tuar\., can varv substantiallr'. Holvevcr, the constric- tion of the mor.rth, er.,ident in the lirnitcd tidal fh.rc- tuations r.vithin the estuary relatir.,c to the sea (Fie.
Estuaries Vol. 20, No. 1, p. 1?4-133 March 1997
Inflowand Estuarine Variability in the Freshwater GamtoosEstuary,SouthAfrica EcrrqRr H. Se me:rsured S.c.tI-'rrv Slnt.'c;rt nl:s Figure ir shorvs the salinity structrlres in the estuarv at spring ticles on Noventber 26 (a) and f)ecember 1() (c), irnd zrt neap tide on f)eccnrber 4 (b). In cach czrsethe high ancl lolv tirle serctionsale gir''en becatrsc of thc rnarked variation incltrcecl bv the clifTerent inprrts of'sealvatcr at t.l)e ticlirl inlet. At the start thc untrsuallv strong flreshrvater flor'r, in the shalkrlv estuarv firrcecl tlrc silltrv'.rter interfirce dorvnstrearn, particularl,v in thc upper reachcs, and, as shown in Fis. 5ra, ltrackish r.r'aterrvith :r salinitv less tharr I5%,o evcn re:rcheclirrt: ancl null point hacl movcd llpstrearn ll'a kilornetcr or n{o (Fie. 5b). Hou'eveq of sreatcr siqnific:rnce is the fact that at lon ticlc the upper freshrvater laver hacl clecreasecl ir-r thickness, thotrgh thcre rvas still lvater n'ith ir salir.rin' of arotrncl l5%o entcring the sea at the srlrface. At hish tide the penetratioll
GamtoosFreshwaterlnflowand Variabilitv
Low Tide
Xl loiat.it
,1,1992 at neaP tide, and c) I)ect:mber 10, 1992,at spring ticle.
129
130
E . H . S c h u m a n na n d M . W . P e a r c e
of lrighcr salinit\' \r'ater reached rnuch further r.rp the estuary than on November 26. On December 10 at spring low tide the freshwater inflow had decreased further and the surface water entering the sea had a salini$' greater than 25Vot, (Fig.5c), though the upstre:rnr pc'rsition of' the rnain halocline had not chirngcd by much. At high tide almost \:ertically hornoseneous conditions occurred all the way to station 4, though with a salinity sradient to the sea, ancl salt\!'ater penetration almost reached station 9. A calculation trsing the tidal prisrn and estuary 407oo) occurrins in surnrner in the upper reaches. Such extreme conditions have not yet been observed in the (;amtoos, ancl the biota reflect the range from marine to freshwater species.Other investigations in the Sundays estuary to the east showed that chlorophyll-a values were elevated in the more highlv stratified sc-'ctionsof the estuary, and were then also depenclent on the freshwater input (Hilrner 1990). The movement of the salt u'eclge up ar-rddown the estuarv will affect different benthic conrnrunities living in different sections of the estuary (e.u., UpogebiaaJricnnain mrrddv sections and Cnllianassa kraussi in sandy substrates).Figtrre 5 shou's that, even with freshrvater inflows of thc order of l0 rn:l s r, the limited mixins in the Gamtoos means thal this rvater rvill exit as a thin surfacerlayer over rnost of the estlrarv dorvnstream of the sand banks at station 7. lt is then primarily in the upper reaches that the be nthic fauna will be affected b1'lar-gevariations in salinin'. If the rnouth closescornplctclv, it is likely that anv freshu.aterinput rvill rernain as an unrnixecl srlrface laver-.since wincl appears to havc a limited cffcct on the (larntoos estuarv. On the othcr hand, closure of the nrorrtlt of an c'sttrary'fbr lone periocls cral] se\rereh afl'ect specics strch as Upogebittafri,cnnan'hich requirc a nrarinc phase of' larval development (\\boldridge 1994). In the end, it is irnportant to put the results presented here in the overall context of longer terrn variability in ttre Gamtoos estuar\r.These restrlts have dernonstrateclthc changes that occtrr in thc
GamtoosFreshwaterInllowand Variabilitv
salinity stnrcturcs as a result of freshwater inflorv cvcnts, with the possible irnplications for the biota. Hon'cveq there h:rve probably'been substantial alterations in thc rrtean ancl floocl florvs this centurl, in particular sinc:e the Kotrga [)arn was built in l9tr4. Corresponcline changes woulcl have ensuecl in the river and estuarv channels and in thc motrth (Reclclering 1988a). but withotrt :r confisuration knowledse of those flows it is not possible to quantif\' the change's. Nonetheless, the results have sho$'n that in its present configtrration, relativelv snrall infltrxes of fresh watcr (-10 m:r s r) can cause rnarked changes in the salinitv structrrres of' the Camtoos cstuary. It is important fbr futurc l-nAnagernent of the estuary that the freshlvater inflow be monitored on a contintrous basis so that the effe ct of ant, firrther changes can be assessed. r\(;xto$.LED(;\tENTS (lourrnission I he \\'nterRestarl r c k r t o w l e d g c c l . T . H . \ \ ' o o k l r i c l g c r n a c l e r r s e f i r l ( ( ) n l n l e r ) t s on tht' rnanttstri1rt.
l,rrr-n-,\rtnE (ltrEo . \ t R R L \ 'l.) . ( 1 . . r r n P . E . S r [ . ] - R .1 9 f t 5 . , \ s r u d v o f n o n l i n c a r t i c l a l prolrau:rtion in sltallorr inlct./t'strurrine svstelns.P:rrt I: Oirserv a t i < r n sI.: s l t r a i n r , ( ) v t s t n ln n d S h t l [ S l ' i t i r L2r l : l 8 i r - 2 0 5 . ( ; . \ R v t \ 1 , R . \ \ ' . . l t . K . l \ I c ( 1 . \ R r r r \ ' . . r r r rK - ( 1 .\ \ k r r t ; . 1 9 9 2 . ' l ' h c ; r x i ; r is : r l i n i t r r l i s t r i l r r r t i o t ri n t l r e I ) e l a r r : t r ee s t u i l r \ ' : r n c li t s r r ' e n k r e s [ ) ( ) r ] steo r i v e r c [ i s c h a r g t f. . t t t r , t r t r t ,t . ' , t i n l , u l d . ! V t t l..\ft i t t t r t ' 1 ; r :l . ) / - I t ) . ) .
( , ( ) s ( : n F . \ .\ \ ' . S . . r r r r E . H . S c u t u . \ \ \ . l t ) l ) r - ) U . P r v e l l i n ga n r l t h e ()((rrrrerlce ol cold i\'atcr irr()un(l ()ePc Recilt, Algoa B:rr, S r r r r t l r ' \ l i i c a . * t u t h , \ f i r a n . l o ur n u l o f , \ l uf i n e S r i s a r r l ( i : 5 7 - 6 7 . I l r u r o t r . . \ . E . F . r s r r K . l - . ' I I \ r . E \ ' . 1 ( ) l J OE. s t r r a r i ets> l ' t h e( i a 1 r t ' I ) a l t l : S v n o p s i so l t l r t ' ( , a p e O o a s t .\ : t t r r l a l f e : r t r r r e sd, v n a r r r r i c s a l t t l r r t i l i s : t t i o r t . ( . S l R R e s e a r c h R e r r o rt 1 1 8 0 .S t e l l e n l t o s c h . \ r r r r t l r- \ l ri t . r . I I t t . t t . t < .T . l 9 1 l ( ) . F : r c t o l si n f l r r c n
