STREAMS IN MEDITERRANEAN CLIMATE REGIONS: Abiotic Influences and Biotic Responses to ...

33 Pages·2009·0.2 MB·English

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September17,1999 17:4 AnnualReviews AR093-03
Annu.Rev.Ecol.Syst.1999.30:51–81
Copyright(cid:176)c 1999byAnnualReviews.Allrightsreserved
S M C
TREAMS IN EDITERRANEAN LIMATE
?
R : Abiotic Influences and
EGIONS
Biotic Responses to Predictable
Seasonal Events
g
or
ws.y. Avital Gasith1 and Vincent H. Resh2
nnualrevienal use onl I1PsIonrlasicetiylt;u&ete-mMfoaarinlN:aagatevumirgeeanCst@,oUnpsnoeisrvtv.etaratsuiio.tayncoR.ifelCs2eaDalirefcpohar,nrTtimael,eBAnvetirovkfeUElennyiv,viCerorasnlimitfyoe,rnTnteailalAS9vc4iiv7e2n60c9;e9,78,
als.aerso e-mail: [email protected]
np
om arjour3/09. For KeyWords flooding,drying,humanimpact
wnloaded frora) on 02/0 MsnoeuAdthibtwesretrsratanecernatnSSoSeuatthr,epAaamfrrtssicoainfawnmedseptdeairtrnetsrNroaofnrecthaennA-trcmalilemrCiachtaiel,epr)aeargtriseoonpfshwy(seaisrcteaaallsnyd,sscuohrureotmhuinAcdauilnslytgr,aaltnihade,
DoEv biologicallyshapedbysequential,predictable,seasonaleventsoffloodinganddrying
30:51-81. e evora (U owthvheeedrrieasbncyhaaanrbngiueoatdileccc(yleicnnlveeis.r,owCnohmricreehnstipasola)nlcsdooinnatgrtloiyml,seathqwauhtadetinocmbciioonmtaictmecduounnrititrnioeglssflu(oeno.dgde.srpgaroreedaraetyidoeuancr,elcydowcmyhpceelne-
99.e d tition)canbecomeimportant.Asthedryseasonprogresses,habitatconditionsbecome
9d
1a harsher;environmentalpressuresmayagainbecomethemoreimportantregulatorsof
u. Rev. Ecol. Syst. by b-on: Universid sastchttuorretueeuaanNmmmteonsrrpptilhoasietprtmtruesnrl.oafHrateWileolpmneirnsoipstafprronaehrdcseeetrescnetdote,mad2wnm5dtietutmhaensfpsiattueaylmrlbasalmtteneruedhscrytptrpupoeerosaeastm.hkibesIel,snyxericsisspopttainrhnritanigratagnsirnpteietlntoaahptkeeutshitrteootScosocytmhuunertcehrhidiennrirogtflnenuirHonreaunesnscmtereieainsanoppm-fuhtystetphorineeef
n streamhydrographonfaunalrichness,abundance,anddiversity;speciescoexistence;
n
A
seasonal changes in the relative importance of abiotic and biotic controls on the bi-
otic structure; riparian inputs and the relative importance of heterotrophy compared
toautotrophy; andtheimpactofhumanactivitiesontheseseasonallywater-stressed
streams. Population increases in mediterranean-climate regions (particularly in fer-
tileregions)resultinanintensificationofthecompetitionforwateramongdifferent
users;consequently,waterabstraction,flowregulation,increasedsalinity,andpollu-
tionseverelylimittheabilityofthestreamstosurviveassustainable, self-regulated
systems.
0066-4162/99/1120-0051$08.00 51
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52 GASITH n RESH
INTRODUCTION
Climaticandgeomorphicsettingstronglyinfluencestructuralandfunctionalfea-
tures of riv?ers and streams (2,45,88). Consequently, lotic ecosystems within a
largegeographicareamayexhibitgreatersimilaritytothoseinotherregionsthat
have similar climates and geomorphology than to those occurring in the same
region(58,93,134). Streamsandriversinmediterranean-climateareasthrough-
outtheworldareexcellentexamplesofsuchconvergence.Althoughconfinedto
relatively small regions (<1% or up to 4% of the continental area according
to different definitions, e.g. 5 and 50, respectively) and widely separated on
org all continents, mediterranean ecosystems support similar types of sclerophyl-
ws.y. lous vegetation (50,51,156,157). Thus, because of the strong influence of the
vieonl climate and catchment vegetation on stream structure and function, streams of
alreuse coastal California, for example, may better be compared with streams in other
nnunal mediterranean-typeclimatesinAustralia,Europe,SouthAmerica,orAfricathan,
als.aerso forexample,withcoastalstreamsoftheadjoiningPacificNorthwest.
