harbour while the main continual source of fresh water entering the harbour is from the mouth of the Hutt River, situated at the north-eastern corner of the harbour. The harbour's only access to the sea (Wellington Heads as in Fig. 1) is located at the southernmost part. The writer's observation and sampling was carried out mostly from the centre of the sea end of the outer T head of Queen's Wharf where the depth of water is between 4 and 4 ½ fathoms. This is a point

Text-Fig. 1.—Wellington Harbour.

about 500 feet from the shore-end breastwork. Figure 1 shows the geographical features which pertain to this study.

Tides

The tides are semidiurnal, having a maximum range of about 5 feet—i.e., from 0.4 to 5.2 feet. At a first approximation the average tidal range is 2 ½ feet, from which it follows that the mass of water displaced in the harbour in an average tidal movement amounts to about 0.015 cubic miles, or about 60 × 10⁶ tons of sea water.

Materials and Methods.

The surface sea temperatures were read to the nearest one-half degree Centigrade, using an ordinary laboratory mercury thermometer. The sea-water samples of which the temperature was recorded were obtained from the top half-metre of the surface waters in a 2-litre glass vessel which had been previously immersed long enough for the glass to assume the same temperature as the sea water.

Salinity was determined on 29 samples of surface (top half-metre) sea water. Twenty-four of the samples were obtained from the Queen's Wharf station, three samples from the nearby Oriental Bay, and two from York Bay, on the eastern side of the harbour not far distant from the outlet of the Hutt River. Salinity determination followed the titration method described by Harvey (1928). Although samples were dealt with in duplicate, a further check to accuracy was obtained in most cases by using a gravimetric technique involving the use of a small pycnometer. There was invariably good agreement in the results obtained from both methods. All salinities in the text are expressed as parts per mille indicated by the symbol 0/00. Tables I and II show the data pertaining to the salinity. Some of the samples were taken during periods of rainfall on the harbour, but the analyses showed no significant decrease in the salinity as a result.

A few determinations were made of the pH of the harbour water using a MacBeth pH Meter in the laboratory within one to two hours after the water sample was taken. No other chemical analyses were made of the sea water.

The extent of pollution was not studied except to note that the Queen's Wharf water was not as pure or clear as that from elsewhere in the harbour.

Temperature.

Table I, below, brings together the writer's observed data of surface salinity and sea temperature and other various data While it is evident from this table that the sea surface and average daily air temperatures are approximately similar, and that there is a seasonal cycle, it is seen that the other data—viz., the hours of sunshine, rainfall, and salinity for those days bear no recognizable relationship with the temperature data.

Table II tabulates monthly averages of air and surface sea temperatures, total monthly hours of sunshine, and the average daily hours of sunshine each month. The relationship between these factors is clearly shown graphically in Fig. 2, below, where it is seen that the sea temperatures closely follow the general temperature of the prevailing air mass for the region. The two temperature curves show roughly the same trend as that for the total hours of sunshine per month. The range of the average monthly air temperatures for the year specified is 8.9° C. and the range for the average monthly surface sea temperatures is 7.0° C. The average air temperature at sea level for the twelve-month period, corrected for the lapse rate, is 13.5° C which is 0.3° C lower than the annual surface sea temperature of 13.8° C.

Table III gives temperature, salinity and pH values for surface waters at Oriental Bay and York Bay. The lower York Bay temperatures possibly indicate that the presence of fresh water from the Hutt River, as evidenced by the lower salinity, has a temperature-lowering effect of about 1° C. in that region of the harbour at the time of the year mentioned.

An average monthly surface sea temperature of 12.2° C. (54° F.) for the winter of 1952 and of 14.5° C. (58° F.) for the summer of 1953 has been noted for the “outside” waters of Cook Strait (D. M. Garner).

Salinity.

There was small variation in the surface salinity of the waters at Queen's Wharf. The lowest salinity found was 34.1 0/00 and the highest was 35.9 0/00.

