Estuaries

297 Pages·2006·10.22 MB·English

by  Edward C. V. Butler (auth.), Peter J. Wangersky (eds.)

HdbEnvChemVol.5,PartH(2006):1–49
DOI10.1007/698_5_022
© Springer-VerlagBerlinHeidelberg2005
Publishedonline:8November2005
TheTailofTwoRiversinTasmania:
TheDerwentandHuonEstuaries
EdwardC.V.Butler
CSIROMarineResearch,GPOBox1538,7001Hobart,Tasmania,Australia
[email protected]
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2 BroadEnvironment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1 HumanInfluence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3 RiverandEstuaryHydrology . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1 RiversandCatchments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.2 Estuaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4 EstuarineChemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.1 SourceWaters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.1.1 Freshwaters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.1.2 Seawaters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.2 HuonEstuary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.2.1 Inputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.2.2 DissolvedOxygen,OrganicMatterandSuspendedSolids . . . . . . . . . . 19
4.2.3 Nutrients,ChlorophyllandMicroalgae . . . . . . . . . . . . . . . . . . . . 21
4.2.4 TraceMetalsandOtherElements . . . . . . . . . . . . . . . . . . . . . . . 27
4.2.5 OrganicContaminants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
4.3 DerwentEstuary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
4.3.1 Inputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
4.3.2 DissolvedOxygen,OrganicMatterandSuspendedSolids . . . . . . . . . . 32
4.3.3 Nutrients,ChlorophyllandMicroalgae . . . . . . . . . . . . . . . . . . . . 33
4.3.4 TraceMetals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
4.3.5 OrganicContaminants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
Abstract The Derwent and Huon Rivers are two relatively small river systems in the
southeast of Tasmania (Australia). They terminate in estuaries that are very similar in
structure and function. Historically, runoff from their adjoining catchments has been
very dilute, but coloured by dissolved organic matter. Their locationin cool, temperate
latitudes results in a maritime climate that is changeable, but delivers regular rainfall,
and therefore river flow, throughout the year. Marked seasonal cycles in nutrient lev-
elsandbiologicalactivityareseen inneighbouring coastalwaters.Dischargefromboth
riversdoesnothavethesameseasonalsignature;itisconsistently enrichedindissolved
organic matter (including nitrogenous forms) and depleted in inorganic nitrogen and
phosphorus. Small variations in chemistry of the riverine end-members seem to ensue
2 E.C.V.Butler
fromintra-catchmentdifferencesingeology,soilsandvegetationinfluencedbylocalised
rainfallpatterns.SiliconmanifestlydisplaysthisbehaviourintheHuonsystem.
Both the Derwent and Huon estuaries are drowned river valleys, with a strongly
stratified (salt-wedge) water column at their head, tending toward partially mixed at
their mouth.Theyareinamicrotidalregion.TheDerwentestuaryconforms toawave-
dominatedsystem; theHuonestuaryisintermediatebetweenwave-dominatedandtide-
dominated. Currents are generally weak for both estuaries(≤0.2ms–1), and weaker in
subsurfacewaters.
Withthe arrivalof Europeans twocenturies ago,the Derwent andHuon estuaryde-
partedfromthesamecourse.ThecapitalcityofHobartestablishedonthewesternbank
of the Derwent estuary, and the catchment was soon modified by agriculture and tree
felling,thenbydammingforirrigationandhydroelectricitygeneration.Duringthe20th
century, the estuarybecame more industrialised, and the urban zone around its shores
grew to support a population of 190000, as did the discharges from these activities to
the waterway. Development in the Huon catchment was slower and more constrained.
Muchlessofthecatchmentwastakenupwithagriculture,andthepopulationhasonlyat-
tained13000inscatteredsmalltowns.TheonlyintensiveindustrycurrentlyintheHuon
catchmentisaquaculture,activeinthewatersofthelowerestuary.However,downstream
processingoftimberfromcatchmentforestsisplannedtorecommence.