np
om arjour3/09. For tmemeIdsni,tea8rr9raencceoeanmntp-rtayervpaieteivwcelimosftauttdehiewesheri(seitnolirs1yt3edosf(u8rbe4jse)e;cantroccnaheteoognfotrmhieeessd)eitasetmrurdoainnegesairnne-vgtoyiolpvneeseawcqouitsahytisca-
ded frn 02/0 ehcaobsityasttseimnst.hTesheereefgfeiocntosfahnudmthaen-nineeflduetoncuendddeirssttuarnbdanthceeoencothloegiinctaelgbriatsyisoffoaqruthaetiicr
nloaa) o rehabilitation prompted us to assess the extent of scientific understanding about
wor
DoEv theorganizationandfunctioningofsuchhabitatsandtheirresponsestokeyenvi-
30:51-81. e evora (U rasotrrenemabmeetntdteayrlnufaanmcdtieocrrsss,t.obSoiodotmi(cee.sgttor.uphciyctudsrrteohlaaotngdwy)eintethexaraanmctoiintohenedsr)si.n(Ame.sge.dairtrieeprsaruarlinta,enawnien-tpoyufptteesnsathnreadadmintos-
99.e d drawonprinciplesandexamplesfromstudiesonstreamsinmoremesicandxeric
9d
1a
nu. Rev. Ecol. Syst. by b-on: Universid rcfa(eoaengl)ndlWittoohrowanehflssetae.tedthnasebrtesryexiestatahrimeeswmsdtaheaeramfirteflnboasienooncgmddaicuendhtsgriaeymratasenhcesdetaesdsderoiisrsneyttci,iinonctchssgtyeolypysftesdeartmriirfeoefsdeainshrme.flansvuItieennfnarmtochdemeiidssd,tirbaitenyenvcrdiarteaocwsntoeheoqwealurneaetcnindmldciemeeswscoaerttfihtebrsesee?esgahaOusmolouawnerr
n
A as, streams in either temperate mesic or xeric areas; (b) the annual, predictable
floodsdeterminethetemporalandspatialdynamicsofthesesystems;(c)thedrying
thatfollowstherainyseasonresultsingradualincreasesinbiotic(e.g. competition
and predation) and abiotic (e.g. low oxygen, high temperatures, loss of habitat)
controls in structuring biological communities; (d) annual variability in rainfall
canleadtoextremeconditionsoffloodinganddrying,andpopulationsandcom-
munitiesresponddifferentlyoveramultiannualscale;(e)thetiminganddynamics
ofriparianinputsinmediterranean-climateregionsdifferfromthoseoftemperate,
forestedstreams;(f)humanactivitiesmayimpacttheseloticsystemsmorethan
theircounterpartsinmorehumidoraridregionsbecauseoftheseverecompetition
forwaterthatoccursinmediterranean-climateregions. Followingadescriptionof
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STREAMSINMEDITERRANEANCLIMATES 53
the environmental setting and the riparian vegetation of mediterranean regions,
we organize our discussion of mediterranean-climate streams around a series of
testablehypothesesabouttheresponseoftheaquaticbiotatothefloodinganddry-
ingseque?nce. Thisfirstreviewofmediterranean-typestreamsisselective;aspects
thatdeservefurthertreatmentarementionedintheconclusion.
CLIMATICSETTING
Onallcontinents,certaincoastalregions(oftenwesterlypositioned)inthemiddle
g latitudes, most extending between 30– and 40– north and south of the equator,
or
ws.y. are governed by a symmetrical atmospheric circulation that produces a climate
vieonl characterizedbycool,wetwintersandhot,drysummers(5). Themoderatingocean
alreuse influence keeps winter temperatures mild, with mean monthly minima ranging
nnunal from about 8– to 12–C, and frosts infrequent except at high elevations or well
als.aerso inland;summermeanmonthlymaximausuallyvarybetween18–and30–C(49).
om arjourn3/09. For p loofctOahtrveedeericnoontnehteihnMaelnfetsdo–iftEetuhrrreaonapereea,anAwSseioaar,lbdAawfsriiidncea(lw(astieittehumdtheaips30min––e1d40i5t2–e)r;Nrah)neaennacnde-,ettymhpebercacoclmiemsmapotaenrlityss
ded frn 02/0 ufosuedrontahmerelifmoritethdeacnldimwaitdeeltyypsec.atTtehreisdcalriemaastoefatnhdewthoerladss(oseceiaFteidgubrieo1mieno9c3c)u:rthine
nloaa) o PacificCoastofNorthAmericafromsouthernOregontonorthernBajaCalifornia
owvor (latitude31––41– N);partsofWestandofSouthAustralia(latitude32––38– S);
DE
30:51-81. e evora (U tccheeepnttsrfaoolurtChAh-uwilseetarsatnelircano,awCsahtpe(erleatRittheuegditeoonp3o2og–f–ra4Sp1oh–uytShi)sAc(fhforairrcaafcut(reltahritezirteudddeebtya3iml2s–o–sd3eee5r–a5t1Se,)r8;e4lai)ne.df,Etthhxee-
99.e d mediterraneanlandsareoftenruggedwithamarkedchangeinelevationand,there-
9d
1a
nu. Rev. Ecol. Syst. by b-on: Universid fCttihenoorerrtenSahc,senleeiaaemqsltasuhonaoernt-eneatieytlnilipsytmyec,tloohicanomelntaiwhdamdtsewiavctotaaeafrct.neioawrdAnbsidtcinolioitltfteleiydosra,onstimtswon6ieabrt5leahso%itmnnrmegfeaaaodlmrnsleitdtslieaosromtfritfavhattneehyeelneyabpn8persi0rh(ine%neocc.riightppo.iaighrtTlhaoemtraieritozoltRenroriveinbvoatuetafoittrlfoee,nndStohipasffearataetilrhnlnaaie;scnine1gwms4hfd5e(aeud5)lr.lri)es--.
