Inter-tidal Zonation — BEVERIDGE and CHAPMAN 
ures 5 and 6 will show that there is a change 
in total hours of exposure and also in total 
continuous exposure at this level. 
The changes in total exposure and total 
continuous air exposure would seem to be 
the causal factors determining the six species 
with limits around -f- 9 feet. The species 
limits at E.(H).L.W.N. are very clearly de- 
fined and include those of three species im- 
portant ecologically, e.g., Pachymenia himan- 
tophora, Gigartina alveata, and Mytilus cana- 
liculus. Probably causal agents at this level 
are the changes in the total amount of air 
exposure and continuous submergence. 
At M.L.W.N. Durvillea is the most impor- 
tant species ecologically and there are changes 
in the total amount of continuous submer- 
gence, the maximum period of continuous 
submergence, and the number of tidal sub- 
mergences. At 2.3 feet the causal factors 
probably include total amount of continuous 
submergence and the total number of sub- 
mergences. At M.L.W.S. the principal tidal 
factor appears to be changes in total air ex- 
posure, though at this level, and with the 
number of species involved, competition may 
be severe. 
SUMMARY 
A general account is given of the inter- 
tidal zonation at Piha on the west coast just 
north of Auckland. Eleven major biological 
communities are recognised, together with a 
few local or seasonal communities. 
A levelling survey of the more important 
species was carried out, and after correlating 
the tides at Piha with those at Auckland, the 
principal tidal phenomena were worked out 
from actual tide charts. In most areas expos- 
ure to wave action elevated the limits of 
certain species, especially those higher up on 
the shore. If allowance is made for this ele- 
201 
vation it is suggested that there are six critical 
levels at Piha; these are mean high water 
spring tides, about 9 feet, extreme (high- 
est) low water mark neap tides, mean low 
water neap tides, -f- 2.3 feet, and mean low 
water spring tides. 
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