The consequences of these disparate recent histories are that the conditions of the
two estuaries are very different. The Derwent estuary has had heavy inputs of organic
matter, suspended solids (wood fibre), nutrients, heavy metals and other toxicants. Of
all these, the metals(especially zinc, cadmium, leadand mercury) residing in the estu-
arine sediments pose the greatest threat. They contaminate at levels as severe as seen
anywhereworldwide.Itmightbefortuitousthatnutrientinputsdonotthreateneutroph-
ication,becausetheestuaryflushesrapidly(∼15days),andphytoplanktonappeartobe
light-limitedinthe middleestuary,where most nutrientsenter. Organictoxicantscause
localisedconcerns,butneedtobemorefullyinvestigated.
In contrast, the Huon estuary has been modified, but its environmental quality re-
mains highwithalmostallmonitoring resultsbelow theAustralianguidelines.Itserves
asausefulbaseline,thoughnotpristine,againstwhichthecontaminationoftheDerwent
estuarycan be evaluated.Nutrients fromaquacultureand the lower catchment, causing
increasedphytoplanktonbiomassandpossiblydepletedlevelsofdissolvedoxygeninbot-
tomwaters,appeartobetheonlyexistingchallengesforenvironmentalmanagementof
theHuonestuary.
Keywords Hydrology·Metals·Nutrients·Organicmatter·Phytoplankton
1
Introduction
TheDerwentandHuonRiversarenotlargeontheworldscale,norevencom-
pared with rivers in continental Australia. What makes them interesting is
thattheyareexamplesofriversthatareunder-representedinglobalscientific
studies:theyareinanislandintheSouthernHemisphere,inacoldtemperate
region,drainingcatchmentsoilsthataregenerallypoorinnutrients.
Thetworivers,andtheirestuariesespecially,aresimilarinmanyways[1].
They can be treated as analogues. However, their modifications since the
TheTailofTwoRiversinTasmania:TheDerwentandHuonEstuaries 3
arrival of Europeans just 200 years ago have followed different paths. The
Derwent estuary has become the centrepiece of the capital city of Tasma-
nia,Hobart.Alongbothitsshoresisnowanurbanribbon,comprising some
heavyindustry,portfacilitiesandmanyeffluentdischarges.Thewatershedof
theDerwent River hasbeen muchmodified byagricultureandotherhuman
activities,andtheflowoftheriverhasbeenmodulatedanddiminishedby16
damsalongitscourse.Thesechanges,andlossofhabitatwithintheestuaryit-
self,haveharmedthenaturalecosystem(e.g.theendemicspottedhandfishis
underthreatofextinction)andseeminglyfavouredtheintroductionofexotic
marinepeststhathaveexacerbatedthenegativetrend.
In contrast, the human influence on the Huon River and its estuary has
been more moderate. Muchof its upper catchment remains as native forest;
sometractsareexploitedbyforestryactivities. Apatchworkofagriculture–
horticulture and livestock grazing – pervades the lower catchment. Human
settlement remains sparse with series of small townships along the water-
course. A recent development is salmonid, and to a lesser extent shellfish,
aquacultureinthemarineendoftheestuary.
Aswellascriticallyexaminingtheavailablephysico-chemicaldataforeach
estuary and its context in regard to catchment and regional environment, I
discuss briefly the possibility of the Huon estuary providing an operational
baseline formanagement ofthemorepollutedDerwent estuary.Theecology
oftheestuaries, apartfromphytoplankton,isnotexamined here; thereader
isreferredtoEdgaretal.[1],Jordanetal.[2],andtheNSR[3]reports.
2
BroadEnvironment
Tasmania hasacooltemperatemaritimeclimatedominated byazonalwest-
erly wind regime [4,5]. The westerly winds are present throughoutthe year,
but fluctuate from weekly intervals (with the passage of anticyclones), sea-
sonally(strongestinlatewinter/earlyspring)andinter-annually.Themarine
influence resultsinmildwintersandcoolsummers withatypicaldaily tem-
◦
perature range of 7 C, but with weather that is very changeable. The island
issufficientlylargetohaveaslightcontinentaleffectonclimateinitscentral
highlands,muchofwhichformtheuppercatchmentoftheDerwentRiver.