n
A ing a few major storm events that may produce flooding (100,102,119). The
strongseasonalityinrainfall(duringthecoolwetseason)andtheassociatedsea-
sonalfloodingclearlydistinguishthemediterraneanclimatefrommostmesicor
xeric temperate climates; in the latter climate regions, storms can occur during
the colder wet season but also at other times of the year. Although the seasonal
precipitationpatternishighlypredictableinmediterranean-climateareas,annual
rainfall can vary markedly in some regions from year to year; a deviation of
30% or more from a multiannual average is not uncommon. Such precipita-
tion patterns are best described as having low constancy (sensu 40) and high
predictability(36,40,126). Relativelylong-termdryandwetcycles(>10years)
havebeendetectedinmediterranean-climateregions(e.g. 48,145),butthepattern
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54 GASITH n RESH
isfarfrombeingconsistentandunderscoresthatshort-termandyear-to-yearvari-
ationsinrainfallareunpredictable.
Becauseannualprecipitationusuallyrangesbetween275and900mm(5,102),
certainme?diterranean-climateregionsfallintothecategoryofsemi-aridregions
(200 and 500 mm annual rainfall; 73,170); however, others may encompass
regions with annual rainfall exceeding 1000 mm, including small amounts of
summer rain (e.g. in Australia and South Africa—50,148,155). Similar to arid
regions, mediterranean-climate regions are naturally water stressed because of
highannualwaterlossesbyevapotranspirationrelativetoinputsfromprecipita-
tion(71,96,170).
g
or
ws.y.
vieonl HYDROLOGICREGIME
alreuse
nnunal The discharge regime of mediterranean-type streams generally follows that of
als.aerso the rainfall pattern, and consequently exhibits both strong seasonal and annual
om arjourn3/09. For p vcfaoalrmli,amwbieilnnittcyeer(,i4na8n,fd1al4el0aor,lr1y4es5ap)r.lryiInnwg.minSeteduribtseaernrqdauneflenoatondd-trsyypoinecgcsutarrneddamudrsein,cgtlhineainhfgeigwflhomflwoownartsheasgbirrnaudpluatltayel
ded frn 02/0 owvheernatpheernioedxtoyfesaerv’esrraalimnsocnothmsminensucme(mseeer,aelnsodi“negxatrbermupet-lwyiinntefar”llcoarteegaorlryywinin7te7r,
nloaa) o and the “perennial” and “intermittent” seasonal winter categories in 171). This
wor
ov seasonal pattern of high discharge coinciding with cool temperatures, which is
DE
30:51-81. e evora (U fobofyltlweomwetpetidenrbgaytaelnoddwesdderirysticnshtgraeroagvmeesrsw,awnithhainwcnhaurcamalnocyrbcheloedti(tseet.mugr.pbe5erd9a)tr.uerpeesa,tgerdelaytlaynddifufneprsrefrdoicmtatbhlayt
99.e d Deviationsfromtheabove-describedpatterncanoccurinmediterranean-type
9d
1a
nu. Rev. Ecol. Syst. by b-on: Universid smistwnettrrmiaeetflyhaapommbewherissag.sitthntuhoIeranmrestetsrehdeddaagiivinnsitedoecsrnhpusrsaabanrrntoogteewferoarskfnaaatfn-chrotesycelatulpionrgmewceausaoqitnltluracogeithgfaimoetmyhrneses(,nem(owt.eganih..nysgie.cSeethl4pxeo)amhv.fiianFabrt,yeaiutdiIrmnstaahroa(rbeeederi.almemg)sr,.oaowltidraeneiart,sglhfemeamsaluaelse)omdbanifanotrnaeeulfldrenrflozatauwisnfncteogetpaufrwnaawst-tittnineayoorttpneenwesrr,
n
A meltinspring(e.g. 4,145).
