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Volume 64, 1935
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Hydrographical Observations in Auckland Harbour.

[Read before the Auckland Institute; received by the Editor November 5th, 1933; issued separately March, 1935.]

In connection with an Ecological Survey of the Auckland Harbour, being carried out by the staff of the Auckland Museum, it has been found that many observations have been taken from time to time for such purposes as harbour improvement and city drainage schemes. While many of the details refer to local problems, there still is a large amount of valuable data filed by local authorities for their own particular purposes. An endeavour has, therefore, been made to select such data as may be of some use to the Survey Staff, and to co-relate various series of observations from the sources available.

Acknowledgments must be made to Mr D. Holderness, Engineer, Auckland Harbour Board, and to Mr H. H. Watkins, Engineer, Auckland Drainage Board, for the use of information regarding various aspects of the Harbour; to Commander E. Lyon Berthon, D.S.C., Naval Department, Devonport, for the use of a series of sea temperatures, etc.; and to the Government Meteorologist and the Inspector of Fisheries for much information of a varied nature. The author also wishes to thank Mr Dilwyn John and Mr G. R. Deacon, Director of Research and Hydrologist respectively, of the R.R.S. “Discovery II,” for their practical help and advice and for the use of valuable literature in the ship's library.

Topography.

The Auckland Harbour is a tidal estuary, situated at the southwestern end of Hauraki Gulf, and running in an east and west direction. Its geographical position is lat. 36 deg. 49 min. 56 sec. S., long. 174 deg. 47 min. 57 sec. E., the observation spot being near the Devonport Wharf on the north side of the harbour. It is officially described as all that area of tidal water within a line drawn from the eastern head of Tamaki River to Park Point, Waiheke Island; thence by high water mark to the western entrance to Owhanaki Bay; thence by a straight line to Home Bay Point, Motutapu Island, and thence by high water mark along the southern and western shores of Motutapu Island, and the southern and western shores of Rangitoto Island to a position where a straight line projected at 270 deg. passes through the Rangitoto Beacon to the mainland. For the purposes of the ecological survey, however, these limits have been exceeded, and work has been carried out to the north-east, the south-east, and along the northern shores of Rangitoto Island. The harbour has a total area of 77 square miles and a water frontage of 198 miles. It has a total length of 54 miles and an average width of 1 mile.

No large amount of fresh water discharges into it, and it may therefore be regarded as a purely tidal estuary.

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The depth of water in the main channels varies from 5 to 12 fathoms, while the bottom is largely mud with smaller areas of rock, sand, and shell, or mixtures of these.

The harbour may be divided into three parts, the Outer, Inner, and Upper Harbours.

The Outer Harbour consists of all the area to the north and east of a line drawn from North Head to Bean Rock Light, thence along the crest of the reef to Bastion Point. It includes the Rangitoto Channel, which runs in a northerly direction with an average depth of 6 fathoms; the Motukorea Channel, which runs in an easterly direction with an average depth of 4 fathoms; and the Motuihi Channel to the eastward of this with an average depth of 9 fathoms. Between Motuihi Island and Waiheke Island lies another channel which receives the tidal stream from an area outside the harbour limits, and which joins the Motuihi Channel to the north-east of Motuihi Island, and passes out between Motutapu and Waiheke to the open waters of Hauraki Gulf. Opening into the Outer Harbour on the southern side is the Tamaki River, which runs north and south, and is a long, narrow, tidal river with mud flats exposed at low water.

The Inner Harbour, which includes the commercial port of Auckland, lies to the westward of the Outer Harbour to a line drawn from Kauri Point to Point Chevalier Reef. Two large bays, largely exposed at low water, open into it, Shoal Bay on the north, and Hobson Bay on the south. The Inner Harbour has an average depth of 7 fathoms, with two well-defined depressions. One is situated off North Head with a maximum depth of 16 fathoms, and the other off Kauri Point with 12 fathoms.

The Upper Harbour lies to the westward of the Inner Harbour, and consists largely of mud flats exposed at low water. The channel runs close to the northern shore and extends for about 11.½ miles past Kauri Point. The depth of the channel varies from ¼ fathom to 5 fathoms. Several large tidal creeks branch off from this and are navigable for small vessels. Each branch has a small fresh water stream at its head, and for this reason there should be a greater range in the salinity of this area throughout the year.

