greywacke, argillite and occasionally quartz, in a matrix of calcific cement which contains smaller particles 2-4mm diam of the same composition. The pebbles vary from 1-15cm diam. with a mean of 3-4cm diam. This very hard rock is susceptible to solution along the^ joint planes giving three major sete of joint patterns, viz. a dominant set 140°/320° and subordinate sets of 170°/350 and 040°/220°. Beds strike in a direction of 260° and dip 4° west, the structural surface being uppermost, but there is also an erosional surface at high water mark. The dark-coloured conglomerate and grit at Cape Rodney consists of calcareous cement, fragments of greywacke and shells. The latter include turbinid and turritellid gastropods, large oysters, pectinid and glycymerid bivalves, echinoid fragments and Cyathoseris, an agariciid coral. Also at Cape Rodney, but just outside of the Reserve, are fragments of Leptoseris and Fungophyllia. These corals are fossil hermatypes, similar to those seen on the seaward side of modern tropical reefs at depths of 10-160metres, and imply a mean annual paleotemperature of around 19-28°C (Squires 1962). At nearby Matheson Bay a fossil barnacle fauna, including Hexalasma aucklandicum and Tetraclitella purpurascens, are indicative of a non-violent, non-turbid paleo-environment (Buckeridge 1975). Between Goat Island and Cape Rodney the conglomerates also include brachiopods, calcareous algae and bryozoans. The latter comprise fragments of reteporiform, celleporiform, erect cylindrical branching and non-branching forms, also suggestive of an ancient sublittoral environment of reduced current velocities (Crabb 1971).

The Ti Point Group, which occurs in the Reserve as a narrow dyke cutting through the middle of Goat Island, comprises vesicular olivine basalt with a trachytic texture. In composition and texture it is like the basalt of Ti Point but with a less pronounced flow structure. Since the basalts baked sediments of the Waitemata Group, and evidently flowed over an eroded Waitemata surface at Ti Point, they are probably Pliocene in age (Hopgood 1961).

CLIMATE Everybody talks about the weather, and probably most people realize that

climate and weather are complex phenomena. What generalizations, therefore, can be made about them? The local weather pattern is dominated by a succession of anticyclones, separated by troughs of low pressure, which pass more or less regularly from west to east at roughly weekly intervals. A typical weather sequence commences with a low pressure trough approaching from the west. Freshening north-westerly winds prevail with increasing cloud, followed by rain for a period, during which winds may reach gale force. The passage of the trough, with its associated cold front, is accompanied by a change to cold south-westerly winds and showery weather. The barometer then rises with the approach of the next anticyclone from the west. Wind moderates and fair weather prevails for a few days as the anticyclone passes over.

This sequence is very common but the situation may be frequently more complex. The anticyclones, of course, vary in size, intensity and rate of movement, while the troughs are unstable systems where depressions readily form at any time of the year, sometimes becoming vigorous storms. Occasionally in summer a cyclonic storm of tropical origin, accompanied by gales and heavy rain, affects the area (Robertson 1968).

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Broadly, the north Auckland peninsula is warm-temperate, possessing a mesothermal forest climate, according to the system of Thornthwaite (Gamier 1951). Local weather statistics are certainly influenced by the sea and by the sheltering effect of a hilly coastline forming the southern boundary of the Marine Reserve. Thus the Leigh weather is generally milder than most other parts of the country. (The geographical location of the laboratory weather station (Fig. 15) is latitude 36 16'S and longitude 174°48'E).

Some of the more immediately interesting aspects of the weather to many people is not only what is the norm, but what sort of extremes have been experienced. People tend to ask, for example: How cold does it get in winter? (Average winter temperature is 12.6°C. The coldest night was that of October 11, 1969, when the screen temperature sank to 2.8°C). Which is the hottest month of the year? (February, with an average temperature of 19.5°C, although January has the highest average sunshine (243 hours)). Does frost ever occur? (Normally one can expect a frost-free winter, but a record —2.9°C was recorded by the grass minimum thermometer for June 18, 1975. The first frost recorded since August 1966 when the weather station began, was in June 1974). What is the windiest month of the year? (July, with an average run of 14.1 km/hr). What was the most rain ever received in one day? (146.9mm (5%") on March 8, 1972). Has there ever been a drought? (The longest periods without rain were 17 rain-free days in February-March 1968; over 22 days in November-December 1970 only 0.76mm was recorded, while the dry est month on record was February 1973, with only 6.8mm total).

These data are for the period 1967-1975. A general discussion of the various components of the weather now follows.

Photograph by M.C. Kampman.