Althoughtheoccurrenceoffloodsduringfall,winter,orspringispredictablein
mediterranean-typestreams,theintensityandfrequencyofthefloodsvarygreatly
fromyear-to-yeardependingonthefrequencyandintensityofrainfall.Lowrainfall
may eliminate significant floods and extend the drying period; we refer to these
yearsasdroughtyears. Fluctuationsofthemeanmonthlydischargerangingfrom
100%aboveto50%belowthemultiannualaveragehavebeenreported(145),and
thisisnotatypicalofothermediterranean-typestreams(e.g. 140).
Theoftenbriefandsometimesintenserainstormsthatoccurinmediterranean
climatesproduceflashyhydrographs. Thestreamsfloodandattainpeakdischarges
shortlyaftertherainstormbegins(possiblywithinhoursinsituationsofasaturated
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STREAMSINMEDITERRANEANCLIMATES 55
soiland/orahighgradienttopography)anddeclinetonormalseasonalflowshortly
afterthestormends(e.g. Figures2and5in43).
Localgeomorphologymayplayamajorroleinhydrographdynamics. Brief,
violentflo?ods(characterizedbyrapidonsetandshortduration)typicallyoccurin
highgradientconstrainedchannels. Mediterranean-typestreamsareexpectedto
beless“flashy”thandesertstreams, whichareoftenrestrictedbysteepbedrock
canyons or alluvium, and more “flashy” than streams in mesic drainages of low
reliefthathaveextensivetributarynetworksandfloodplainsandwherethefloods
buildinintensityoverperiodsofhourstodays(e.g. 77,109,126).
Inthewetterareasofmediterraneanregions,streamsusuallymaintainperma-
g nentfloworatleastholdsurfacewaterthroughouttheyear. Inthedrierranges,
or
ws.y. mediterranean-typestreamsoftenhavesectionsofthestreamthatlackcontinuous
vieonl surfacewaterandarecomposedofaseriesofisolatedpools(e.g. 43,56);werefer
alreuse to this mosaic of isolated pools and dry areas as intermittent reaches. In Israel,
nnunal for example, drying of the headwater tributaries is characteristic of many of the
als.aerso coastal streams, and the duration of intermittency and proportion of the channel
om arjourn3/09. For p tochhfaagtnrdnoreuilensdwwgeetannteeersr,sarleliyntuitrnhnceorelfoaawsgeersircfrureolatmucrhaneloswrtohaftetsoro,smaoneudtoheffafltshuereasnietndfsatirsllecahdmaercsgreedaumsreiasny.gmStheaeeinpdtaargiyne,
ded frn 02/0 soufmthmeemrepdeirteiorrda.neSatnrecalmimsaitnemaraeyasatlhteartnaatreebinettewremeendpiaetreeninniathleflowwetinnesrasingyraydeieanrst
nloaa) o and intermittency with surface water limited to isolated pools in drought years
wor
ov (e.g. somecoastalmountainstreamsinCalifornia,43,140).
DE
30:51-81. e evora (U THESTRUCTURINGROLEOFFLOODINGANDDRYING
99.e d
9d
1a
nu. Rev. Ecol. Syst. by b-on: Universid Fnadtgeorrldryaonrrdioumynudpigaensteldgaidinprtperebromrcoyrchcoaeraenssarstesiasnamoc,n(tvie-we.etseryrhii.cspetatheriaecegnraridddsvairedsdyttruerussymaru(libnp1mrage3enmrd9ciau)neet.cermetSpivdoeteeerndrnesiieaotitsnremdtri.dnrssaitsstnrhcteerrhaeaoamanurm-ggsteyhsopoaivneunetdbsrttowhrateenheatwmmatenosednrdiluasidtareaelaralrcc)saoy;onncheultoeainnwndui-eseovaruingensrdo-,,
n
A Floods have a variety of effects on the stream ecosystem, depending on their
frequency, intensity, timing, andduration. Theseeffectsincludescouringofac-
cumulated sediment and debris, and redistribution of streambed substrate and
organic matter in the channel; changing channel morphology and forming new
erosional (riffles) and depositional (point and mid-channel bars, pools) zones;
washingawayin-channelandencroachingriparianvegetation;restoringchannel
connectivity;andhomogenizingwaterqualityconditionsalongthestreamchannel
(48,78,90,93,145).
Watershed-stream interactions in mediterranean-type streams are most pro-
nouncedduringtherelativelyshortwetseason. Thefirstfloodsusuallyflushthe
tributariesofaccumulateddebrisandcarryahighloadofdissolvedandsuspended
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56 GASITH n RESH
matter from the drainage basin into the stream. Following this and continuing
throughout the wet season, the concentration of total dissolved solids is usu-
ally relatively low and that of suspended solids relatively high (e.g. 145), wa-
ter quality?conditions along the stream channel are least variable, and diurnal
rangesinvariablessuchastemperatureanddissolvedoxygenareusuallylowest
(e.g. 32).
