A biological study of Durvillaea antarctica (Chamisso) Hariot and D ...

Nave force inside the OhC!m Point channel is more concentrated
than on an equal area of rock platform at Tautuku.
shallow slope off shore nor any wide D. wiZlana band.
There is no
Waves begin
to break as they enter the channel, and their total energy is dissipated
within its narrow confines.
The channel is "L" shaped and the
wash rebounds from side to side so that the water becomes vexy agitated.
The impact component of wave force is much higher at Ohau Point.
Unlike Tautuku, the marked plants are consta.ntly tossed aJ)out in the
waves even on the calmest days.
The channel bottom is filled tllit.h
large boulders and rocks, a.nd these grind u.p and dONn during storms"
Drifting logs and dunnage become trapped inside the channel and severely
bat.ter the fringing' D. antarctica.
the channel \Vera larger than 1 cubic metre.
226
Some of the rocks tossed about in
A high mortality of small and very large D. antarctica plants
recorded at Tautuku was not unexpected (Fig. 7.16),
Many of the
small plants were attached to rotten rock, or patches of red algae such
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as Po~ysiphonia spp., and were easily dislodged during storms. Large
specimens were also easily dislodged because their holdfasts ,\lere
completely exca.vated by burro\'ling animals.
Yet at Ohau Point the
mortality increased with increasing plant size (Fig. 7.19),
This was
because ma.ny of the plants \'lere dislodged by loose boulders and rocks
and drifting timber.
The larger the size of a plant the greater are
the chances that its holdfast will be scraped off the rock, or that
its frond will be momentarily entangled and torn loose.
10.7 ~RKS CONCERNING LONGEVITY
By measuring the gro"lth of marked plants for three years, it
became apparent that many of the largest specimens ",ere at least 5
years old and probably older. 55% of the marJ{ed D. antarctica plants
at Tautuku were detached in 30 months, and on the assumption that about
half the sample was lost every 2~ years, some plants ",ould probably
survive for ten years"
Longevity can be calCUlated more accurately than this by making
use of the annual layering of conceptacles. In June 1972 lamina
samples ~'lere taken from some of the larg-est marked Do antai'ctioa plants
at Tautuku. These, when sectioned, revealed three and sometimes four
concepts.cle layers. Since plants do not become fextile until they
are at least 1:\-10 years old, these marked specimens were not less than