For the purposes of the Ecological Survey, the whole harbour was divided into the following areas:—

A.

The Upper Harbour.

B.

The Inner Harbour.

C.

The Motukorea Channel and the northern half of Motuihi Channel to the Harbour boundary.

D.

The Tamaki Passage and between Motuihi and Waiheke and the southern half of the Motuihi Channel to the Harbour boundary.

E.

The Rangitoto Channel.

F.

The area along the northern shores of Waiheke and to the north-east of area “D.”

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G.

The area along the northern shores of Rangitoto and Motutapu.

H.

The area to the north of the area “C,” including the channels and shores of Rakino and the Noises.

The areas F, G, and H form part of the waters of the Hauraki Gulf, and have been used chiefly for comparative purposes.

Tides.

The tides in Auckland Harbour are of the semi-diurnal type and do not exhibit any general difference from similar tides in other localities.

They vary in height from 5 feet 6 inches (neaps) to 12 feet (springs). There being no great influx of fresh water, the period of ebb and flow is approximately equal, and any variation is due to wind or barometric pressure.

During January, 1933, the automatic tide gauge at Queen's Wharf showed that flood tides varied from 5 hours 58 mins. to 6 hours 46 mins., and ebb tides varied from 5 hours 34 mins. to 6 hours 32 mins. The interval between successive high waters during the same period varied from 12 hours 17 mins. to 12 hours 42 mins., and low waters from 12 hours 11 mins. to 12 hours 38 mins. No related observations of wind and air pressure were taken.

The time and height of high or low water is, of course, greatly affected by the prevailing wind. Thus at the outset of a typical cyclonic disturbance, when the wind is from the north-east and the barometer is falling, higher and earlier tides may be expected. After the centre of the disturbance has passed and the winds are westerly and the barometer is rising, lower and later tides may be expected.

The extreme difference in barometric pressure observed at Albert Park during the years 1928–31 was approximately 1.4 inches. This could account for a difference of about 1.4 feet in the height of the tides.

The prevailing winds at Auckland are westerly to south-westerly, and the ebb, therefore, should be generally rather more rapid than the flood. The maximum velocity of the tidal current is about 3 knots an hour, but varies considerably in different parts of the harbour. It is more rapid off projecting points and slower off open bays. The maximum rate recorded off the centre of Shoal Bay is 2.½ knots. The maximum velocity is reached during the second hour after high water. Insufficient tests have been made to be more precise in the time of maximum. The flood-tide is rather less rapid. The tide gauge at Queen's Wharf shows that there is little period of slack water in the Inner Harbour, and the same applies to the Upper Harbour. Float tests off Motukorea Island in the Outer Harbour, however, showed no slack water at high water, but nearly two hours at low water. Problems involving tides and tidal velocities, therefore, must be dealt with according to the tidal conditions of the local areas concerned.

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As would be expected, there is a greater range of tides in the Upper Harbour, the spring tides being eighteen inches greater at Riverhead than at Auckland.

An interesting series of simultaneous water level observations from Auckland to Riverhead was taken by the Harbour Board staff in 1914. The tidal wave does not become constricted until after it passes Birkdale. The tidal levels at Auckland and Birkdale show, therefore, only slight differences. Beyond this point the tide is confined through a narrow strait which leads to several long, narrow creeks. At Greenhithe, at the entrance to this narrow portion, a considerable variation is not unexpected. Again, at Lower Albany, where the tide is confined to a long, narrow channel, the rise and fall is probably greater than at Riverhead, though it is one mile nearer Auckland. Unfortunately, the water leaves the bottom of the gauge towards low water, and accurate readings are not available.

The velocity of the tidal current is also greatly increased in the Upper Harbour. A very rapid current can be observed in the narrow strait previously mentioned, but no current meter readings have been taken in this locality.

Only a general description of the course of the tidal stream can be given here. The stream enters the harbour from both entrances. That which enters by way of the Motuihi Channel is divided into two parts by Motuihi Island. The southern portion turns south and passes up the Clevedon River outside the limits of the survey. The northern portion passes up the Motukorea Channel and is divided by Motukorea Island. The southern portion passes up the Tamaki River, and the remainder goes on to meet the stream coming southward by way of Rangitoto Channel. The combined streams move in a westerly direction up the Inner Harbour. At the eastern tide deflector the stream is deflected slightly northwards towards Stokes Point, and from here follows a course rather to the north of the centre of the harbour until it enters the upper reaches. In the Upper Harbour it is confined to the deeper water and follows the course of the channel. The ebb tide follows a somewhat similar course.