Dryinginvolvesagradientofeventsfromreductionsinflow, toformationof
isolated pools, to complete drying of substrate; a temporal sequence of this is
illustratedforadesertstream(160). Thephysicaleffectofdryinginvolveshabitat
contraction,whichisessentiallytheoppositeofthehabitatexpansionthatoccurs
g duringflooding. Dependingontheseverityofdrying,thefollowingconditionsmay
or
ws.y. occurinstreams(55)andtypicallyoccurinmediterranean-typestreamsduringthe
vieonl dryseason: highlyfluctuatingoxygenconcentrationscausedbyacombinationof
alreuse factors(elevatedtemperatures,highdaytimephotosynthesisandnighttimerespira-
nnunal tion,highbiochemicaloxygendemandassociatedwithincreasedalgalproduction
als.aerso andenhancedmicrobialrespiration,reduceddilutionofeffluentsinstreams,and
om arjourn3/09. For p rdceredopuaoccsheitdmioetnunrtobofuflfievnneecgese)et;adrtieimodneuncitensdt;odlositlsrusetaioomfnccoohfnannneunctertliisveni(tttysh;awtiniftchurrettahhseeerdheynspaholairnnhiceteyics; sziniolctnareet;iaosenen)d-;
ded frn 02/0 andDirnocuregahstesdcacnhaenxnagelgeerroatseiotnhebaevcearuasgeeodfrtyhiengprsoelqounegnecdedarnydinngotoofnblaynrkess.ultinthe
nloaa) o elimination or reduction of scouring flows but also result in an alteration of the
wor
ov temporalandspatialdynamicsofhabitatstructure. Consecutivedroughtyearsmay
DE
30:51-81. e evora (U hepxarovpdeaunacsnieoemnffaoejfcotdrsecipmhoaisnliagtrieotsnoinzthocanhteaosn,fnbwealantmkeroerdrpoivhseoiorlsonig,oyna,niidnncvtlheuagdteiantacgtciarouncaolemnocfprlfioenatecehcsmhedoeiknmitnemgnatosyf,
99.e d thechannel(e.g.78). Therealsomaybeanindirectresponseofflowtoriparian
9d
1a
nu. Rev. Ecol. Syst. by b-on: Universid vtmtdlohaeegeegptdoheosioetestaneaitvtrie(dreo6dagun7ners)teitahn.aanrgtnodi-odut(nyrgeoph.tguhec.gash8hntar5tent)yhag;emeetahsdsriisimsnrbetaguceyetrconbaemseuffcstoehoercemptthshle.oeosrwlIeeoneagirssrymoe;ignnotiahssolutilnsyfrsfiemoacwraciaeyhhnnebintrneeuwtoaobafletlageyanrrcdehetiaosletoceswnorbgnacuasnoihtenledcsad,tew,eqcdcauolyfoearnssotthecmadeel
n
A Anincreaseinevapotranspirationinsummermayresultinanincreaseincon-
centration of dissolved minerals (16,32,145). The diurnal fluctuations in water
qualityconditionarehighestduringthisperiod(32),andaspatialgradientinwater
qualityconditionscandevelopalongthestream,particularlyinstreamswherethe
surfaceflowisfragmentedintoisolatedpools. Thechangesinwaterqualitycon-
ditionsthatoccurduringthedryingprocessinmediterranean-typestreamsextend
overaperiodofseveralmonthsandthushaveamoregradualimpactonthebiota
thanistypicalofmorearidstreams. Asinotherstreams,waterqualityconditions
in mediterranean-type streams are influenced by stream-specific attributes such
aslocalgeomorphology,edaphicfeatures,andhumanimpact,andthusmayvary
spatiallyirrespectiveofclimaticconditions(e.g.139).
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STREAMSINMEDITERRANEANCLIMATES 57
RIPARIANVEGETATIONANDINPUTOF
ALLOCHTHONOUSORGANICMATTER
RiparianVeg?etation
The vegetation in mediterranean climates is typically sclerophyllous and ever-
green, adapted to water stress during the dry summer period, and able to grow
oninfertilesoils(22,25,50,83,91,103,156,163).Theavailabilityofyear-round
moisturenearstreamsenablesdeciduouswoodyvegetationtooccurintheriparian
zoneasseeninmediterranean-typestreamsintheNorthernHemisphere(86),with
g equivalentspeciespairsoccurringindifferentmediterraneanregions(e.g. Israel
ws.ory. andCalifornia,150).