Fig. 15: Leigh Laboratory weather station. Little Barrier on horizon at right.

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Wind. Winds from a westerly quarter prevail in all seasons, and in summer months are almost equalled by northeasterlies and easterlies. From May to October nor'westerlies may be as common as sou'westerlies, while winds from a northern quarter are always scarce, especially in February. The hills adjacent to the Marine Reserve shelter it from the south, and accordingly, winds from a south to easterly quarter are negligible. In the colder months of the year winds from the east are reduced compared to summer, while for September, southeasterlies, normally scarce at any time, have never been recorded. Overall wind directions for the years 1967-75 are shown in Fig. 16.

January February March

October November December

Fig. 16: Relative importance of wind directions at the Leigh Laboratory weather station. Each bar represents the total number of days, from 196/-1975, in which wind from a certain direction has occurred.

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In general wind strength is not more than a gentle breeze (73% of the time), but this is sufficient to give a daily average run of 317.8km or 13.2km per hour (9 year average). This is relatively high compared to the country as a whole, and is no doubt due to the coastal situation, and the location of the weather station on a ridge open to wind from most quarters. Only 3% of the days in a year are calm at 9am when the observations are made, although gales are very infrequent. Days when wind strength is gale force or stronger are <1% of the total. (There have been only 27 such days in 9 years). The most gales have been recorded in August, which has probably contributed to this month's having a high average number of kilometres run, while the highest number of calm days have occurred from May to July. 1972 was the calmest year since 1967, with the lowest number of kilometres run and a relatively low amplitude of seasonal fluctuations in wind strength. Since January 1973, when recordings of gust velocities commenced, the highest gust speed was 81 knots, from the north-east, June 14, 1975. Precipitation. Some rain can be expected to fall on at least 25% of the days in any month of the year, and up to 50% in winter months. Mean summer rainfall is 67mm per month, with 117mm in autumn, 122mm in winter and 96mm in spring. In general, June tends to be the rainiest month and January the dryest. 1972 was an exceptional year, however, when 26% of the total yearly rainfall fell in March alone. Average yearly rainfall is 1222mm (cf. Kerikeri 1617; Auckland 1243; Wellington 1206; Christchurch 669; Milford Sound 6433; Dunedin 790). Calculated 'normal' rainfall (based on long-term estimations from the Met. Office) indicates that the two nearest stations, Mokohinau has less, and Warkworth more, average precipitation. 'Normal' for Leigh is 53mm in January and 122mm in June.

There is not a perfect correlation between the average amount of rain and the average occurrence and extent of cloud cover in any month. There is generally more cloud cover in spring and least in autumn, although the differential is relatively small. The clearest month, April, has only about one fifth less cloud, on the average, than the cloudiest month, October.

Hail is rare, having been recorded on only 10 occasions in the past 9 years, in the months June through October. Thunderstorms. Thunder is not often heard (1-11 times a year). It may occur in any month (though apparently with least frequency in February and October) and is accompanied by visible lightning three-fifths of the time.

Temperature. The Northland climate is generally pleasant and mild, with a relatively small seasonal temperature range. For Leigh average summer tempera-tures are around 19°C and in winter 12.6°C. The average monthly temperature range varies from 5.4° (August) to 6.7° (January), while the yearly range is 12.85° (mean of highest maximum — mean of lowest minimum). The rate of decline of temperature from mean February max. to mean July min. is faster (1.56° per month) than the rate of increase after winter (1.11°). The warmest day encountered was a balmy 28°C on February 11, 1971. 'Normals' calculated by the Met. Office, based on long-term data, indicate that Leigh is intermediate

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between Mokohinau and Warkworth. Moreover, the temperature range is damped by the coastal situation, hence mean 'normal' June temperatures, for example, are exceeded only by three other stations in New Zealand, whereas mean 'normal' January temperatures are exceeded by several stations.

Frosts almost never occur. Not only is gently sloping ground with a northerly aspect least affected by frost (Robertson 1968), but the proximity of the laboratory weather station to the sea (26.5m above mean sea level and <50m from mean high water over a horizontal distance) moderates temperatures somewhat. Only twice, since August 1966 to the present, have ground frosts (grass minimum — 1.0°C or lower) been recorded, in June of 1974 and 1975. Further inland, frosts are much commoner.

total actual average % Month hours per hours hours per possible

month 1975 day sunshine

January 448 243 14.46 54 February 371 211 13.24 57 March 388 181 12.51 47 April 340 159 11.33 47 May 319 148 10.28 46 June 290 124 9.66 43 July 309 145 9.96 41 August 336 149 10.83 45 September 358 178 11.94 50 October 406 201 13.11 50 November 424 197 14.13 47 December 454 213 14.66 47

Total 4443 2149 x 12.17 48 Table 10: Sunshine hours, 1975 and the total possible (calculated from N.Z. Nautical Almanac 1976) for the Leigh Laboratory climate station.