^*

the range for the year being 1.8°/00. Although the value 35.9°/00 seems to be unduly high, it is nevertheless reported since no defect in procedure could be detected upon close scrutiny of laboratory notes. The salinity determinations of three samples taken from Oriental Bay on three different days gave values of 35.0, 34.2, and 33.5°/00. The two samples taken from York Bay gave rather lower values—viz., 31.85 and 32.0°/00. Although not observed by the writer, there are undoubtedly some regions of the harbour, particularly near the outlets of fresh-water drainage culverts, where low surface salinity prevails, but these regions probably imply local aberrance and therefore should not be considered as ecologically important with reference to the harbour as a whole. It is suggested that surface salinities of the order 32.5 to 33.5°/00 prevail generally in a wide region embracing most of the north-eastern portion of Wellington Harbour.

The data in Table II does not demonstrate any obvious effect from the amount of rainfall per month on the average monthly salinity. Although the number of salinity determinations is few, it seems that if the effect of rainfall had been

Figure 2. Surface Air and Sea Temperatures and Hours of Sunshine for the Queen's Whar [ unclear: ] vicinity, Wellington Harbour

noticeable on the surface salinity it would be apparent in the results because of the randomness of the sampling.

It is seen from Table III that the values of five pH determinations for Oriental Bay and York Bay ranged from 7.55 to 8.30. The values of other isolated pH determinations of Wellington Harbour water were found to be within the above range.

Fresh Water Entering the Harbour.

The Hutt River furnishes the chief source of fresh water entering the harbour. While there seems to be no readily available information regarding an average rate of flow of this river into the harbour, figures have been obtained for a minimum and maximum flow. A minimum flow of 1,000 cusecs has been recorded from gaugings at Trentham and a maximum flow of 70,000 cusecs has been mentioned for a significant flood in 1939. It is probable that a flow of 40,000 cusecs is a high rate. In order to consider the effect of Hutt River water in relation to daily movements of the harbour sea water the above figures are converted to daily figures. Thus the minimum amount of Hutt River water entering the harbour in any one day is about 2.6 × 10⁶ tons, whereas the maximum amount is about 180 × 10⁶ tons of fresh water in any one day. The amount of fresh water usually entering the harbour from the Hutt River would be within the stated amounts, but probably nearer the lesser, since the floods are known to be of sudden occurrence, of short duration, and of fairly rapid recession.

Rain water as a source of fresh water entering the harbour is of secondary importance. Tables I and II give information concerning rainfall for the period July, 1953, to June, 1954. At Wellington it is predicted that on the average a maximum rainfall of 2.3 inches will occur on one day in a year, and that once in ten years there will be a fall of 4.1 inches per day, and that once in every one hundred years a maximum fall of 6.0 inches may be expected. This rainfall, expressed as tons per day falling on the harbour water area, is 4.6 × 10⁶, 8.2 × 10⁶, and 12 × 10⁶ tons respectively. The maximum rainfall ever recorded in Wellington is 6.3 inches, or not more than 12.6 × 10⁶ tons per day on the surface area of the harbour. In order to arrive at the maximal possible added amount of fresh water attributable to rainfall over the harbour area and to the concomitant run-off from the surrounding hills, one-half of 12.6 × 10⁶ is added to this figure of maximum recorded rainfall, thereby giving a value of 18.9 × 10⁶ tons per day. Since 6.3 × 10⁶ most certainly represents an exceedingly gross run-off coefficient, then the figure 18.9 × 10⁶ can be taken as a safe extreme.

The addition of fresh water from artesian sources in Wellington Harbour has been a matter of recent speculation. And it has been estimated that there is a daily flow into the harbour in excess of 30 million gallons. In order that the effect of this water on the ecology of the harbour will not be minimized a maximum of not more than 100 million gallons per day or 0.5 × 10⁶ tons per day is suggested.

From the above data—viz., for the maximum recorded flow of Hutt River water in flood into Wellington Harbour, for the maximum recorded rainfall in Wellington, and for the estimated maximum addition of artesian water, it is calculated that the total amount of fresh water entering the harbour, in an hypothetical instance where these maximal conditions obtain, is 199.4 × 10⁶ tons per day.

Pollution.

Visual observation of the Queen's Wharf sea-water samples showed signs of pollution such as slight oil surface film and varying degree of turbidity and odour. Some of the pollution of the Queen's Wharf area is due to the refuse emitted from ships in port and from wharf latrines. Other prominent sources of pollution are city sewage discharges and effluent from a major meat-freezing works. Pollution due to anti-fouling paints and such items as copper sheathing

probably constitutes a factor of local importance only near the source of these pollutants. The stirring up of bottom sediments by the almost constant shipping activity probably accounts for much of the turbidity. It seems unlikely that pollution in Wellington Harbour causes distress to the fauna, especially the bottom-dwelling forms, but from an ecological point of view, pollution from such sources as sewage outlets would be expected to be serious near the points of discharge.