vieonl Withincreasingaridity,theriparianvegetationofmediterranean-typestreams
alreuse becomesshorter,morescattered,morerestrictedtothesideoftheactivechannel,
nnunal and markedly different from the upland regions (58,103). Because the riparian
nals.aperso vliethgoeltoatgiyon(ei.sg.a5ls6o,1re1l1a)t,eidtmtoasyiteex-hspibeictisfipcataitatlrivbaurtieasbisluitcyhinasspeelecvieastiroinch,nsleosps,ec,oamnd-
m arjour09. For p(5o6s)i.tioSnh,ruanbdsdaenndsihtyerfbraocmeotuhsehpelaandtwsaoteftresnatphriegdhomeleinvaatteioonntoetxhpeolsoewdlabnadnkres,acphaers-
o3/
ded frn 02/0 tsitcruealamrlbyedins aorfeiansteirmmpiattcetnetdsbtryeahmums a(5n6s,).aAndlthtohueigrhrirciphanreisasnivnecgreeatasteisoninoefxhpuomseidd
nloaa) o regionscannotwithstandevenmildfires(121), vegetationalongmediterranean-
owvor typestreamsisfrequentlyexposedtonaturalandhuman-madefires(22,23,158,
DE
30:51-81. e evora (U 1fi6re8.)andthereforemaybemorefire-adaptedandshowmorerapidrecoveryafter
99.e d
9d InputsofAllochthonousOrganicMatter
1a
nu. Rev. Ecol. Syst. by b-on: Universid Rsbiostarirgspkeaeaanndmroiiwacmsnnmowsiaaontbtlrptyoleudurotwtinnmirdpispaeutuayt(rdieinbia.egenls.o,itiwn1hnp1e-tou0erpt)mds.reipItmrotessramtarrrtoyeeeladeefmiontiesnrererirssgnattynsrtehe(saeaeomn.wug-rt.eeyct3cepto,ee1rstoy1srtas0rtnc,eeog1amne7mss4osu)fi,m.stcAhewaerlnestmohllpoienyurdegiclcthooeowrvgfrae-narorinzrlaedeenaesddnsr,
n
A climate are apparently as important as they are in temperate, forested streams
(e.g. TableIIIin103).
mediterranean-typestreamsdifferfromtemperatedeciduous-foreststreamsin
thetiminganddynamicsofallochthonousdetritusinput.Inthelatter,thebulkof
litter fall occurs in a short pulse in autumn (ca. 70%–80% in 1–2 months, e.g.
60),withadditionalmaterialenteringthestreamsbylateralmovementoverthere-
mainderoftheyear(e.g. 10,42,112). Thefewstudiesthathavemeasuredorganic
matter input to mediterranean-type streams suggest a more protracted period of
litterfallthaninnon-mediterranean-typestreams,withpeaksoccurringatdifferent
timesoftheyearintheNorthernandSouthernHemispheres(autumnandspringcf.
summer,respectively,22,91,103,163). IntheNorthernHemisphere,thepresence
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58 GASITH n RESH
of fall-winter or summer deciduous trees along with sclerophyllous evergreens
in the riparian community of mediterranean-type streams (e.g. 43,86,103) can
partiallyexplaintheautumnpeakandtheextendedperiodoflitterproduction. In
theSouthe?rnHemisphere,protractedlitterfallresultsfromevergreenphenology
wherebynewleafgrowthisbalancedbyalmostsimultaneousleaffall(157);per-
hapsthesummerpeakresultsfromtherelicphenologyofthetropical-subtropical
vegetationthatoccursthere(155,157). InNorthernHemispheremediterranean-
type streams, most input occurs when discharge is high and the temperature is
relatively low. In contrast, most leaf fall enters mediterranean-type streams in
theSouthernHemispherewhendischargeislowandwatertemperatureisusually
g increasing.
or
ws.y. Riparianlitterthatentersstreamsbylateralmovement,i.e.blownbythewind,
vieonl sliding in from the stream banks, or being carried in by receding flood water
alreuse (10,136), may vary considerably in quantity and quality, but it may be espe-
nnunal cially important in areas of low plant cover, such as in higher-order streams or
als.aerso mediterranean-typestreamsindrierregions. Thelateralmovementoftheriparian
om arjourn3/09. For p lnwiatittneudrr,ebhlaoonwwdendveoenrr,sfliistoyoedoxwfpetahcteteerudtnrtadonevsrspatrooyrrtyleodvc.eaglleytadtieopnen(3d4in)g,aonndgwehoemthoerprhthoelolgitytearnfdaltlhies
ded frn 02/0
nloaa) o RetentionandTransportofAllochthonousOrganicMatter
wor
ov Theavailabilityofallochthonousdetritustoconsumersdependsnotonlyoninputs
DE
30:51-81. e evora (U btchhueatrragelesteoannotdinvpetohcseiatpiivnaescltiyrteyianomflfudtehinsectresitdbruebatyimorne,tewonfhtiitvchheessietsruignceptnuuertersas.l(lTiy.ehi.enrvloeacrtktseesrlayinsdreadlaefutbenrdicsttdiooandmiossf-)
99.e d wherecoarseparticulateorganicmatter(CPOM–>1mmindiameter)accumulates
9d
1a
nu. Rev. Ecol. Syst. by b-on: Universid (atCHdyenoPe.pdwgmOe.tniMrs1ssattp3rnreh3esdaepa,umr1more3irs,nt6sgipme,nav1tore4hbtsr9ieotac,tluhw1olm5faeh3tottee,hnms1mete5hiaas4sasptl)otalh.eonferDtc.redheurIotsnrhiwitoanmnrengesnoetttdurlheieiserktaseemwdrlteryhe;atetrtnthiosetseeuatrearsneexs-faohitosmynirbp,eesies,.xtplplosIaoentwtrwceeN-tseaeosomsdrtcdrstoteshoutriearnmrnbntdehethidHenteirgsetaetmSrnrseorstiaaosupmnpctohehkrbaeteeeonrrdndef-
n
A mediterranean-typestreams,theresidencetimeoftheallochthonousmattermaybe
muchshorterbecauseitentersthestreamsshortlybeforeoratthetimeofflooding.
