Recent Saltmarsh Changes and Sedimentation Rates in the Sado Estuary, Portugal Maria Eugenia Soares de Albergaria Moreira Centro de Est lidos (;eogrc.~lti('()s Faculdade de l.ei.ras Universidadc de Lisbon 1699 Lishoa Codex, Portllgal ABS'rRA(:l~ "_"_""""" """". MOI{EIH,A, M.KS.A., ]!l~l~. Hen'1I1 salt marsh ehallgl'H lind sr dunent.at ion rateH in t hr- SlIdo EHluHrv, ,tflllllll:. Portugal. .luurn al of ('ul1l;fal U/'."I'{I,.("h. l'{C\), I);l] 1;40. Fori Lllllderdllk (Florida l, ISSN ()749-()~OH. ~ The j idal mudflats (If t he r-st u.rrv of t hi' Sado l i ivr-r an' COtllpoSl'd (If silt and cia v. wit h intr-rspersed sandv units that mnv repn'spnt n'sl rictl'd "all'o I ida) chanll('L" [lr Wldl'Spft'Hd (Iuviul flood d"l\Osils. The min­ (·ralogy (If t hr- day is primarily mont motillouue. wlwft,!!.,-, tilt' silt and silnd grains .ire Jllq~f'ly quartz. euus~ ~ ~ Estuarine and lagoollal salt marshes rovr-r 'i'll', 100' I (If the llppn m udflat , forming- narrow platforms -+- ;,-- disst'etf'ef hv neeks and pol,vgel\t'llt· sail pallS. The lowr-r mudtlnt is dis~wett'd liv Ilw<lndl'ring and alias t omoaiuj; liefal channels. ;lIld Is charilclerizl'd hv nu nu-rous small marshv island- The spat iul changes of tilt' I I(lailla! ft'al\lrf'~ are dOlllinated hy Ilw rl'tr;'at of the llppn mudfla! surface, r.iuse-d hv notching of its margin. llnder t II(' root lave-r: which rpslllts ill llw CIl[IHp~f' ,lllei ~Iippag(-' sPrlwmd of peaty blocks of marsh. As II cons('qw'IlCf' of t hi'st' lJnrln('llt tIllg pro("('SS('S. I ht' upper I idal IIBI wet la nd margins art' [('Ireating al fates of O,H 10,;\ crn/vr. Wlu-r« tidal pans Oil t hr IIPlwr Sllrfact' art' hreachr-d, t /1(' scarp h(,("(llll('S disst'c1(·d quick lv and the horizoll!;]1 rl'l n'at of ! ht' scarp ft'!lchpS ,I;),:) rm/vr. The maximum measured relrf'at of '71;:; r m/vr IS c<lllspd h~' anthfl>!HIg'('lIic art ivitv. In plal'l's, prograd;lIiOll of tht' saltmarsh edge is ohsr-rved , [('aching 1 nr/vr. TIlt' VNI iellJ sedimenlarv record is I'haracll'~I/,f'd hv (lcnd 1011 r<J II'S I hat, du ri nj; t he last df'eade (I ~I7H 1!IH9), roached D.I'< fl.;; mm/vr 011 Ilw lJPIH'r mudtla: (including t hr- saIl paw.,) and 101; mrn/vr on the sanrlv lr-vr-es. III .4t'ver,1I placl's il IH'gal ive rate was o!Jst'rv('d, hut not 1lll'llsurt'd . On till' lower mudflat. si lt v sr-dirnr-nt s Hlld shrll hash accullllIlilf(' IH'JH it...; inland margin, dose Lo the scarp of t he high marsh, wherr-ns sand /lats and sand /t,\l'(,s ,It'l'\J1111Jiall' along I hI' rll!trgill~ of I h« est uarrru. chun nr-ls. However, ill sr-vr-rn] plact's prt'st'nf d,p/ t hill dl'posits of ~alld ovr-rlav I ht, inlier murgin of the low tidal III lid flat as wr-l] as some portions of t lu- high marsh sllffal'f' These s.mdv a('ClJrIlIJlations and tilt' gt'lH'ral n-t.n-at of 11)(' marsh sca rp ,Irt- rl'lau·d It) I he local sea I{',,"('I r ise. ADI>ITIONAL INDEX WORDS: Est uurin e ..,,,,d'llll'nllll/ull. ("U(/SIIl!u,I'f/IlTlds. J.:1'fJm(}r{J}w/ug\-', SI'{1··!('/'e( IN'TR(llllJ(:'rI()N 1he marsh ('rosiollal pn)('('ssps (!\/l()HEIHA-LoI'Es, t ~r,~): M()HEIIL\, I !)Hi). The coastal wetlands of the Sado estuary", Oil the southwestern coast. of Portugal (Figure 1l, oc­ l{('('ognition of the h.\'drological rpgilne is irn­ cupy 7,170 km'. The wetlands are part. of a Na­ portend. tot he u ndr-rst and ing of t he sedimentarv ~H' tional Natural J{eserv(' whose surface has hr-en processes Oil t est uari IH' we-tlands (V EHC!':I{, 19()H; ~: decreasing because of progressive reclamat ion and HAN \V ELL, t Hi (' II A I)1\'1 A \J, 1974 ). r l' hp flu v iaI re ginH' of Sado i~ of a I\;']edit('rraIH'(\n-type (AM­ developtnent. on its rnargins (MOHEIHA and ()LI­ the VEIHA, 1988). rrhe estuary is sheltered seaward b.v HAH ('I ul., IDHO), hut at pres('nt the cool season ~In<lll a northerly-extending sandy spit.. It incorporatps floods are controlled h\' IlllllH'rOllS darns in wide tidal fiats and narrow salt. rnarsht's which arc the dist.rihutarips. Several titnt's during the wet characterized by a dense Mpdit.erranean-1yp(' \vinters Of during ('arl:'t' spring, the controlled dis­ t III llSP halophytic vegetation (MOHEII{A, J9H7). eharges fforn he d a s ca a n a rti ficia I Hood­ rrhe clirnate is suhtropical, wit h wann and dr:'t' illg'. 