ii. Tide Levels
In any water mass partly enclosed by land considerable variations in both time and height of the tide may be expected to occur. Many workers on intertidal ecology have found from experience that, owing to a frequent deviation of tidal behaviour from the normal for the nearest port of reference, each locality needs to be studied on its own. In the Hauraki Gulf extreme spring tides range from about 14. Oft. in Coromandel Harbour to about 8 Oft. at Cuvier Island. Taking Auckland as the standard port of reference, the following figures taken from the Nautical Almanac (1950) give the ratio of the tide ranges for localities where it is known:
The writer's brief observations of tidal behaviour in the Gulf (Table V) varied from that made over a weekly period (Station 24) to those over a single 6-hourly rise or fall of the tide (Stations 17, 30, 32).
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|Station||Tide Range in Feet|
|17||[ unclear: ]||1.9.50||8.1||8.4||7.9||(Auckland)||Spring|
|24||Ti Titoki Flat||17.10.49||4.7||5.0||4.0||(Tryphena)||Neap|
|24||Ti Titoki Flat||24.10.49||6.5||8.0||8.0||(Tryphena)||Spring|
|28||Fletchers Bay||17.5.50||4.0||5.2||5.1||(Cuvier Is.)||Neap|
In the above table the Coromandel Harbour figures are obviously not applicable to Long Beach, which is only 5 miles away, but is situated on the open coast. Results from Ti Titoki Flat, Little Barrier, also appear anomalous in comparison with the expected ranges at Tryphena. The neap range is greater but the spring range is smaller, if splash is not taken into account. The splash zone (Colman, 1933) was particularly hard to estimate here owing to the broken surface topography occasioned by the loose stones and boulders. Figures from Fletchers Bay agree substantially with the expected record for Cuvier Island; likewise Whitianga with Mercury Bay. and Arran with Auckland. The range at Port Fitzroy appears on superficial examination to be slightly less than at Tryphena,
but the record cannot be accepted as significant in the absence of a synoptic figure at the latter port.
Tide watches were confined to days when wind and waves were at a minimum, so that differences in water level could be judged as accurately as possible. Half-hourly records were kept and the vertical sequence of species and communities was established as each became uncovered, after the method of Evans (1947, p. 281). At Stations 17 and 32, wooden jetties provided vertical faces easy of access on which to make measurements. In other localities observations had to be made directly on rocky headlands. All watches began at high water, for the practical reason that the highest water mark shows quite clearly in fine weather against the dried out line of demarcation of the upper midlittoral or supralittoral, whereas the exact line of low water mark is often hard to ascertain. Actual high water mark was taken as the mean between maximum and minimum swell.