40 
PACIFIC SCIENCE, Vol. VI, January, 1952 
Melarhaphe oliveri f 
Suaeda maritima o 
Triglochin striata var. filifolium r 
This is principally a community of about 
M.H.W.S.T. In the shelter of the dominant 
angiosperms is a Rhizodonium hookeri socies. 
2. Avicennia officinalis consocies 
This is a mid-tidal community. Although 
the vertical range within which mangrove 
seedlings will develop is narrow, a mature 
plant occupies a wide vertical range by virtue 
of its size. A community of Rhizodonium 
hookeri, Catenella nipae, Caloglossa leprieurii, 
and sediment-fixing Myxophyceae occurs on 
the pneumatophores. In some areas, Hormo- 
sira hanksiiis attached to the pneumatophores, 
and to scoria where this is present. Gradlaria 
secundata f. pseudoflagellifera appears to be 
free-living on the surface of the mud. 
3. Zostera nana consocies 
This is the community of the lowest littoral 
and the upper sublittoral. 
LEVELLING SURVEY 
A numerical account of distribution was 
provided by means of a levelling survey. The 
species were treated individually because of 
the wide variety of vertical and horizontal 
ranges which produces the observed com- 
plexity of community groupings. 
The upper and lower limits of the inter- 
tidal organisms were determined as relative 
to tide levels, both for the elucidation of 
causal factors and for correlation with similar 
work elsewhere. There being no bench marks 
on the island, all levellings were referred to 
marks established above E.H.W.S.T., these 
marks in turn being tied to E.L.W.S.T. as 
determined for the island (see ''Tidal Fac- 
tors”). The levels given below are based on 
E.L.W.S.T. as 0.00 feet. 
Relation of Communities to Levelling 
(Figs. 5, 6) 
The levels for an apparently normal zona- 
tion confirm the observed communities. 
There is a definite tendency for the majority 
of the upper and lower limits of species to 
occur in the vicinity of the apparent limits of 
formations and communities. 
It is also possible to postulate critical levels 
at which significant changes in species com- 
position occur. Five such levels were recog- 
nised on Rangitoto. The lowest critical level 
on the shore was placed at 1.50 feet, being 
the average of the seven upper limits and 
eight lower limits of species which were 
located between the levels 1.00 feet and 2.00 
feet above E.L.W.S.T. Others were placed at 
3.75 feet (five upper and three lower limits 
between 3.50 feet and 4.00 feet); at 8.00 feet 
(five upper and four lower limits between 
7.50 feet and 8.50 feet); at 9.30 feet (seven 
upper and five lower limits between 9-00 feet 
and 9.75 feet); and at 11.00 feet (nine upper 
and three lower limits between 10.50 feet and 
11.50 feet). 
The numerical basis of these levels (to the 
nearest 0.5 foot) is shown in Figure 6. The 
principal levels of change in species compo- 
sition are clearly in the vicinity of 1.5, 3.5, 
9.5, and 11.0 feet. 
FACTORS INVOLVED 
Tidal Factors 
Tidal Information. The tides at Auck- 
land are of the normal semidiurnal type, with 
a diurnal inequality. Tidal measurements are 
made at Queen’s Wharf by the Auckland 
Harbour Board’s tide instrument. The zero 
for these measurements is E.L.W.S.T., taken 
as 3 feet above the Auckland Dock Sill. The 
Admiralty Datum, upon which the tidal data 
in the New Zealand Nautical Almanac are 
based, is 3.5 feet above the Dock Sill. 
Tidal Behaviour at Rangitoto. Since it 
was intended to use the tide charts prepared 
by the Auckland tide machine in the calcu- 
lation of submergence and emergence, and 
also because the correlation of littoral levelling 
with similar work elsewhere had of necessity 
to be through tide levels, a determination of 
