Seasonal oceanographic surveys of Macquarie Harbour suggested the existence of a three-layer system: a surface layer responsive to seasonal heating and cooling and to river runoff, a slowly changing mid-level layer, and (in the deeper basins) a marine layer fed by tides flooding over the sill

Barker, BA

Cresswell, GR

Edwards, RJ

GR Cresswell (17323777)

RJ Edwards (11054997)

BA Barker (17323807)

The Francis Crick Institute

Macquarie Harbour, Tasmania-seasonal oceanographic surveys in 1985

University of Tasmania Open Access Repository

Papers and Proceedings (Jf the Royal Society of Tasmania, Volume 123, 1989 63 MACQUARIE HARBOUR, TASMANIA - SEASONAL OCEANOGRAPHIC SURVEYS IN 1985 by G. R. Cresswell, R. J. Edwards and B. A. Barker (with three text-figures) CRESSWELL, G.R., EDWARDS, R.J. & BARKER, B.A., 1989 (31:x): Macquarie Harbour, Tasmania - seasonal oceanographic surveys in 1985. Pap. Proc. R. Soc. Tasm. 123: 63-66. https://doi.org/10.26749/rstpp.123.63ISSN 0080-4703. CSIRO Division of Oceanography, Marine Laboratories, GPO Box 1538, Hobart, Tasmania, Australia 7001. Seasonal oceanographic surveys of Macquarie Harbour suggested the existence of a three-layer system: a surface layer responsive to seasonal heating and cooling and to river runoff, a slowly changing mid-level layer, and (in the deeper basins) a marine layer fed by tides flooding over the sill. Key Words: oceanographic survey, Macquarie Harbour, Tasmania. INTRODUCTION Macquarie Harbour in western Tasmania is one of Australia's largest estuaries. It has inputs from both nature and the activities of mankind the sea, rivers and rainfall (1.8 m annually at the coast and 2.6 m in the highlands); mine slurry discharged into the King River for over a century and cold water discharged from turbines deep in the Gordon Dam. A recent review of the state of knowledge of the hydrological and hydrodynamic system of Macquarie Harbour found this to be "totally inadequate for making any statement on the environmental effects of any development projects which will physically alter the rivers or the harbour" (Waterman and Matthews, 1979). 1985 SURVEYSThe present study was modest in both effort and expense; it was designed to get an initial feeling for the seasonal variations of standard oceanographic water properties in the harbour. Four oceanographic surveys of the harbour and the Gordon River were made in January, March, July and October 1985, using a powered catamaran chartered from the National Parks and Wildlife Service; the Gordon River was monitored because of its importance as a source of fresh water for the harbour. As shown in figure 1, four stations were sited along the harbour axis (named according to the nearest landmark) and three on the river - "down-river" at the mouth of the Gordon River, "mid-river" 8 km upstream, and "up-river" 35 km upstream at Warners Landing. (The undammed Franklin River flows into the Gordon River 5 km upstream from Warners Landing.) Nansen bottles (with reversing thermometers) were spaced at 2.5, 5 or 10 m depth intervals. The lead weights on the Nansen bottle wires were used for sounding at the stations. Water samples were analysed for salinity, oxygen, nitrate, and silicate content (Major et al. 1972). The January and October surveys were each completed in one day. Both the others required part of an extra day because of bad weather on the March survey and bad weather and short daylight hours on the July survey. The authors' conclusions about seasonal variations take no account of high frequency variations due to tides, passing weather patterns, varying flow through the Gordon Power Station, etc. However, the harbour is expected to have a long time constant as compared to, say, a river, so that the coarse temporal sampling used will be suitable for identifying gross seasonal variability. RESULTS Macquarie Harbour The sections (fig. 2) and the temperature, salinity, and oxygen profiles at the Sophia stations (fig. 3) enable three layers to be identified in the harbour: (1) A surface layer in the upper 10 m, dominatedby seasonal heating and cooling and river runoff.The annual surface temperature range at the Sophiastation is nearly 10°C. The runoffs from the Gordon64 G. R. Cresswell, R. 1. Edwards and B. A. BarkerFig. 1 -- A chart of Macquarie Harbour and the Gordon River, showing the station positions.River and, to a lesser extent, the King River bringswaters low in salinity and high in oxygen andsilicate into the harbour.(2) A long-residence-time layer centred at about20 m depth, having a small annual temperaturerange (0.8°C), low oxygen content and, in general,a peak in the nitrate content.