It is interesting to note that the two depressions occur off projecting points of the northern shore. Similar depressions on a smaller scale occur under similar conditions in the upper reaches. Both have long been recognised as good fishing grounds, and are therefore of particular interest to the Ecological Survey.

Harbour Improvements.

The construction of two tide deflectors on the southern side of the harbour has tended to throw the tidal stream to the north of its previous course. The velocity is also somewhat increased. The deflectors appear to have made little difference to the foreshore except in Stanley Bay, where Zostera beds extending to the end of the wharf have disappeared and the rocky shelf on the western side of the bay is now almost completely exposed to low water level.

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The area to the west of the western tide deflector is, on the other hand, silting up.

Again, by the building of the Water Front Road across the mouth of Hobson Bay, the Zostera beds in the area have apparently been silted up.

On the other hand, the bed from North Head towards Devonport Wharf seems to have been extended during this period. Unfortunately no previous records of the extent of the Zostera beds in Auckland Harbour have been made, and it is impossible to give more than an opinion on this matter. The importance of the relation of Zostera beds to animal life is recognised by marine biologists in other parts of the world, and a detailed survey of local areas is indicated.

Apart from the above-mentioned changes, very little difference in the contour of the harbour is shown in the various surveys taken since 1907. The difficulty of keeping a boat in exact position would easily account for the small variations which occur.

Meteorological.

Meteorological observations have been carried out at Auckland for many years, and a summary of the previous year's observations is given in each edition of the New Zealand Official Year Book and in the Annual Reports of the Meteorological Office, Wellington.

These observations have been dealt with very fully in a series of papers and reports by the Government Meteorologist, Dr Kidson, and only a summary need be given here.

Rainfall and Humidity.

The mean annual rainfall at Auckland is 44.73 inches, occurring on an average of 175 days in the year. The precipitation is generally greatest during the months of May, June, and July. During November, December, and January, the average totals are small, but the lowest occur in December and January. The month of maximum precipitation, however, varies from year to year, and there cannot be said to be any definite “dry” and “wet” seasons.

An interesting series of observations of the incidence of rain storms of various intensities has been compiled by Mr H. H. Watkins, Engineer to the Auckland Drainage Board, covering the period 1909–30. From this has been worked out the probability of the occurrence at Auckland of rain storms of a given intensity and duration. This may be conveniently summarised as follows:—

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Incidence. Times per year. Intensity. Inches per hour. Duration in Minutes.
6 105 14
5 1.5 18
4 2.0 25
3 2.0 30
2 2.5 38
1 3.0 60
1 in 2 yrs. 4.0 60
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More severe storms occurring once in 4, 5, and 22 year also recorded.

The relative humidity is high. During the years 1905–27 the monthly averages of relative humidity per cent. are as follows:—

Jan. 77 May 81 Sept. 80
Feb. 75 June 80 Oct. 79
Mar. 78 July 82 Nov. 79
April 79 Aug. 80 Dec. 77

The yearly average is 79%.

Barometric Pressure and Wind Velocity.

The yearly variation in barometric pressure is about 1.4 inches. The highest monthly averages occur in April, and the lowest in December and January, with a secondary maximum in September and a secondary minimum in June.

Wind velocities recorded by Robinson Anemometer average less than 180 miles per 24 hours. The greatest velocity ever recorded at Auckland was 947 miles per 24 hours.

The prevailing wind is south-west. The direction of the wind for the period 1865 to 1930, reduced to a percentage, is as follows:—

N. N.E. E. S.E. S. S.W. W. N.W.
7 16 7 7 12 28 13 10

The greatest number of days of south-westerly wind occur during the same months as the greatest average velocity, i.e., in September, October, and November. This is fully discussed by Dr Kidson in his paper on air pressure in New Zealand.

Hours of Bright Sunshine.

The average yearly total of hours of bright sunshine is 1914.5 hours. The highest monthly averages occur in January, February, and March, and the lowest in June and July.

Air Temperature.

Auckland has an average temperature of two or three degrees Fahrenheit above its expected isotherm. The daily range in temperature in the shade averages about 7.0° C., and the average yearly range is 26° C. The highest temperature recorded in the shade was 32.8° C., and the lowest —0.5° C. The mean annual temperature is approximately 14.4° C.