Rainfall is broadly highest near the west coast and declines steadily to
the east. The rugged topography of many parts of the island creates micro-
climates; higher altitude features produce localised rainfall patterns super-
imposed on the general trend. In the west of the Huon catchment, annual
average rainfall peaks near 3000cm; it is 500cm and less in the southeast
of the Derwent catchment [6]. Precipitation is generally uniform through-
out the year, although toward the west coast it peaks in late winter. In the
east, it is slightly higher in autumn and spring, affected by intermittent cy-
4 E.C.V.Butler
clonicdepressionsofftheeastcoast[5]. Atmosphericdeposition,wetordry,
throughout Tasmania is typically uncontaminated, because to the west, over
theSouthernOcean,isoneofthelongestfetchesanywhere.
The Derwent and Huon catchments (Fig.1) are located in the same geo-
logical province characterised by post-Carboniferous cover (Permo-Triassic
mudstone, sandstone and shales into which large sheets of Jurassic dolerite
have intruded) over pre-Carboniferous basement rocks [7,8]. Only in their
headwaters do they differ, with the Derwent draining highlands of similar
geology to its lower catchment, but with scattered outcropping of Tertiary
basalt, and in the far northwest Quaternary glacial and periglacial deposits
overlying the dolerite. A few western tributaries of the Derwent River have
their sources in an Ordovician limestone sequence. The upper Huon catch-
ment sits in an adjacent geological province of mostly Precambrian rocks
(unmetamorphosed sandstone, siltstone, conglomerate, etc., withoccasional
metamorphic quartzite). The hills between Cygnet (Fig.2a) and the Huon
estuary are a local anomaly, with Upper Carboniferous glacio-marine sedi-
ments intruded by alkaline, Cretaceous syenite. The Central Plateau and the
higher altitudes of the western Huon catchment were repeatedly glaciated
duringtheQuaternaryperiod,andclassicalglacialandperiglaciallandforms
arecommon.
Geological faulting during the early Tertiary period has established the
dominantNW–SEalignmentofvalleysseeninthecourseoftheDerwentand
Huon Rivers. The soils reflect the partitioning of the region’s geology and
some influence from vegetation. The lower catchments of both rivers have
typically acidic, organic-rich podzols [10]. Skeletal soils and moor podzol
peats overlie the ancient rocks that give rise to the Huon River. The upper
Derwent catchmentisdifferentwithalpinehumussoilsandmoorpeats.The
soilsoverlyingthePermo-Triassicsedimentaryrocksareparticularlyimpov-
erishedinbothnitrogenandphosphorus[11].ThosewithJurassicdoleriteas
parentmaterial aremarginallybetter(typicallylowNandmedium P),butit
isonlythesoilsderivedfromTertiarybasaltintheupperDerwentcatchment
thathaveadequatenutrients(typicallymediumNandhighP)tobedescribed
asfertile.