'I'his pro('('ss is irnportilllt to thp lllodern day summers of high evaporatioll rates causing- dps­ est ll<lrinp aCCf('tiOl1al pro('pss('S because of t he high t urhid it.\, of t.he relensed watt'rs. 'l'hf' lnaxilnuln iccation of the BHld flat clays and prpci pi tat ion of cOI1(,Pllfrat ion llfsllSJH'lH!f'd sedin)f\l1t in these dis-, salt crusts on t.he unvpgetated saltlnarsh surfiH'(' '7:~ and on the hot Lorn of dr:'t' t.idal salt -pans. I)uring charges has rpitched 1r). g/I ill 1ht, t Ilrbid plllrne I97~) the winter rainfall is concentrated and acceleratps IlPilr Abul (Figllre t I in February' (M()HEIHA- LO!'I<::--:, I ~)7~) I. I hlri ng the sUlnrner Pi'­ rind, the thlVicl! dischaq.u· into th(' ('stuary IS 111­ 9110<" rNl'il'('d {lnd (l('I'I'{)fl'd 29 NOI'I'f/lllI'r 1991 signiticant. 632 Moreira SURVEY METHODS -------- 1 1-- '''I, - \ --1 The linear horizontal retreat or progradation of '.[ J~~ ~j ::~: __ the salt.marsh scarp was observed by surveying i ~.:-' ~f",:"~--:---_J the distance from the scarp-line to fixed bench l~ __ marks. In some cases, profile lines were run to 2 -~lL JJ ~."'1 :', include aspects of the vegetation dynamics. Ver­ D 't.j . , r lJG tical accretion rates on the high and low marsh ~-.~.;~~:=:.:. surfaces were determined through a combination . of surveying, using marker horizons of brick dust ~~{~~:;,.,:} (STEERS, 19:38; RICHARD, 1978), and stakes to r'~ r, 0 .o identify specially-designated core sites in a range I 0 i.otdeu o , \:)\dPTrO'r;B of wetland morphologies (Figure 1). Two cores \:~" " were collected at each site and the results of the '11'Kld" A, --p analyses represent the average of the two samples. o «)"'01'0 -<;' Brick dust was applied to sites after the first spring tide in September in each of three different years (1978,1982, and 198~3). Recovery of the brick dust layer in the entire study area was incomplete because several of the sampling sites were dis­ turbed by fishermen, by cattle, and by other traffic through the marshes. Some of the other sites, al­ though not apparently disturbed, did not retain ~~~O'nporlr1 a recognizable brick dust layer in their cores. .:...Jt'dOlt:Qtl'fT"\,t r. ...:...JSCllt-rnUr':::lh jdUflt! The inorganic component of the collected sam­ r 1 r 1 .~ ,--./:hluf' l~]~·ke l~g(]tP 1 (1 r I n 9 <; I t ples was determined by treating the samples with Figure.I. Location of the core sites on the intertidal platforms H20~ and expressing the remaining sediment as of the Sarlo estuary. a percentage of the total dry weight. The major surficial geomorphological features and their distributions were identified on aerial The tidal regime is sernidiurnal and mesot.idal, photographs (1:25,000 and 1:15,000 scale) and on The tidal range at spring tide is ;1.7 m at the satellite imagery (Landsat MMS and TM), but entrance to the estuary (Troia) and 4 m at loca­ the micro-geomorphological features and the pro­ tions within the estuary (Comport.a, Palma, Zam­ cesses at the micro-scale were measured, moni­ bujal, and Mitrena). Wave heights in the estuary tored, and analyzed in the field. are usually less than 1.0 m. During the summer, maximum wave heights are associated with winds from the north, Nortada. In the winter, winds GENERAL GEOMORPHOLOGICAL AND from the northwest are funneled through the Ma­ SEDIMENTOL()GICAL CHARACTERISTICS rateca estuarine channel and generate waves OF THE STUDY AREA reaching 1.0 m. These waves produce considerable The intertidal platforms and shoals of the Sado erosion of the dikes that are exposed to these estuary develop between the elevations of - 1.8 greater fetches. Oceanic swell does not pass ill and +2.0 m along the margins of the lagoon through the inlet into the estuary. and the estuarine channels (Figure 1). Most of the The objective of this study is to describe and wetland system has been modified by human ac­ analyze the sedimentation rates of the last decade tion. The ecotone of the inland margin of the on the intertidal platform and to relate the rates intertidal platform is against a constructed dike to the variety of natural and anthropogenic pro­ along 80(';, of its length.