Sea Temperatures.

Sea temperature observations are part of the routine of the Navy, and are taken at the end of every watch, i.e., seven times a day. The observations at the Naval Depot, Devonport, were available from March, 1931. A series of monthly averages of sea temperatures at different places in the Auckland Province is given in the Annual Report on Fisheries, Marine Department, Wellington. This includes two stations in Auckland Harbour—Tamaki River and Nelson Street Wharf.

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During the year 1913–14, the Auckland Harbour Board conducted a series of observations at the end of Queen's Wharf. This series was taken not more often than three times in the month, and therefore shows some irregularities. It may be said, however, that the monthly averages follow closely those given by the Fisheries Department during the years 1929–32.

No regular observations of sea temperatures are carried out in the open sea near Auckland, and the only ones available are those taken by the Fisheries Department staff at irregular intervals. None of these were taken more often than seven times in one month, and then not always at the same place. They are, however, sufficiently accurate to indicate the difference in range between the open sea and estuarine waters in approximately the same latitude.

The daily range in sea temperatures in Auckland Harbour is not great, and does not on an average exceed more than two degrees Centigrade. The daily temperature appears to reach its maximum at about 16.00 hours. This is maintained until after 18.00 hours, when it begins to fall, reaching its minimum towards 4.00 hours. As would be expected, during bright calm weather there is a sharp rise of as much as 5° C. On the other hand, during a period of heavy wind, the temperature will be depressed, irrespective of wind direction. The Naval observations show that winds of a velocity of over 4 on the Beaufort Scale may depress the temperature as much as two degrees Centrigrade.

Sea temperature observations at Auckland have been carried out for so limited a period that it is impossible to give more than an indication of the range throughout the year. It seems, however, that the temperature of the sea in Auckland Harbour follows closely the mean temperature of the surrounding land. From Table I it will be seen that there is a sudden drop in temperautres during April and May, and a sudden rise during September and October. The maximum is reached during January and February, and the minimum during July and August.

A comparison of the sea temperatures in Auckland Harbour with those of other estuarine waters and the open sea in the vicinity is given in Table II. It may be said that the more shallow the estuary, the greater will be the yearly range in temperature. During the years 1929–32 the average yearly range at the various points of observation was:—

Kaipara 11.3° C.
Tamaki 10.2° C.
Auckland 9.2° C.
Russell 7.9° C.

Of these, Kaipara is the shallowest and Russell the deepest estuary.

The open sea range for the same period was:—

Cape Brett 7.3° C.
Mokohinau 7.9° C.
Cuvier Island 6.5° C.

and for all open sea observations, 7.2° C.

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No figures are available for the upper reaches of Auckland Harbour, but it seems reasonable to expect that the yearly range will resemble that at Tamaki or even Kaipara. It will be noted that the estuarine temperatures follow each other closely, but the open sea temperatures appear to reach their maximum a month later, i.e., during March.

Both air and sea temperatures follow closely the hours of bright sunshine, though the higher altitude of the sun during the summer months will also have an effect in increasing the temperatures.

The general increase in hours of bright sunshine, and the rise in temperatures should have some effect on the activity of the phyto-plankton during September and October, but so far no observations have been carried out to confirm this.

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Table I.
Sea and Air Temperatures and Hours of Bright Sunshine at Auckland. Temperatures in degrees Fahrenheit. Hours of Sunshine Total Hours per Month.
1929–30. 1930–31. 1931–32.
Month. Sea. Air. Sun. Sea. Air. Sun. Sea. Air. Sun.
May 57.9 57.2 155.5 61.2 55.0 169.1 58.5 55.0 123.1
June 55.6 54.9 103.3 55.4 52.3 121.6 55.0 52.8 141.5
July 54.1 51.0 135.1 52.0 50.3 144.1 54.5 53.1 106.1
August 52.9 52.1 163.4 53.1 53.8 140.9 53.2 52.8 155.1
Septmbr. 57.2 53.2 146.1 35.2 53.5 147.4 55.5 53.1 175.9
October 60.1 58.0 174.8 57.4 55.3 154.3 60.1 56.4 192.1
November 63.5 61.9 147.3 59.9 58.0 178.6 66.9 63.1 220.4
December 65.8 64.4 179.4 65.1 65.5 237.3 67.1 63.7 198.5
January 65.8 66.6 223.2 68.7 66.5 213.8 69.6 65.0 268.7
February 68.4 65.8 196.9 68.4 63.0 246.7 70.7 66.7 204.8
March 68.4 64.0 253.1 67.1 62.6 195.9 69.3 65.3 188.0
April 65.1 59.6 171.5 63.7 60.9 131.2 66.6 61.8 177.8

Sewage Disposal and Pollution.

