-57-
AN OBSERVATION
ON THE
TIDES A T
HAURAKI
OTATA
ISLAND,
GULF
by A. Haughey
Zonation on the sea shore is p r i m a r i l y determined by the tides.
A s these
vary f r o m place to place it is n e c e s s a r y to know the tidal range b e f o r e an e c o l o g i c a l
study can be undertaken.
See article by D. H. Wood f o r position of Otata Island.
F i g . 1. is the tide curve taken at Otata Island, N o i s e s Group, on 23rd August,
I960 by the following m e t h o d :
A 12ft. long rod was marked off in tenths of a foot and affixed to a heavy c a s t iron weight, sufficient to anchor it at high tide.
This was then placed at an arbitrary
level at low tide and a note taken e v e r y 10 o r 20 minutes of the height of the tide.
These heights were later c o r r e c t e d by c o m p a r i s o n with r e c o r d s taken at the same time
by the Auckland Harbour Board at Queen's Wharf.
Values for high and low water at
this station are indicated by heavy a r r o w s in Fig. 1., Mean sea level at each station
being regarded as the same height above datum.
This gives the amplitude of the
tidal wave at the station and f r o m this can be calculated the values for extreme high
water spring tide (EHWS), mean high water (MHW), etc..
These are shown in Table
1.
CALCULATION
OF
TIDAL
RATIO
The tidal ratio i s ; tide range at s t a t i o n : : tide range at place of r e f e r e n c e .
Tide range at Auckland was
A r b i t r a r y range at Otata
9. 8 5 ' — 1. 95'
9. 20' — 1. 85'
T h e r e f o r e tidal ratio for Otata is
=
=
7. 90'
7. 35'
7. 35'
=
7. 90'
0. 93.
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CORRECTION
OF
HEIGHTS OBTAINED
Auckland
ON GAUGE
Otata Is.
Difference
High tide
9. 85"
9. 20'
+ 0. 65'
Low tide
1. 9 5 '
1. 85'
+ 0. 10'
Sum
+ 0. 75'
0. 75'
Therefore 2
must be added to heights r e c o r d e d at Otata.
Thus the tidal
range on 23rd August was 9. 60' - 2. 2 5 ' , c o m p a r e d with 9. 85' - 1. 95' at Queen's
Wharf, Waitemata Harbour.
Waitemata
Harbour
E x t r e m e high water springs (EHWS)
Mean high water springs
(MHWS)
Mean high water m a r k (MHW)
Mean high water neaps (MHWN)
E x t r e m e (Lowest) high water neaps. E(L)HWN
Mean sea level (MSL)
E x t r e m e (Highest)low water neaps E(H)LWN
Mean low water neaps (MLWN)
Mean low water m a r k (MLW)
Mean low water springs (MLWS)
E x t r e m e low water springs (ELWS)
AUCKLAND HARBOUR BOARD DATUM
11. 95'
11. 4 1 '
10. 35'
9. 28'
8. 88'
6. 22'
3. 96'
3. 29'
2. 29'
1. 12'
0. 60'
0. 00'
Otata
Is.
11.
11.
10.
9.
8.
6.
4.
3.
2.
1.
0.
53
03
04
05
68
22
10
47
54
45
97
T A B L E 1 : Values f o r tidal phenomena in Waitemata Harbour and those calculated
for Otata Island. F i g u r e s f o r Waitemata f r o m "Intertidal Zonation at Narrow N e c k "
U. V. Dellow in S. P a c i f i c Sci. C o n g r e s s 5, 1953.
Although there was a lapse of m o r e than six hours between s u c c e s s i v e high
and low waters the time permitted only a c u r s o r y note of the l e v e l s of exposure o r
flooding of the main o r g a n i s m s on the s h o r e .
Ranges f o r s o m e of the main types
are superimposed on the tide chart (Fig. 1).
Where upper a n d / o r l o w e r limits were
not determined an oblique line has been drawn, e. g . , Salicornia australis.
COMPARISON
OF
SPECIES
RANGES:
With the datum values of MHW etc. calculated for the station a c o m p a r i s o n
with other stations may now be made which allows for differences in tidal range.
F o r example U. V. Dellow gives the range of Saxostrea glome rata at Narrow
Neck as 3. 4' — 6. 5'.
i. e. 6. 2' minus 2. 8' to 6. 2' plus 0. 3'
The equivalent range at Otata would b e :
6. 2" - (2. 8' x 0. 93) to 6. 2' + (0. 3' x 0. 93)
= 3. 6' — 6. 5'
Thus the l o w e r limit of o y s t e r s in sheltered places at Otata is found to c o r r e s pond exactly with that at Narrow Neck and not be elevated as it would appear if no
allowance w e r e made f o r differences in tidal range.
REMARKS
1.
There are slight variations in published values f o r M. S. L. at Auckland but these
are not of the o r d e r to materially affect results.
2.
Datum values f o r mean sea level vary widely f r o m place to p l a c e but these are r e lated to chart datum (i. e . , 0. 00' on Admiralty charts of the area) and in no way
affect comparative zonation studies e x p r e s s e d by the range EHWS to ELWS.
Of
c o u r s e M. S. L. must be given the same numerical value for each station compared,
but this is arbitrary.
3.
The difference afforded by c o r r e c t i o n f o r tidal differences may appear trivial, and
probably is for most o r g a n i s m s in this c a s e .
However tidal ranges recorded in
New Zealand are f r o m 3. 4' (Wellington Harbour) to 14. 5' (Tasman Bay).
Also
in an autecological study where the limits of the organism are near EHWS or ELWS,
e. g . , Salicornia australis, this c o r r e c t i o n may be an important guide in reaching
valid conclusions.
BRIEF
DESCRIPTION
OF
LOCATION:
The ranges of m o s t o r g a n i s m s shown in Fig. 1. are f r o m o r g a n i s m s growing
either on a gently shelving greywacke substrate facing NE o r on a steep wall facing
N. W. protecting the f o r m e r .
With o r g a n i s m s c o m m o n to both areas no noticeable
FIG 1 +
TIDE
CURVE
-
OTATA
ISLAND
differences in range w e r e o b s e r v e d .
In F i g . 2. the shaded portion of the inner c i r c l e indicates the exposure of the area
to direct wave action.
The outer c i r c l e shows the distance and direction to surrounding land m a s s e s .
O c c a s i o n a l heavy seas f r o m the Gulf during Northerly storms
produce moderate surf conditions.
Types marked with an a s t e r i s k in F i g . 1. were
not present in the area d e s c r i b e d above but o c c u r r e d in m o r e exposed a s p e c t s ,
Ecklonia, Zonaria and Saxostrea facing m o r e exposed E a s t e r l y direction, while
Codium and Mytilus w e r e m o r e exposed to the South.
FIG. 2 .
EXPOSURE -
OTATA
ISLAND.