~!
II, Op~ionfo'rdumping.
Near eelgrass beds
Near intertidal flats
,
\
\
I
I
..
Effect.
','
.
.'.
Possib1edeterioration
of eelgrass
Decreased productionbyx:nicrophytoberithos
I
I
depending on sedimentation on
. intertidal flats
Mort ality amon g m eio faUna dependin E 0 n extent
i
.'
";:;,
"
Nematodes are likely to survive deposition of sediment up to ca. 10 em provided the deposited sediment is
nbt too different from the locally present one. In tidal
channels, nematQde assemblages are usually impoverished; assemblages in intertidal flats are much richer
(Bouwman 1983). As most. dl.lrripirig of.-dredged
sediments takes place in tidil channelsorothe:r subtidal
locations in the coastal zone,. negative effects on inteF
tidal
, nematOde populations, are not very likely .
Deposition 00 Oem of fine sediment, hbwever, does'
have clearneg-,ative effects on bivalves, such as Mwi/us,
of $ediment'at
10\Ver
,
,
cnannels
and
Mortality
subtidal areas
r,::,~~~~.
on intei'tidal flats
among
,
,
,
,
sit.e
\,pumping
diSU1'buted
,
,
'over large area
Use site each year
n
~
c
th
o~
tO
i ~ ~~
"~
;
.
1
I
,.
.
~.~:
,
.
.
..
. .
.
.
.
.
.
,
. .
impact
,
.
.-
..
Little
.
,.
.
.
on benthos
lncompletereco~ery
of dumping
.
..
,
,
.
.
area
in-betWeen dilmping occaSions
Each year different site Better recovery of benthos at dump site
esca~ing from beirlgburied.,is $rnaller.,J}1!g1E~~R.~
.
sediments inatitumn, when water.wmper.atU)ie~~
~allX.&~~~~cQiiliffi.Qrt;f~ostanima1s
is I!ood'(being
af,JheYendofth~b~ing'''seasOI;)''-;6~]1~r[JI~~~9-
~fQmrs-mgoft1~1'?rm~,,~~~pt"''"'' "-"
.
...'
~~fW'~i1"dumPing,occasions
recovery of ma.crozoobenthos a.t the dUi1}psite may occur proviq~dthat the
time for :recovery is long enough and includes the period
of reproduction ano settlement of recrtiits. One might
consid~i a benthic c6Il1II1unitytob~ recovereqwhen at
least,80% of the species diversity and biomass has been
restor~cj.(cf.}LL. KJeef eJal.InCrep. 1992; Newelletal.
1998). Regular use of a dump site more than 1 - 2 times
per yearislikely to cr~at~ a'long lasting impoverishment
of benthic fauna atthe location(s)wher~ ,the dumped
sediment will de;positRecovery wilfva1-y from species
to species.
'
'
Fro!D the ob~ained results and considerations several
options cah be derived regarding dump site location,
,
and time and frequency of dumping (Table 4).
.General Discussion
, Most &edg'ingand dumping activities take place in
estuarine and coastal waters; These.'areas are generally
relatively rich in n).lments (cf. McComb 1995). The nutri.
ent release experimentS andecosystel1f model simulations
performed ih the Ems estUary indicate only little.,if any"
influence of dr~dging and dumping on nutrient dyp.amics
and therefore-on Pby1oplankton prirnaryproduction. How, . the 'E~mshaven'
ever.
.
sediment
,.'.
,",' tested was, fine sand,
I"
"
!z
,(
'
_
Ch~ce for macrofauna
to escape fTomburial
.
._
Normalwinterrnigrarion6fjuveni]eMacomabalthica [,
.
Great impact on bent.hOsof sinalI area
.
at small
on eXtent,
on intei'tidal flats
~:
~f:r;,~2"~l:~ ~;~:.~
;
,
Dumping
meiofaijria'depending
=
I
,
,
,
:
.
than at
,
and type of sedimentation
:
'
,
,
.
tidal
I. Inother
,
,
,
,
Mort:llity among macrobenthos depending on
extent and sedimentation type on imei'tidalflats
\
According to K Bijkerk (In!. rep. 1988) Cerastodem1£Z{
- edule can 'riot surVIve burial' by more thap 8 ;. 12 cmof
sediment. Field obserVatioris arid' experiroentsby J. van
, DaIfseri{Int. rep.l'994), however, indicate a greater SI.fS-'
,
ceptibility of this ,species. Mort:alitywa.s' fOl,lhdto otcur
after deposition of 5-8cm of fme sediment. Thesuscepti:
. bility of rnacrozobbenthic spedesto l:?urialby dumped
dredged sediments 'may 'Varyto soine extent dependent on
localc6nditibns, In the Baltic Sea, Qlenin (1992) found'
Nereisdi)iersicolor ~lJ)dMacomabalthicatQ be arnofig
the most resistant spe(:les, where~ Corop7ziumyolutator
andE:ygpspio tiegans were the .most sensitive species.