Five sewage systems discharge into Auckland Harbour. The Auckland system discharges at a point 875 feet north-east of the Orakei Jetty. Sewage is discharged during each tidal period of 6.½ hours commencing 2.½ hours before high water. Provision is made, however, for automatic discharge during excessive rain storms occurring during the remaining period of the tide. On Saturdays, Sundays, and public holidays the discharge is confined to night tides. Before being discharged all sewage passes through a screening apparatus of ¼ inch mesh. The soffit of the outfall sewer at the point of discharge is 22.2 feet below ordinary high water level, 14.0 feet below ordinary low water level, and 11.3 feet below the lowest recorded tide level.

The Devonport system has two outfalls. Both discharge crude sewage continuously. One outfall is situated at North Head and discharges at a depth of about six feet below L.W.S.T. The other outfall is off the southern end of Narrow Neck Beach and discharges into Rangitoto Channel at a point about 9 feet below L.W.S.T.

The Takapuna system has also two outfalls. Both discharge crude sewage from storage tanks. The northern outfall discharges

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[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

Table II.—Monthly Average of Sea Temperatures, 1929–33, in Degrees Centigrade.
Auckland. Tamaki. Russell. Kaipara. Cape Brett. Mokohinau. Cuvier.
29–30 30–31 31–32 29–30 30–31 31–32 29–30 30–31 31–32 29–30 30–31 31–32 29–30 30–31 31–32 29–30 30–31 31–32 29–30 30–31 31–32
May 14.4 16.2 14.7 15.8 14.2 14.1 16.6 16.7 15.4 14.5 15.1 16.0 18.3 16.2 15.5 18.5 17.0 14.7 18.5 17.0
June 13.1 13.0 12.8 12.8 11.9 11.9 15.0 14.4 14.1 13.3 12.4 14.3 15.7 14.7 14.5 16.5 16.0 13.7 14.5 15.5
July 12.8 11.1 12.5 10.8 10.3 11.3 11.8 14.5 13.2 11.9 12.4 11.2 11.5 14.6 13.5 13.2 14.0 14.0 14.5
Aug. 11.6 11.7 11.8 12.2 11.2 11.4 12.7 14.3 12.2 11.7 12.3 12.2 14.0 15.0 12.0 10.7 14.0 13.5 12.0 14.5 14.0
Sept. 14.0 12.9 13.0 13.6 13.2 13.2 15.0 14.4 13.1 13.6 14.2 13.6 15.2 14.7 13.2 17.0 15.5 14.0 17.5 14.5 13.7
Oct. 15.6 14.1 15.6 15.5 14.8 15.9 17.0 15.5 14.7 15.4 15.0 13.8 17.5 14.7 15.5 13.0 15.0 14.0 14.0 15.0 15.0
Nov. 17.5 15.5 19.4 18.1 16.8 20.8 18.3 16.2 17.1 17.3 15.7 19.5 16.7 16.2 16.5 16.7 14.7 16.0 14.5 16.3
Dec. 18.8 18.4 19.5 19.0 20.1 19.6 19.2 17.7 18.2 19.7 20.4 21.6 17.2 18.9 16.6 18.5 16.1 17.3 17.7 15.6 17.3
Jan. 18.8 20.4 20.9 19.8 21.0 21.2 19.0 20.8 20.8 21.1 23.4 23.6 17.6 20.5 17.7 18.5 19.0 18.2 18.5 19.5 18.0
Feb. 20.2 20.2 21.5 20.8 20.1 20.8 19.5 19.7 19.7 21.6 21.2 23.7 19.3 19.7 19.1 18.2 18.0 19.1 19.5 18.0 18.5
Mar. 20.2 19.5 20.7 20.1 19.3 20.4 20.4 19.1 19.1 22.0 19.2 20.2 19.9 19.5 18.5 20.5 21.0 20.4 17.8 20.0
Apl. 18.4 17.6 19.2 17.4 17.1 19.1 19.0 17.6 17.6 19.4 19.0 18.5 19.0 17.7 18.0 18.9 18.5 20.0 18.5 18.5 20.0
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into Rangitoto Channel at Black Rock near Milford Beach. The storage tanks at this point can hold 360,000 gallons, and at present sewage is discharged every second day. The point of discharge is 12 feet below L.W.S.T. The southern outfall also discharges into Rangitoto Channel at a point off St. Leonard's Road to the south of Takapuna Beach. The storage tanks at this point have a capacity of 250,000 gallons, and sewage is discharged at present every fourth day. The point of discharge is 6 feet below L.W.S.T.

