42 
PACIFIC SCIENCE, VoL VI, January, 1952 
and low water accurately determined. The 
times of high and low water were almost 
identical with those at Auckland, but there 
seems to be a slight local variation in the tide 
range at each place as compared with that at 
Auckland. Full-tide curves made at Rangitoto 
Wharf (south coast) show no irregularities. 
For the comparison of the levellings at the 
various places with the levellings at Scoria 
Flat, E.L.W.S.T. was approximately deter- 
mined on the basis of the tide readings made 
at each place, equality being assumed be- 
tween the Auckland and the local tides. 
IE -s- 
— NUMBER OF UBPER UMIT% 
NUMBERS OF LOWER LIMITS 
•• ■*“ - ' TOTAL NUMBERS OF LIMITS 
Fig. 6. Distribution on the shore of upper and lower 
limits of organisms. Total numbers of limits (upper 
plus lower) for each level also shown. 
Principal Tide Levels* The limits 0.00 
feet and 12.00 feet (above AHB Datum), rep- 
resenting E.L.W.S.T. and E.H.W.S.T., are 
never normally exceeded by tidal fall or rise. 
Certain levels for Queen’s Wharf for 1947 
were calculated from the Nautical Almanac 
predictions (in each case the calculated level 
is followed by the average of the actual re- 
corded tides at Queen’s Wharf for the 3 
years 1945-1947, by courtesy of Miss U. V. 
Dellow). These levels are: E.H.W.S.T., 11.90 
feet (11.95 feet); M.H.W.S.T., 11.01 feet 
(11.41 feet); M.H.W.N.T., 9-22 feet (9.28 
feet); M.L.W.N.T., 3.31 feet (3.29 feet); 
M.L.W.S.T., 1.36 feet (1.12 feet); E.L.W.S.T, 
0.60 foot (0.60 foot). 
The lowest high water recorded is extreme 
(low) high water of neap tides (E.(L.)H.W. 
N.T.), and the highest low water is extreme 
(high) low water of neap tides (E.(H.)L.W. 
N.T.). All levels between these (8.40 feet 
[8.88 feet] and 4.00 feet [3.96 feet], respec- 
tively) are subjected to alternating submer- 
gence and emergence twice in every 24 hours. 
For all levels above E.(L.)H.W.N.T. there is, 
therefore, a varying amount of continuous 
emergence; and similarly, for all levels below 
E.(H.)L.W.N.T., a varying amount of con- 
tinuous submergence. 
The correlation of tidal information with 
critical levels may be given as: 
The critical level 1.5 feet is just above 
M.L.W.ST. 
The critical level 11,0 feet is just below 
M.H.W.S.T 
The critical level 3.75 feet lies between 
M.L.W.N.T and E.(H.)L.W.N.T. 
The critical level 9-3 feet is approximately 
M. H.W.N.T. 
The critical level 8.0 feet is below E.(L,)H. 
W.N.T 
Significance of Tide Levels. Some of 
these levels appear to represent thresholds. 
For example, E.(L.)H. W.N.T. is the highest 
level of regular, semidiurnal, alternating sub- 
mergence and emergence, while E.(H.)L.W. 
N. T. is the lowest level regularly uncovered 
by the tide. To determine the significance of 
such levels, the tidal factor was analysed with 
respect to submergence-emergence relations 
and to continuous emergence and submer- 
gence. 
The submergence-emergence behaviour of 
the tides at Auckland was determined from 
the tide charts prepared by the instrument at 
Queen’s Wharf. These charts are graphs of 
sea level against time. Calculations were made 
of the relative submergence and emergence 
of the different levels between the extreme 
limits of inundation, expressed in terms of 
the percentage of the total time considered ; 
during which each 1-foot level was not sub- 