(3) A deep marine layer in which the salinityexceeds 31 %0 and the oxygen content increaseswith depth. The annual variation of the temperatureof this layer (1.5°C) has a phase similar to that ofthe ocean, being warmest in March and coldest inOctober. The layer is probably replenished by floodtides bringing in marine water, which is thenmodified by mixing across the harbour entrancebefore sinking to the deepest basins. The entranceto the harbour is about 7 m deep and the speed ofthe flood tide ranges from 2 to 6 knots (Waterman& Matthews 1979).The Gordon RiverThe sections (fig. 2) revealed the Gordon River tobe a source of fresh high-oxygen waters. The up-river station was mixed from top to bottom, withno sign of saline waters. The salt wedge alwaysreached the mid-river station, where the maximumsalinity was 20.8%0 in March.DISCUSSIONA testable hypothesis arising from this work is thatthe water at about 20 m depth in the harbour hasa long residence time and is low in oxygen. Abovethis, the water properties are controlled by runoffand seasonal heating and cooling, while below, inbasins where depth exceeds 20 m, can be foundmarine water that has been modified by mixing.Macquarie Harbour ocea,l1oenlDI~ic surveys in /985 6540~20-EOctg 10£; 20~300---5-12"-~---21-28-38km----~......46--73Gould ~i:; ~~~ R~r40L:J,-~7~§ 30 -:- -3~ / Mar '~/40--~600:-~'00 : woo\-~. 400 • '- Y~ , "/:,---. ~oo Jul "-...Since 20 m is not an uncommon depth in theharbour, a large part of the harbour floor will becovered by water low in oxygen. The chemistryand biota may reflect this.Waterman & Matthews (1979) took temperatureand salinity profiles around the harbour on fourdays in early September 1979. Their temperaturesection along the harbour axis was very similar tothat for July 1985, except that it was restricted to26 m depth by the length of their instrument cable.The salinities measured at these depths were lowerFig, 2 - Sections along the axis of MacquarieHarbour and the Gordon River for the foursurveys: (A) temperature, (B) salinity,(C) oxygen, (D) nitrate, (E) silicate,-_":-31~20 .. ~\.-5_:, '-'\ / ;-30~~O ,/Jan 1985 ~/0"-5-12-- 21-28-38 km--46--73Coal GaUd ~~ Ri~~ R~er~;oo:~:25g~o./2~ \..-TJan 19850-5-12--21-28-38 km -· 46--73Down Mid ~King 8qJt1ia Coal Gould River River Riverto0j2'~;Q,""'~''"'~ .'5. 20- . . <150' '\, 50/~ 30 . ·150 Jul "-.../40 ,Strahan King ~l;:~~::~1:1 30 15'~'---:--'/4066 G.R. Cresswell, R. J. Edwards and B. A. BarkerREFERENCESACKNOWLEDGEMENTSCONCLUSIONS(27%0) than the value of 31 %0 measured on all fourof these 1985 surveys.In the Gordon River, the salt wedge was alwayspresent at the mid-river station. Kearsley (1978)found that the length and depth of the wedgedepended almost entirely on the flow in the riverand concluded that in high flows the salt wedgewould be almost entirely flushed out of the river.King & Tyler (1981) have expressed a concern thatthe controlled flow from the Gordon Ri ver PowerStation prevents the natural upstream excursions ofthe salt wedge, which are probably essential toprovide salt water to the meromictic lakes alongthe river. Certainly, the salinities at the mid-riverstation 8 km from the mouth were much less (20%0versus 30+%0) than the pre-dam values reported byKearsley (1978).KEARSLEY, B.V., 1978. THE PHYSICAL BEHAVIOUROF SALINE WATER IN THE GORDON RIVERESTUARY. Lower Gordon River Scientific Survey,Hydro-Electric Commission, Hobart.KING, R.D. & TYLER, P.A., 1981. Meromictic lakes ofSouth-west Tasmania. Aust. 1. Mar. Fresh. Res. 32:741-756.MAJOR, G.A., DAL POINT, G., KLYE, J. & NEWELL, B.,1972. Laboratory techniques in marine chemistry. Amanual. CSIRO Aust. Div. Fish Oceanogr. Rep. No.51.WATERMAN, P. & MATTHEWS, LH., 1979. A Review ofInformation on the Macquarie Harbour Environment.SWTRS Discussion Paper No. 20, NPWS, Hobart.The work could not have been done without theassistance of the National Parks and WildlifeService and for this we thank the Director in Hobartand Nick Bates, Jack Gibson, Shane Hunniford andRussell Wills at Strahan. From CSIRO we had theassistance of Ed Butler, Peter Herbert and CarlNilsson.The series of surveys along the harbour axis hasproven useful in identifying gross seasonalvariations. The biological implications of theapparent long-residence-time, low-oxygen layer atmid-levels could be a fruitful subject for futurestudy.CBA. .Temperature ( °C)12 14 16 18 20, ,,,~ ---.JOct....-_.... ~ ~........ r;; ~~ an.........!e ~~\ .\ \c\ i1\\I "\i il,MarI joelL Ju,MJanSalinity (practical units)5 .."" 10 (;- ~~ 15 ,s 20 25 30 35o ~I---i~k~='-~O=-'_~__'= ~>-----L__-.L__-"__-.J._--~,,,~,~\\\\~il104010304050:§~ 20o.c~o 30_ 20.5Oxygen J,Jg atomll100 200 300 4000, L-----....L--c:= -;"'"--'-'_,," ....JIMar Oct .../JU!Ja t::"/' /<:.10 .w;(.:..~.....·o.120] ~30 .;;0\~\~40 UFig. 3 - Profiles for the four surveys at theSophia station for (A) temperature, (B) salinity,and (C) oxygen.(accepted 10 February 1989)

Macquarie Harbour, Tasmania-seasonal oceanographic surveys in 1985

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