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

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that DurvilZaea extends several miles up Lyttelton a.nd Akaroa Har.bours
into quite sheltered water.
Wave action there is presmnably grade
II or less. lUthough DUY'viZlaea does grollT in increasingly sheltered
localities sou"thNards, this statement is apt to mislead because it.
could convey the impression that there is a continuous Duroillaea band
fringing the rocky sho:r'eline of these harbours.
At Lyttelton both
species extend no further than Mechanics Bay, less than 500 m inside
Godley Head,
At Aka:ma D. antarctica grows at Wainui Cfig. 5.6g-3.)
B kIll inside the harbour, but the plant:.s are sparsely distributed an.d,
confined to the end of a small promontory that is fully exposed to
southerly swells. The situation in Otago Harbour is similar to tllat
of Lyttelton. Both species extend no further than just inside the
Heads (Bat-ham 1958:656 and own observations) .
The distribution of DurviZZaea is frequently patchy even in
areas subjected to grade IV and V waves. This probably reflects
subtle diffex:ences in wave force. The horizontal distribution of
the t;wo species overlap, but D. antarctica is 'che more \... idely distributed,
In areas subjected to ~~treme
wave action, and in moderately sheltered
areas, D. wilZana is relatively uncommon.
On the very end of Seal
Reef I
and in the semishelter of the Fisheries Ttfuarf at Kaikoura,
the kelp band is almost exclusively D. antaretiea.
(ii)
Shore slope
Wave force and shore slope are closely interrelated.
As the
shore becomes steeper, 'the kinetic energy of the ",aves is dissipated
over an increasingly smaller area.
On steep slopes, wave force has
a high impact component, \oJhereas on more gentle slopes dr:ag forces
are more important.
On the north side of Tautulm Peninsula D. IlrUlana gl:OWS
abundantly on slopes less than 60 0 , but is rarely found on steeper
slopes. It is difficult to decide whether the relative scarcity of
this species on the steepest slopes is due directly to their steepness,
or indirectly to very strong Have force that they receive.
Observations at many places suggested both factors may be important.
On many cliff shares there is often a no_rrow ledge at 1m" tide level,
and any D. wiZZana are invariably confined to these ledges" This
suggests that the kelp is unsuccessful on exceptionally steep
Yet in some areas subj ect to severe 'Vlave force, plants growing on
these ledge s are badly ba t-tered.
They have fell' lateral l