Vegetation of the catchments comprises three main assemblages: alpine
(austral-montane), temperate rain forest and sclerophyll forests dominated
by Eucalyptusspp. [12]. Within these assemblages are many different veg-
etation types – Kirkpatrick and Dickinson [13] identify about 30 over the
Derwent and Huon catchments. This diverse mosaic is a result of the com-
plexityoftopography,regionalgeologyandsoils,overlainwithmicroclimatic
variability.TheDerwentRivermarksanapproximatetransitionbetweenrain
forestandwetsclerophyllforeststothewest,anddrysclerophyllforest,grassy
woodland and grassland to the east. The latter two vegetation types were
probablymaintainedbyburningpracticesoftheindigenouspopulationinthe
past;littlenowremainsafterclearingforagricultureandgrazing.Morethan
TheTailofTwoRiversinTasmania:TheDerwentandHuonEstuaries 5
Fig.1 MapofDerwentandHuoncatchmentsinsoutheastTasmania.Catchmentbound-
aries,majortownsandtributaryriversareindicated.Thecross-hatchedareaoftheGreat
Lake/OuseRiversubcatchment isthatpartdivertedtotheSouthEskRiversystem. The
urbanzoneofHobartisalsoindicatedalongtheshoreoftheDerwentestuary
6 E.C.V.Butler
Fig.2aMap of Huon estuary.Samplingsites for theHuon EstuaryStudy [9] areshown,
asarethelocationsofthemajorland-baseddischargestothewaterbody.Individualsites
oncross-estuarytransectsarenumberedfromwesterntoeasternshores(e.g.A1––A9);
axial sites at the head of the estuary are numbered upstream (R1––R5). R6 is on the
tributaryKermandieRiver
TheTailofTwoRiversinTasmania:TheDerwentandHuonEstuaries 7
Fig.2b Mapof the lower Huonestuaryand adjacent D’EntrecasteauxChannel depicting
thelocationandareaofmarinefarmleasesattwojunctures–April1999(towardtheend
oftheHuonEstuaryStudy)andDecember2003
aquarteroftheDerwent Rivercatchmenthasbeencleared(27.6%including
urbanareasandwaterstorageimpoundments),whileamuchsmallerfraction
oftheHuonRivercatchment(5.6%)issoaffected[1].
Coloureddissolvedorganic matter (CDOM) isleached readily fromsome
vegetation types (e.g. buttongrass moors Gymnoschoenus sphaerocephalus),
8 E.C.V.Butler
plant species (e.g. tea-tree Leptospermum spp.) and also soils (e.g. peats) in
the Huon catchment, and to a lesser extent the Derwent catchment. Conse-
quently, the waters of many tributaries of both rivers are strongly coloured.
Generally, the concentration of the nutrients nitrogen and phosphorus in
runoff to Australian rivers is relatively low, especially if catchments retain
theiroriginalforestcover[14].Thisisfurtheraccentuatedinriversofsouth-
east Tasmania by low nutrient concentrations in precipitation and naturally
impoverished catchmentsoils.
2.1
HumanInfluence
Aborigines are believed to have arrived in Tasmania more than 35000years
ago. The two tribes of the southeast coast (Oyster Bay and South East) were
hunter-gatherers,whousedtheestuariesoftheregionextensivelyasasource
offood,particularlyshellfish[15].InlandintheDerwentcatchmentwerethe
Big River Tribe. They hunted terrestrial animals as a stapleof their diet, but
also ventured to the coast regularly for shellfish. The imprint of these first
Tasmaniansuponthelandanditsdrainagesystemswasminimal(basepopu-
lationestimateof3000–4000fortheentireisland),exceptperhapswherethey
modifiedcatchmentvegetationbyuseoffire.
From1804 at Sullivan’sCove(Fig.3) –stillahub ofpresent-day Hobart–
thefirstEuropeansettlement inTasmania rapidlyexpanded. The population
was∼5000bythe1820s,∼35000by1901,andispresentlyjustunder200000
in Greater Hobart (not all within the Derwent catchment). During develop-
ment, the western shoreline of the Derwent estuary was modified by land
reclamation, aswere anumber ofthesidebays later [17]. Further upstream,
convict labour built the Bridgewater causeway in the 1830s. Settlement of
much of the farmed land in the Derwent River catchment had happened by
the 1850s. The first-class deepwater port became one of the most important
in Australia fromabout that time, and maintained that status formore than
a century. Industrialisation of the Derwent estuary in the 20th century was
promoted by the advent of hydro-electricity. First came the electrolytic zinc
refinery at Risdon just north ofthe cityin 1917; the newsprint mill at Boyer
followed in 1941. Smaller industries such as abattoirs, tanneries, foundries
andhorticulture(e.g.hopsandmarketgardens)haveoperatedfromthemid-
to-late1800s.Thesewerejoinedduringlastcenturybytextilemanufacturing,
fertiliserproduction,timbermillingandfoodprocessing.