Details of the sewage systems in the Northcote and Otahuhu districts are not available. They are, however, smaller than the other systems, and an approximate amount of daily discharge has been estimated on a population basis. In both cases a septic tank system is installed and only the effluent is discharged. The amount of pollution from these two sources is probably negligible, but the estimated daily amounts have been included in the total for the whole harbour.

During wet weather, the sewage discharge is greatly increased and a large amount of storm water flows direct into the harbour. A certain amount of crude sewage is also discharged from vessels in the port. These details have not been considered in the following table.

District. Method of Disposal. Daily Average Discharge in Gallons.
Auckland and Suburban Screened sewage from Orakei Outfall for 6.½ hours every tide commencing 2.½ hours before high water. 14,500.000
Devonport Crude sewage discharging continuously from one outfall at North Head and one outfall Rangitoto Channel. 400,000
Takapuna Crude sewage discharged from storage tanks every second day. Two outfalls in Rangitoto Channel. 130,000
Northcote One outfall at Stokes Point. Septic tanks with tidal discharge of effluent. 70,000*
Otahuhu Two outfalls into Tamaki River. Septic tanks with effluent discharging continuously. 100,000*
Total 15,200,000

Sewage pollution of harbour waters has been the subject of much discussion, and from time to time statements are made of the serious deterioration of animal life from this cause. The recent work in Copenhagen Sound by Blegvad and Aage, however, shows that the effect of sewage pollution on the fauna and flora of estuaries has been somewhat exaggerated. If any serious deterioration has taken place, it must be pointed out that this is more likely to be due to general harbour improvements than to sewage pollution.

[Footnote] * Estimated on a population basis.

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It has been estimated that approximately 254,560,000 gallons of tidal water per minute pass over a line drawn from Orakei Wharf to the old wharf at Devonport, i.e., roughly north from Auckland sewer outfall. From the above table it will be seen that sufficient tidal water passes over this line in one minute to dilute the average daily discharge of sewage into Auckland Harbour more than sixteen times. There can be no doubt of the complete dilution of this discharge over two tidal periods of 12.½ hours each.

The author has been unable to detect any difference in salinity and hydrogen ion concentration between samples taken near the Orakei outfall and those from other parts of the harbour.

A series of analyses carried out on February 2nd, 1929, by Mr K. M. Griffin for the Auckland Drainage Board, gave the following results:—

1 2 3 4
Sample. Between Orakel Outfall and Beacon Rock at 10.45 a.m. Off Bastion Beacon and edge of discharge fan south side at 10.55 a.m. Outside edge of discharge fan off North Head at 11.5 a.m. In flow of sewage 150 yards from outfall.
Smell at 100° F. Faint Faint Faint Faint
Chlorinity 1988.0 1959.6 1995.1 1988.0
Nitrogen as Nitrates. Nil Nil Nil Nil
Nitrogen as Nitrites. Nil Nil Nil Nil
Ammonical nitrogen from free and saline ammonia. 0.0010 0.0145 0.0015 0.0012
Albuminoid nitrogen. 0.0060 0.0110 0.0075 0.0100
Oxygen absorbed in 4 hrs. at 80° F. 0.0370 0.0480 0.0400 0.0400

Results expressed in parts per 100,000.

All four samples are rather cloudy in appearance, and on standing gave large deposits. One or two organisms other than normal diatoms were detected.

The free ammonia in No. 2 is high and indicates a fair amount of pollution by sewage, but the other three samples are normal harbour water.