Discussion.

The Queen's Wharf average monthly surface sea temperatures for the 1953-1954 period indicated in Fig. 2 agree closely with those reported by Ralph and Hurley (1952) for Oriental Bay for 1949-1950. In general the inner harbour has slightly greater extremes of mean monthly surface sea temperature than the “outside” waters of Cook Strait. Winter mean monthly surface sea temperatures at Port Nicholson are approximately 2.5° C. cooler than mean monthly surface sea temperatures of the region outside of the harbour entrance. The inner harbour in summer is warmer than the outside region by about the same amount. Since this difference is not great, one would not expect temperature to be a significant limiting factor to either slightly or markedly eurythermal animals of the shelf area just beyond the harbour entrance which do not normally populate the harbour. For these animals other limiting factors must be sought in explanation of the discrepancies in distribution. Obviously temperature is not directly a limiting factor for the animals which are common inhabitants to both locations. However, the difference of 2.5°/00 C. might be critical for stenothermal species which are more usually to be found in the deeper waters beyond the shelf.

It is suggested that the small temperature difference mentioned above for the two regions is evidence of quite efficient tidal mixing between the two water masses, for otherwise the harbour seems to be sheltered sufficiently to lead one to expect a wider range of temperature due to the direct effects of solar radiation, and to indirect effects of radiation and convection from the surrounding geographical features. Although only surface temperature measurements are considered, it does not seem precocious to postulate that the differences between surface and bottom temperatures are relatively small, say up to 2° C., having regard to the shallowness of the water and the supposed efficacy of tidal action in preventing gross thermal stratification. The data in Table III suggest that the Hutt River water entering the harbour in summer is probably slightly cooler than the harbour water, but this is not likely to be a factor affecting harbour ecology.

Consideration of intertidal hydrology is precluded from as general a discussion as this, for intertidal locations are more subject to changing conditions than are the larger water masses.

Results of the salinity analyses of Queen's Wharf water samples showed a sufficient constancy to evoke the suggestion that fresh water from the Hutt River has little influence on the salinity of the sea in the Wellington Harbour as a whole. It is also suggested that the salinity of the harbour is not significantly disturbed by pollution. The estimated maximum amount of Hutt River water entering the harbour—viz., 180 × 10⁶ tons per day, is a small amount in contrast with the 1320 × 10⁶ tons of water of the harbour, and therefore the salinity of the harbour as a whole will not be seriously affected by fresh water from this source.

The salinity data, in Table III, for York Bay tend to support this idea. However, in the areas adjacent to the mouth of the Hutt River there will be all degrees of dilution and a tendency to stratification as the fresh water flows out over the salt water. In this region there are undoubtedly great changes in salinity accompanying the tidal cycle. It is regrettable that a detailed knowledge of the salinity characteristics of the waters adjacent to the mouth of the Hutt River is not available as large numbers of Echinocardium cordatum, the common heart urchin cosmopolitan in distribution, are found in regions quite near the fresh water discharged from the Hutt River. Powell (1937) writes that the salinity factor is evidently the cause of the omission of Echinocardium from the muds of the Upper Harbour and from most of the Inner Harbour at Auckland; the range in salinity for the Upper Harbour being reported to be from 29.63 to 32.80 °/00. If we assume that this factor is truly the cause, then the presence of E. cordatum near the mouth of the Hutt River can be taken as further evidence that the Hutt River water does not significantly dilute the bottom layers of the Wellington Harbour water, even near its point of entry. On the other hand, it may be that E. cordatum is a more euryhaline species than Powell's statements imply, and that the exclusion of this species from Auckland's Upper Harbour is due to other causes or to some indirect effect of lowered salinity. The writer's results, as yet unpublished, of experiments on the viabality of the larvae of E. cordatum in media of different salinities, suggest that this species tolerates considerably lowered salinity. But it is not yet known for this species whether or not the larvae are more tolerant to extremes of lowered salinity than the adults.