The Huon region has always been sparsely settled from the arrival of the
first Europeans about 160 years ago. The present population is ∼13000;
most are dispersed in small towns. It was not until the second half of the
1800s that land in the lower catchment was seriously taken up for forestry
and agriculture, mainly horticulture (apple and pear orchards), grazing and
dairying [18]. By the late 1800s these activities extended into sawmilling,
TheTailofTwoRiversinTasmania:TheDerwentandHuonEstuaries 9
Fig.3 MapofDerwentestuary.SamplingsitesfortheDerwentEstuaryStudy(CSIROun-
publisheddata,1993–1994)andalsoforseveralmorerecentsurveys(see[16])areshown,
asarethelocationsofthemajorland-baseddischargestothewaterbody.Individualsites
oncross-estuarytransectsarenumberedfromwesterntoeasternshores(e.g.A1––A7);
singlesitesintheriverchannelabovetransectHarenumberedupstream(U1––U23)
shipbuildingandfruitprocessingalongthebanksoftheHuonestuary.Trans-
portfortheregionfromsettlementuntilwellintothelastcenturywasmostly
provided by the waterway. For part of that time, it had two important ports
(PortHuonandPortCygnet,Fig.2a)servingnationalandinternationalmar-
10 E.C.V.Butler
kets. Modification of the estuary included the draining and reclamation of
wetlands around Franklin, and the dredging of the western channel around
EggIslands toallowwater traffictoFranklinand Huonville. TheHuonestu-
aryhasnotexperienced major industry,apartfromthreestintsontheshore
ofHospitalBay(PortHuon).The“largestsawmillinAustralasia”wascutting
localtimberstherefromtheearly1900suntilitsclosurein1929[19].Onthe
same site, a neutral sulfite semi-chemical pulping mill (processing eucalypt
hardwoods)operatedfrom1962–1982and1986–1991.Marinefarmingbegan
inthelowerestuary(Fig.2b)inthemid-1980swithsalmonidfinfish(nowex-
clusively Atlantic salmon Salmo salar) and to lesser degree shellfish (mostly
bluemusselsMytilusedulis).Amajorwoodprocessingplantupstreamatthe
junction of the Huon and Arve Rivers (Fig.1) is planned to begin operation
inlate2004.
3
RiverandEstuaryHydrology
3.1
RiversandCatchments
Characteristic data for both rivers and their catchments are presented in
Table1. During the last four decades, headwaters from an area of ∼10%
of each catchment have been diverted for hydroelectricity generation. This
amounts to 12% of the total water yield from the Derwent catchment. Dis-
charge at the river mouth has fallen ∼30% from the 1920s as a result of
diversions, extractionsforirrigation,fishhatcheries andotherchanged land
practices,aswellasdrier climateintheintervening years[20]. Medianflows
at Meadowbank Dam have decreased from 100 to 75m3s–1. Moreover, the
16impoundments alongtheDerwent River,andtheiruseinhydroelectricity
generation, havesmoothedouttheriverflows,bothseasonallyandannually.
Incomparisonwithpre-damrecords,currenthighflowrates(5thpercentile)
havehalved,butlowflows(90thpercentile)havedoubled[21].
IntheHuoncatchment,diversionofitsheadwatersaboveScottsPeakDam
(1972)hasreducedtheannualdischargeoftheHuonRiversystemfrom3000
to 2600E6m3 (Gallagher [23], and references therein). Livingston [24] esti-
mated that after the dam was built, the median flow decreased by 15% and
lowflowsbyabout8%.TheHuonRiver attheflowgauging stationatFrying
Pan Creek (annual median 41m3s–1) presents a monthly flow record simi-
lar tothe Derwent River, withthehigher flowsinlate winter/early spring in
line with the rainfall pattern (Fig.4). No power stations are sited within the
modifiedHuoncatchment.Theuniformsummerflowispossiblymaintained
by either reserves of groundwater, or retention in the button-grass plains of
theuppercatchment,orboth.