Bacteriological examination gave the following results:—

Sample No. 1.—Sample slightly turbid and colourless. Earthy matter and infusoria present in deposit. B. Coli present in 1.0 c.c. No. of colonies on Agar at 37 deg. C. 15 per c.c.

Sample No. 2.—Sample somewhat turbid and yellowish. Earthy matter and many bacteria in deposit. B. Coli present in 0.1 c.c. No. of colonies on Agar at 37 deg. C. 93 per c.c.

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Sample No. 3.—Sample slightly turbid and colourless. Earthy matter in deposit. B. Coli present in 0 2 c.c. No. of colonies on Agar at 37 deg. C. 58 per c.c.

Sample No. 4.—Sample slightly turbid and colourless. Earthy matter and infusoria in deposit. B. Coli present in 0.1 c.c. No. of colonies on Agar at 37 deg. C. 106 per c.c.

High water was at 8.20 a.m., and all samples were taken whilst the Orakei storage tanks were discharging.

Other chemical analyses gave the following results:—

A. B. C. D.
Sample. Middle of Harbour off Stanley Bay. 1.½ miles below Riverhead. Rangitoto Channel during sewage discharge. Orakei Outfall during discharge.
Free Ammonia 0.0014 0.0020 0.0016 0.1840
Alb. Ammonia 0.0070 0.0116 0.0078 0.0320
Oxygen absorbed 0.0320 0.1250 0.0420 0.1100

Sample D is very high in free ammonia and albuminoid ammonia, showing the presence of sewage.

Sample B contains more organic matter than is normally present in sea water and is also lower in salinity. The sample probably represents the normal water of this part of the Auckland Harbour, and these features are no doubt due to the intermixing with fresh water from streams in the vicinity.

It will be seen that with the exception of Sample D taken immediately above the sewer outfall at Orakei, all samples are normal estuarine waters. It must be remembered, however, that bacterial pollution of the harbour is possible from decaying animal and vegetable matter on the foreshore and from the washings of agricultural lands, streets, etc., during wet weather, and care should be taken with regard to the consumption of edible shellfish and similar marine animals in the Inner Harbour area.

Pelagic animals such as fish will probably avoid highly polluted areas and are unlikely to be disturbed by the present amount of sewage discharge.

Chemical Composition of Harbour Waters.

The information available is practically negative. Nothing has been done with the exception of two chlorinity determinations by Phillips and Grigg (1925).

The analyses conducted by Griffin for the Health Department are unsuitable for biological or hydrographical purposes. Further, the samples had obviously been stored for some time previous to

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analysis, with the result that the salinity figures are too high and the supply of nitrates used up by the microplankton present. Similar results in salinity were obtained from stored samples collected by the Survey Staff, and these have been rejected.

In the limited time at his disposal, the author has carried out a small number of salinity and hydrogen ion determinations. Unfortunately, equipment is not available to carry out the salinity tests by the method of Oxner and Knudsen (1920), and the determinations were made volumetrically by titrating with silver nitrate solution standardised against pure sodium chloride using potassium chromate as an indicator. The results are, therefore, approximate only.

The hydrogen ion determinations were carried out in the field by means of a “B.D.H.” colorimetric set using cresol red as an indicator.

No determinations have been made of the water in the Upper Harbour, but it is to be expected there will be a considerable amount of variation in the salinity of this area throughout the year. No details are available to show the effect of rain on the salinity and hydrogen ion concentration of the water or to show how long it takes the water to return to normal. Further, no determinations have been made of the seasonal variations in the nitrate, phosphate, and oxygen content. This work would entail the full time services of a hydrologist, and such is not at present available in Auckland.

In Table III the two observations of Phillips and Grigg, collected in 1923, have been placed at the head, and for convenience have been converted to total salinity figures by Knudsen's formula So/oo = 0.030 + 1.8050 Cl.