Relative humidity. Humidity is commonly between 73% (January) and 80% (June-August). It varies inversely to temperature, falling to a minimum when temperature is highest. The diurnal variation is greater, in fact, than the difference between summer and winter. In general, the low seasonal variation is due to the coastal situation, and is correspondingly higher than most inland situations. High relative humidities of 97-99% may occur in any month of the year, though the lowest percentages are higher in winter (c. 56%) than at other

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times of the year. Low percentages of 4243% have been encountered in autumn months and October. Sunshine. The sunniest parts of New Zealand are favoured with over 2400 hours of sunshine per annum, and indeed, a large portion of the country is favoured with at least 2000 hours (Robertson 1968). The Marine Reserve generally receives somewhat over 2100 hours p.a. (48% of possible), though it has ranged from 1923 hours (1969) to 2285 hours (1974). January has, on the average, the highest number of sunshine hours (243) which is almost double that of June (124), although expressed as a percentage of the possible (Table 10), February has the most sunshine, with 57%. Only four months (January, February, September, October) have 50% or more of the total possible. The month of June has only 43% on the average. The overall sunniest month on record (as percentage possible, not total hours) since July 1967 was February 1973 which had 74% of possible. The least sunny month, on the other hand, was May 1969, with only 25% (Table 11). The increase in the percentage of summer sunshine is related to the passing of anticyclone centres over or near this region.

sunshine % sunshine % Month hours possible year hours possible year

max sunshine min sunshine

January 289 65 1974 193 43 1971 February 276 74 1973 136 37 1969 March 220 57 1969 146 38 1972 April 197 58 1975 127 37 1971 May 171 54 1971 79 25 1969 June 154 53 1975 88 30 1969 July 183 59 1967 113 37 1968 August 174 55 1974 127 38 1973 September 210 59 1972 117 33 1970 October 213 53 1967 145 36 1969 November 261 62 1971 131 31 1967 December 268 59 1974 138 30 1968

Table 11: Highest and lowest number of sunshine hours for each month (1968 - 1975) as percentage of possible, for Leigh Laboratory climate station.

Atmospheric pressure. The mean annual sea level pressure at the Marine Reserve is 1016.06 millibars (101 606 Pa), ranging from 1018.2mb in March to 1014.5mb in June and December (8 year averages). Extremes since 1968

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(inclusive) have been 1038.0mb (July 1969) and 979.4mb (November 1071). Evaporation. Mean annual monthly evaporation is 111.5mm. There is a sequential decline in mean monthly (9 year average) evaporation from January (192.3mm) to June (44.5mm). The highest daily evaporation was 20.8mm in December 1971. One particular feature of note is that zero daily evaporation occurred as much as three times more frequently in 1975 than in any previous year since records started in mid-December 1966 (Table 12). Compared to other parts of the country evaporation is relatively high, for although humidity is also high, so is temperature, and wind effectively moves air above the pan evaporimeter.

No. of days months in which of zero days of zero

Year evaporation evaporation occurred 1967 0 -

1968 2 February, June 1969 2 May, July 1970 0 -

1971 1 June 1972 3 March, May, July 1973 3 January, September, October 1974 2 August, September 1975 9 April through December Table 12: Occurrence of days of zero evaporation at Leigh Laboratory climate station.

HYDROLOGY From where is the water mass of the Marine Reserve derived? The Reserve

lies outside the Hauraki Gulf proper, which is defined as being within a line from Cape Rodney to Cape Colville, beyond which is an area, designated the Hauraki Gulf Approaches, bounded by Bream Head and Mokohinau Islands to the outer Mercury Islands in the south-east (Taylor 1973), but inside the 100m contour and comprising 5480km 2 (Paul 1967, 1968b). The main ocean current of this region is the East Auckland Current, which is part of a large body of subtropical water deflected by the Northland peninsula (Brodie 1960). This current is derived from the Trade Wind Drift and no doubt influences the occurrence and distribution of marine organisms in this sector of the North Island, which is part of the so-called Aupourian Province of transitional warm-temperate waters (Powell 1961, Knox 1963). The East Auckland Current produces a net movement of water south-eastwards across the entrance of the Gulf and the Approaches, which is believed to be more pronounced during summer.

Within the Gulf the tidal stream sets southward with a rising tide and northward with a falling tide (Admiralty Hydrographic Dept., 1958:164). The

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