In streams with broad valleys, litter may either enter the stream or be deposited
inthefloodplainduringtheriseandfallofthehydrograph(44,136,151). Thus,
fluctuatingdischargesduringthewetperiodmaysequentiallyaccumulate,replace,
ordistributeCPOMinstreams(47); however,theorganicmaterialisrearranged
onlyduringhighdischargeperiods.
Theretentivecapacityofforestedstreamsismarkedlyenhancedbyaccumula-
tionsoflargewoodymaterialoriginatingintheriparianzone(120),whichforms
debris dams (12). Because of sparser woody riparian vegetation (except for re-
strictedheadwaterreachesinthemoremesicmediterraneanrange),mediterranean-
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September17,1999 17:4 AnnualReviews AR093-03
STREAMSINMEDITERRANEANCLIMATES 59
typestreamsarelikelytohavefewerdebrisdams(onanarealbasis),lowerretention
capacity, and less allochthonous detritus available than their temperate forest
counterpartswithsimilarphysicalstructure(e.g. 91). Humaninterferencethattyp-
icallyocc?ursinmediterraneanregions(deforestation,grazing,fire,channelization,
urbanization)furtherreducesriparianinputsofwoodymaterialandthelikelihood
ofnaturalformationofdebrisdamsandbackwaterhabitatsinmediterranean-type
streams.
IntheNorthernHemispherewherethelargestamountoflitterfallapparentlyen-
tersmediterranean-typestreamsinautumnandwinter(e.g.ca.60%inaMoroccan
stream;103),CPOMisexpectedtoberetainedinlow-orderreachesforonlyashort
g periodinautumn(1–2months)untiltherainyperiodcommencesandtheseasonal
or
ws.y. successivefloodsoccur. Inshortandsteepgradientmediterranean-typestreams
vieonl withlowretentivecapacity,asignificantportionoftheCPOMthatentersthestream
alreuse duringfallmaybeexporteddownstreamandpossiblyevenleavethesystemincom-
nnunal pletelyprocessed(energy“leakage”sensu172). Thislossmaybepartlycompen-
als.aerso satedforlaterintheyearbytheinfluxoflitterinspringandsummer(e.g.ca.40%
om arjourn3/09. For p osthpfehthederiesacwnhnhauergareleilniitnpteumrtfeiandlilatpeMrroraodnruoeccatcnioa-ntnypspteeraeksatsmrein,a1mla0st3e)iss.pIdnrienccgorenaatnrsdainsstgu,,minCmtPheOerMSatotuchtaehnteirbmneeHewfefhmeecin--
ded frn 02/0 tCivPeOlyMremtaaiynebdeirnedlouwce-dorbdyerhrigeahcnhuemsfboerrsaopfesrhioreddodfer4s–t6hamtoonctchusr(t2h2er,e92(1),62bu,1t6th3i)s.
nloaa) o
wor
ov
DE
30:51-81. e evora (U ETCOOFSLYOSOTDEMINAGN,DDRAYQINUGAT,AICNBDIOHTUAMRAENSPIOMNPASECT
99.e d
9d ExpectedResponse
1a
nu. Rev. Ecol. Syst. by b-on: Universid Bapwpsaraesetsotdweecidrocinatusaotlebodndlfeetixshnteprepeaeunscctodhtontsafh.araaalWltclcttoehhecraephinsbrtgothiioepocstosnasoiooenuffs2mhto5harebetgdeiaitshtanteyatirdcbrcrlamooenngahedtraytaientppir-ooh,tatynhnopsedfesatesmnhtsdreee(dssareieemtaqsessouroruseanhrnnacoteleiuaasllcandlvay-ftraoycnillpiualteoymbwiosbltfiecrtweyree,aradtmttaehiisrnne,,
n
A brackets)organizedaround(a)abundanceanddiversity, (b)richness, (c)abiotic
andbioticpressures,(d)speciescoexistence,(e)riparianinputsandtheimportance
ofallochthonousorganicmattercomparedtoautochthonousproduction,and(f)
competition for water and the impact of human activities. After presenting the
hypotheses,weexaminewhattheliteraturepertainingtofloodinganddrying,and
the status of stream ecosystems in mediterranean areas, can tell us about actual
influenceofenvironmentalandhumanfactors.