Repeated surveys at the Ems estuary dump site by
RL.K1eef et at (Int. rep. 1992) have shown tha.tseveral
species were more susceptij:He to deposition by dredged
sediments than was known from the literatUre (R Bijkerk
Int. rep. 1988). Nevertheless, it s~ems warranted to conc}ude' that .most 'roacrbzoobentJ}.ic species. other tha!!
Mytilus edulis, Mya arenar;ia,Osr,-ea spp:, SagartiCLspp.
a.ndsome staifisbspecies will Mtbe seriouslY affected as
::longas sediment deposition is iesmcted!oO.2- 0.3 m.
In view of tbe great susceptibility of Mytilus edulis,
and Ostrea spp.to' deposition of sedimel1Hfa~al depth:
1-2 cm) it would be advantageous to avoid sedimenta.
tion of dredged sediments after dumping on. intertid~l
arid shallow' subtidal. mussel and. oyster beds. ,Also the
cockle would benefit from such a.n 3,I>proac'h:.These
bivalve species represent considerable econorriicvalues
. in many eStuarine and coastal areas..
M~crozoobenthic species werefoundto have a greater
higher ternperatures~ At lower temperatures, however,
their cra}"ling activity, and therefore their chance of
and type of sedimentation
.,
.
I
edu'lis, Mya_a;el~aria and Ostred spp., on theanth~zoan
~"Sirfariia -;;1'1'.;- andbn~some'staifishspeties' (cf.Fig. 4).
to. dep6sit:ion
time and
i: :!ect$ Qfenhancedse lment epbsItlononbenthkfauna:
..
tolerance
.lor the location.
fr~quency' of dumping of dredged sediments in view cif.:
~
'
Discussion and Conclusions
77
;\
E.,sinK, K.
.
/
"
Dumping of the sedirnentmainly in secondary tidal
channels has led to increased sedimentation ori some
Intertidal flats. As a consequence. steepness in theestu~
aryhas increased. i.e. shallow subtidalareas.have iargely
disappeNed, and the total area of intertidaL flats has
decreased. Furthermore, the mean height of intertidal
flats increased as well as the proportionofhighdynarnic
'flats 'within thein~ertidal aJ,'ea..Although Gorp.plex by
nature, these morphologica.l. a.ndsedimemological
changes did influence the compositiohof
the.
.
macrobenthic COm1):luni
ties, and thereby the numbers of
birds feeding on the estuarine intertidal flats QJ.Coosen
per's; coinrn.). The latter examples merely indicate that
dredging may very well haviother, f~-reacl;1ing. effects
.
than those dealt within this paper.
Most of the options for management a.sgiven at the
end of each section in tb.ispaper, are qualitative ra.ther
than quantitative. No attempt is made towards an inte"
grated preseptation of the various impacts, with differ.
entweighin.g factors given to copepods, nematodes,
cockles,eel!Zrass etc. The reason for this is that the
applicability~ of the$~optioris will very much depeIlc on
the 'loca.l' hydrological (e.g. dispersal of dumped seai- /
rnents) ',and ecological (e.g. presenceofe~lgras,s beds,
commercial shel1fish stocks) cbnditions present in different water bodies. As an example, it may.for sC7veral
whereas mostharbouTsedimentswill be finer and coi1t~jn'
hifherarnoums of organic matter and nutriems.. The
effect of nutriem release from dredged sediments is much
dependem ofadsorption.ancj desorptiotl.proce.sse~ and ,the
rOleofinQrganic arid organiC suspended partlc1es during
transport in the estuarine gr,adient.
With both. dreq.ging and dumping;segimentgoes
into suspension,. changing the SPM-cbncentra,tion and
tUrbidity of the water column. Forproper management
9fdredgingand dumping operations quapv.tativelnsight
intathe spatial and temporal distribution of suspended
, sediments would be advantageous. To this enq.,usea.nd
Aurtherdevelopment ofmathema~~m~roramn,.