Table III.
Salinity and Hydrogen Ion Concentration of Sea Water at Auckland.
Date. Temp. ° C. Sal. o/oo p.H. Locality. Time.
5/9/23 34.95 Inner Harbour
5/9/23 35.26 Queen's Wharf
6/12/32 16.9 34.87 8.30 Hobson Bay 9.10 a.m.
" 17.4 35.00 8.25 Off Orakei Wharf 9.20 a.m.
" 17.2 34.90 8.20 Between Rangitoto Wharf and Tamaki 9.40 a.m.
" 17.0 35.00 8.40 Off Emu Point, Motutapu 10.5 a.m.
" 17.1 35.22 8.30 Between Motutapu and Waiheke 10.30 a.m.
" 16.5 35.25 8.24 David Rocks, Noises Islands 11.20 a.m.
14/12/32 18.0 34.52 8.26 Queen's Wharf 8.30 a.m.
" 18.5 35.00 8.29 North Head 9.50 a.m.
" 18.75 35.10 8.34 Rangitoto Beacon 10.15 a.m.
" 19.00 35.12 8.33 Between Rangitoto and Tiri Tiri 10.45 a.m.
" 21.00 35.13 8.27 Tiri Tiri Island Wharf 11.40 a.m.
7/4/33 19.75 35.00 8.32 Rosaria Channel, Kawau 9.15 a.m.
" 20.25 35.09 8.37 Off West Coast of Moturoa Island 11.40 a.m.
" 20.00 34.90 8.37 Whangaparaoa Passage 1.00 p.m.
" 20.25 34.95 8.33 Between Tiri Tiri and Rangitoto 4.5 p.m.
" 20.00 34.90 8.33 Rangitoto Beacon 4.50 p.m.
– 270 –

Summary.

From the data at present available it would be unwise to state any definite conclusions. It is apparent, however, that the Auckland Harbour does not differ greatly from similar areas in other parts of the world, and it may, in fact, be described as a typical tidal estuary.

The hydrographical features vary considerably in different parts of the harbour, and this fact must be taken into consideration when the ecological details of a particular area are being worked out.

The temperature in Auckland Harbour is 2° or 3° F. above its expected isotherm.

The sea temperatures follow closely the air temperatures, and both follow the hours of bright sunshine.

There is a rapid rise in sea temperatures during September and October and a corresponding fall in April and May.

The present amount of sewage entering the harbour should be insufficient to cause serious pollution of the water, and apart from possible bacterial infection of mollusca, is not a serious menace.

The improvement schemes of recent years have destroyed several areas of Zostera, and this may have some effect on the biological features of the harbour.

The salinity and hydrogen ion concentration determinations show a close similarity to related observations in other places.

References.

Blegvad, H., and Aage, J. C. J., 1932. Report of the Danish Biological Station, Vol. XXXVII.

Fowler, G. H., and Allen, E. J., 1928. Science of the Sea, Oxford University Press, London.

Harvey, H. W., 1928. Biological Chemistry and Physics of Sea Water, Cambridge University Press.

Kidson, E., 1930–31. Variation of Rainfall in New Zealand. N.Z. Journal Sci. and Tech., Vol. 12.

——, 1931–32. The Annual Variation of Pressure in New Zealand. Mean Temperatures in New Zealand. N.Z. Journal Sci. and Tech., Vol. 13.

——, 1932. Average Rainfall in New Zealand for Period 1891 to 1925. Report N.Z. Dept. of Sci. and Tech.

Oxner and Knudsen, M., 1920. Chloruration par la methode Knudsen. Bull. Comm. Internat. pour l'Expl. sci. de la Mer Mediterraneenne, No. 3. [See also Glazebrook's Dictionary of Applied Physics, Vol. iii, 1923.]

– 271 –

Phillips, W. J., and Grigg, F. J. T., 1925. The Salinity of Inshore Oceanic waters of Australasia in Relation to Fishes. Proc. Linn. Soc. New South Wales, Vol. L, Pt. 4.

Watkins, H. H., 1924–25. Rainfall in Auckland. Proc. N.Z. Soc. Civil Engineers, Vol. XI.

——, 1931. Report on Sewerage and Stormwater Drainage for the Auckland Metropolitan District. Auckland Drainage Board, Auckland.

Description of Text Figures.

1.

Map of Auckland and Vicinity showing Harbour Survey Areas and Sewer Outfalls.

2.

Graph showing Relation of Sea and Air Temperatures and Hours of Bright Sunshine at Auckland.

3.

Graph showing the Relation of Sea Temperatures at Auckland, Kaipara, and Cape Brett.

Picture icon

Map of Auckland and vicinity showing Harbour Survey Areas and Sewer Outfalls.

Picture icon

Graph showing Relation of Sea and Air Temperatures and Hours of Bright Sunshine at Auckland.

Picture icon

Graph showing the Relation of Sea Temperatures at Auckland, Kaipara, and Cape Brett.