[1.1]Unlessthestreamdriescompletely,theabundanceofthebiotaisexpected
tobelowestduringthewetseasonwhenfloodingoccurs. [1.2]Inspring,following
thecessationofflooding,thebiotawillrapidlyincreaseinabundancefromindi-
vidualsrecolonizingfromrefugia(e.g. fish)orfromindividualsmigratingfrom
P1:FHS/FHR/FGO P2:FLM/FGP QC:FLM/arun T1:FhN
September17,1999 17:4 AnnualReviews AR093-03
60 GASITH n RESH
nearbyaquatichabitats(e.g. insects,amphibians). [1.3]Themostflood-resistant
specieswillremain,andtheresilientspecies(the“pioneerspecies”thatareeffec-
tive colonizers after disturbance) will be the first to colonize after floods cease.
[1.4] Thes?e are gradually augmented with other species that are less resistant to
floodingandslowertorecoverbutthatcanusethewindowofopportunitypresent
duringlatespringandearlysummer,whenhabitatandresourceavailabilityarehigh
andhabitatconditionsarerelativelymoderate,tobecomeestablished,grow,and
reproduce. [1.5] Correspondingly, the abundance of the biota in mediterranean-
typestreamsisexpectedtobehighestattheendoftheintermediateperiodbetween
floodingandextremedrying.
g [1.6]Withtheprogressionofdryinginlatesummerandfall,andtheassociated
or
ws.y. declineinhabitatavailabilityanddeteriorationofwaterquality,thebiotagradu-
vieonl allybecomesdominatedbyspeciesthattoleratelowdischarge,warmerwater,and
alreuse poorwaterquality. [1.7]Correspondingly,diversity(i.e. theinteractionofrichness
nnunal andevenness)shoulddifferbetweenwinterandsummerassemblages,reflecting
als.aerso temporalchangesinspeciesdominancethatresultfromchangesinresourceavail-
om arjourn3/09. For p appbeeirnlidiotidyngwanohdnenhthaeebxiettrxaettmecnoetnpodhfiytdisroyincisan;lgea,vnoedvnepnrheaysllssaiscbhouocnuhdleadmnbciecealcloawcnoernsedtmiataitoitnnhsehipegrnhed,viaoniflc.rthe[a1es.d8er]eyDvineegn-
ded frn 02/0 fdurrytihnegriwshraepniddraynidnghaibsistalotwcoanndditiiosnosladteedterpioooralstemoaripnotaoilnsdwrayt.e[r,1.o9r]dFeucrltihneermwohreen,
nloaa) o thespeciescompositionandabundanceofthebiotainmediterranean-typestreams
wor
ov is expected to shift rapidly from the late summer-fall assemblage to that of the
DE
30:51-81. e evora (U wusuaimlnttmerarenarssiastseiosmenmbtlhbaaglategowec.ictuh[r1st.h1fe0roo]mnTshetheteocfheflnandog-ooedfs-f;flrtoohmoisdtihwseiinnwtmeirnatfreakruenadsacsloeamnstsbrealmastgbteolatgtoheethtgoartathodef-
99.e d the summer assemblage occurs earlier in drought years as a result of the earlier
9d
1a
nu. Rev. Ecol. Syst. by b-on: Universid dfydtooeeercasb[emlr2ei,rn.twa1recse]htrlorAeoaefritlaenittvmavhheseoeslrutythchgeaapybhntedrofarrbasfoteueigtssnestrmaehainlnpoptchrm,eto.erremared.[tflpe1itoe.eh1csru1itrtima]inongiCnedaotcshnrthre-reateysneappvgmoeoesnlssudctotraiicenoncagnumblayrers,,ylsootphcanecedgcuaeicrperosstamaorntviimdcoehnrtuhnntaeohitstfeysostfihcssteuaeelcmebxcipeopoesfetcsaroitaoneitdnnee
n
A mediterranean-typestreamstofloodinganddrying. [2.2]Moreover,speciesthat
evolvedinstreamsthataredisturbedpredictably, asmediterranean-typestreams
are, may be expected to have life-history adaptations that maximize growth and
reproductionduringtheperiodofmoderatehabitatconditionsandminimizeex-
posureofvulnerablestagestoextremeconditionsoffloodordrought.
[3.1]Withinanannualcycle, theregulationofassemblagestructureinmedi-
terranean-typestreamsisexpectedtotemporallyalternatebetweenmostlyabiotic
controls during flooding (“reset” periods) and toward the end of the drying pe-
riod (e.g. extreme physical-chemical conditions, such as high temperature, low
dissolvedoxygen,lossofflow),andbioticcontrols(e.g. predation,competition)
thatoccurwhenisolatedpoolsform. Thispatterncorrelateswiththehydrograph