Despite the fact that coloured, silty, Hutt River water can at times be observed either as a broad ribbon, indicating a flow along the eastern portion of the harbour towards the direction of Wellington Heads, or as a spreading, fan-shaped expanse, it is not the author's opinion that this layer or tongue of fresh water constitutes an ecological menace. These phenomena are considered indicative of water masses devoid of marine life and of masses which would be avoided by organisms which are capable of active voluntary movement. A small percentage of the total number of harbour zooplankton organisms would probably perish at the edges of the surface fresh-water current as a result of being swept into the frictionally induced eddies Lucas and Hutchinson (1927) made the interesting observation that Diatom optima exist where the Fraser River water and the sea water in British Columbia are mixed. The abundance of marine bottom-dwelling invertebrates which can be dredged from the Wellington Harbour areas underlying the fresh-water tongues could possibly indicate that silt sedimentation is not a factor harmful to their welfare. The disappearance of these fresh-water layers is probably attributable to tidal movements of water and to the action of winds on the harbour surface.

The figures given for maximal amounts of fresh water entering Wellington Harbour are summarized below:

These figures have been derived from estimates which tend not to minimize the significance of the relationships between each other, and the figures are not

to be taken as absolute values of calculations based upon accurate measurements. However, it is maintained that despite the approximate nature of the estimates, the relative proportions are probably of sufficient accuracy to allow for conclusions to be drawn with respect to animal ecology in the Wellington Harbour. Thus the hypothetical maximum total daily addition of fresh water to the harbour is estimated to amount to about 15% of the total harbour mass, and to amount to roughly the same quantity as a maximum tidal displacement. On the basis that, “If one kilogram of water of high salinity, S, is diluted by adding n kilograms of distilled water, the salinity of the dilution, S_D, will be S_D = S/n+1” (Sverdrup, et al., 1942), then the effect of the above daily addition of fresh water to the harbour, neglecting effect of tidal exchange, would be of lowering the salinity from 35 0°/00 to about 30.5 °/00. However, this hypothetical maximum total addition of fresh water assumes the character of a freak of nature and should therefore not be considered in general ecology. A more reasonable estimate could be based on a figure of, say, 40,000 cusecs for a Hutt River rate of flow which is still probably high even during a period of heavy precipitation over the Hutt River catchment area. An estimated lowering of salinity in the harbour, neglecting tidal action, calculated from this amounts to about 2.5 °/00, i.e., from 35.0 to 32.5 °/00. But, if we consider the action of tides, then reductions in salinity such as these could not be expected to occur. Therefore, it seems reasonable to presume that, from an overall ecological aspect, reduction in salinity of the Wellington Harbour is not a significant limiting factor.

Although I do not have figures for the amounts, it is known that Wellington Harbour is subject to pollution by human wastes such as faecal matter discharged into the harbour as sewage. It has been stated by Vaccaro, et al., (1950) that in any pollution survey which considers the distribution of coliform bacteria the bactericidal activity of the sea water must be taken into account if the distribution is to be explained and misleading interpretations are to be avoided. The importance of this is obvious in the light of the finding that Escherichia coli populations are unable to survive when placed in raw sea water (Vaccaro, et al., 1950). Also, it has been concluded by these workers that the most probable cause of the death of E. coli populations in sea water is an antibiotic effect which requires the presence of the normal marine flora. If the abundant zooplankton fauna of Wellington Harbour is indicative of abundant marine flora, then such bactericidal properties must be present, in which case pollution would not be of sufficient concentration or toxicity to prohibit the growth and development of zooplankton. It is concedable that bacterial pollution may be regarded as an addition of food for some of the marine organisms. As previously pointed out by Hounsell (1935), Belgvad and Aage, from work done in Copenhagen Sound, showed that the effect of sewage pollution on the fauna and flora of estuaries has been somewhat exaggerated.

The reputed existence in Wellington Harbour of several species of Copepoda usually considered as open ocean forms could possibly be taken as further evidence of a condition of efficient tidal mixing and exchange of water in the harbour.

By eliminating temperature, salinity and pollution, notwithstanding their convenience as factors readily accounting for the difference in distribution between marine species of Wellington Harbour and those of the shelf outside, it remains that the principal limiting factors are either constitution of substratum, types and availability of food, degree of shelter from wave action, or a complex interdependence of these factors.