252
J. A. KITCHING AND F. J. EBLING
Saccorhiza in fast current on the Sill is remarkably elongated (Ebling
and Kitching, 1950), but this does not seriously affect this relation.
Our information is summarized in Table 111. Patina pellucida had a
marked preference for South Rapids and the three seaward stations to
North Rapids and the Sill. The results confirm a previous conclusion
(Section IV) that in the Rapids area Patina prefers a moderate current.
They add, however, the important fact that the numbers of Patina
were much greater at South Rapids and a t all the seaward stations than
a t North Rapids or the Sill. The greatest numbers were at Cave Island,
so that moderate wave-action appears to favour Patina in the sea, just
as moderate current does in the upper Rapids, but it also appears that
some additional adverse factor supervenes from the Sill northwards. It
is possible only to speculate what this may be. For instance, the current
flows almost continuously on the Sill and at North Rapids, whereas a t
South Rapids there are much longer periods of relatively quiet water
with slow turbulent movement rather than continuous flow. Thus the
opportunities for the settlement of Patina may be much more restricted
at North Rapids and the Sill than anywhere else.
The fronds of Succorhim proved a more favourable substratum for
Patina than either Laminaria digitata or L. hyperborea (Table 111).
VI. ZONATION
A. INTRODUCTION
Restriction of each species to its own zone, extending horizontally a t
a characteristic level, is the principal and most widely observed feature
of the distribution of plants and animals in the tidal zone. The period
of exposure to air is the obvious primary environmental cause of such
zonation, though wave-action, substrate, aspect, climatic conditions,
the time of low water of spring tides, and miscellaneous biological interactions may all be modifying factors (Lewis, 1964).
Zonation continues below the limits of the lowest tides. In the shallow
sublittoral the intensity and quality of light change with depth, and
there is no doubt that illumination is of cardinal importance in the
distribution of algae. The decrease in water movement due to waveaction may also be important. For instance, the upper limit of Laminuria.
sacchurina in the Sound of Jura is depressed by increased wave-action
(Kitching, 1941), and this relationship has been confirmed by Kain
(1962). As wave-action and current lessen, the bottom deposit also increases in amount and the quality may change. Many other factors may
play a part. The existence during summer of a thermocline within
Lough Ine involves the establishment between a depth of 20 and 30 ni
of gradients of oxygen concentration and pH as well as of temperature.
J. A. KITCHING AND F. J. EBLING
Saccorhiza in fast current on the Sill is remarkably elongated (Ebling
and Kitching, 1950), but this does not seriously affect this relation.
Our information is summarized in Table 111. Patina pellucida had a
marked preference for South Rapids and the three seaward stations to
North Rapids and the Sill. The results confirm a previous conclusion
(Section IV) that in the Rapids area Patina prefers a moderate current.
They add, however, the important fact that the numbers of Patina
were much greater at South Rapids and a t all the seaward stations than
a t North Rapids or the Sill. The greatest numbers were at Cave Island,
so that moderate wave-action appears to favour Patina in the sea, just
as moderate current does in the upper Rapids, but it also appears that
some additional adverse factor supervenes from the Sill northwards. It
is possible only to speculate what this may be. For instance, the current
flows almost continuously on the Sill and at North Rapids, whereas a t
South Rapids there are much longer periods of relatively quiet water
with slow turbulent movement rather than continuous flow. Thus the
opportunities for the settlement of Patina may be much more restricted
at North Rapids and the Sill than anywhere else.
The fronds of Succorhim proved a more favourable substratum for
Patina than either Laminaria digitata or L. hyperborea (Table 111).
VI. ZONATION
A. INTRODUCTION
Restriction of each species to its own zone, extending horizontally a t
a characteristic level, is the principal and most widely observed feature
of the distribution of plants and animals in the tidal zone. The period
of exposure to air is the obvious primary environmental cause of such
zonation, though wave-action, substrate, aspect, climatic conditions,
the time of low water of spring tides, and miscellaneous biological interactions may all be modifying factors (Lewis, 1964).
Zonation continues below the limits of the lowest tides. In the shallow
sublittoral the intensity and quality of light change with depth, and
there is no doubt that illumination is of cardinal importance in the
distribution of algae. The decrease in water movement due to waveaction may also be important. For instance, the upper limit of Laminuria.
sacchurina in the Sound of Jura is depressed by increased wave-action
(Kitching, 1941), and this relationship has been confirmed by Kain
(1962). As wave-action and current lessen, the bottom deposit also increases in amount and the quality may change. Many other factors may
play a part. The existence during summer of a thermocline within
Lough Ine involves the establishment between a depth of 20 and 30 ni
of gradients of oxygen concentration and pH as well as of temperature.