'-o'L11tti!l!I~,J;~~~rnentS-;ncrsin1u:
~~9fn~S2l.9.gi~al.~f£$Gts,{e.g. Bache'taT:"'i~'~~
"delt"ok1992, 1995; 'Moritz & Randall199~~,." mendeq,. Amodel
~~~!'~"'.,
,',',"
approach Seems essential m theselec-.
',".',..',
.,",
I
tion.of dump s}tesin, coastal systems, such as the interri~tiol1al Wadden Sea; where.commercial" exploitation'
of natUral resources -(e.g. shellfish) and 'nature protec"tion are clearly outlinedpolicies. However, the predictability of functional system responses has certain limits
(cf. de Vries et al. 1996).
Negative effects on benthos,at durn' shes can be minimi~ifc)~<:w
1C a;~.oor'l'
.
.
~'o~~
~cts,.catl. s;Q~be.feduced.b~,
: dumping in such'a w,aythat the layer of deposited~edimen(
-does riot ~xce1mr.r:tJ~:rm~wMatllemau2arm5deis"d~b:"
'ilig the d,ensltYcharlg"esQ'(dfedgedsediments during dump"
.
,
reasons
dump in the outer part of ~
particular; estUary, but if In that outer part important
.stocks' of cockles or eelgrass occur: that option 'will not
{lOld.Furthermore, t~e'options also relate to differences
in amount and typ~ of dreq.ged sediment, locality of
(optional) dump site and of the method of dumping.
Therefore, managers and , authQritieshave '. to' tboos.e,
ing, the impact on the sea..bedand the subsequenthorizon~
tal dispers~ of the,sediments as influenced by water cur-
rentsand bottomtopography,~ch as developedbyT; van
Heuvel (lnt. rep. 1988) and further evolved since then,
maybe a us~ful tool. Sedimem trartsport models have the
. potential of becoming importahtmanagemept tools in the
prediction of effects of dredging and dumping in <ti'easwith
commercially important shellfish stock (mussel cultivation
10tS,cockles, SpisuZa, shrin1ps).
.
':"'Arral~~may~.;fo: dum~g;'~~~I1
tidal chan?els wou~ 5'e'S§~~;,l.~~ti..?:1?oy~.t~,~~jg~_.
r-w~m'.~liJ$"W6iiiCt
however;dras~~'
''''-''''l'l'arum.'ctrartr
.,£,);!.Qx~ar,>.,;.".,.~.:ia'sr"fi'6m; nence with the 'depot de Biiho' in the
""""'f:;oITe'
estUary we know that building up such a depot will
have a great spatial effect on sediment composition,
.benthos. fish and birds (Gallenne 1990).
Another example of changing the Ems estuary's
morphology and accompanying SPM-regime by dredging, wa.s already referred to (de longe 1983)..11'1the
Western ScheIdt (SW Neftlerlands), increas~d dredging.
of shipping channels to the port of Antwerp has changed
themorpho~ogyofthe estilary. Since ca. 1970 dredging
efforts increased., ~speciallyin the eastern part of the
estUarY (Buys 1995; J. 'hooD et at 11'11.rep. 1997).
berecomihendedto
.?betw~~~'.Fh~'~ws~~re:a5'PI!~.n~~
aCC;,o~c!an~~wltb.1n:e
~~iw..~~.Recently,Dutch
harbour managers in the Wadden Sea andEmsestijary,
iri tbei~fPphcation for new dumping permits, have to
make dea.r to Dwch and. Germanaijthorities that the
.
qump si~ethey wai:1t,to use, and Fheirpractice
of dump~
irig, does not seriously affect specific natural resOurces.
Acknowledgements. Thanks are due to all participants in the
BAGHWAD project, especially Hans Baveco, Ronald
Bijkerk, Lies bethDeGroodt, MerinoE.ngelke~. Jetry EpPinga.
ViclOrde longe, Tjarkvan Helivel, Hans Kleef, Aukje Mol.
Herman Mulder,. Harry Peletier, Klaas Roggen, Karin
Rorneyn, Frank Steyaert, Peter Tydeman, El~eWeUing. as
well as employees and shipc.rews
of the 'Mee.tdienSl
Groningen' of :Rijkswaterstaat, Directorate' Groningeh. Rob
.
Jungcurt prep~ed the illustrations:
...
~
.,,'
...
.
...
'- Ecologic.:,!
'(:l.t1'I:'::\'
().i'dllmpil1g
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'"Intethal reports in DUtch mentioned in the ,text are available
through the'auth.or.
Received 14JuJy 1998:
Revision received 14 April 1999;
Ac~eptGd 4 May 1~99.