ECOLOGICAL STUDIES AT LOUGH I N E
209
level with Mark 7 on the quay (Fig. 4); to the north and south of this
point the greatest speed attained falls off, and is less than 1 m sec-l in
mid-channel at each end of the Rapids. The current is weak in places
where the shore recedes from the main channel at either end of
the Rapids - Renouf’s Bay, Eddy Creek, and off Nita’s Rock - and
these places are occupied by eddies when their end of the Rapids
is downstream.
The bottom of the Rapids is covered by a forest of the large brown
algae Saccorhiza polyschides and (over the Sill) Laminaria digitata
(Fig. 7). The fronds of these algae form a canopy held up a little distance
above the bottom and populated by a characteristic but limited fauna
and flora (p. 222). The current speeds at the upper surface of the canopy
at several stations are given in Fig. 8. They are little different from those
at the surface of the water.
The canopy partially protects the bottom from the current. The
current speed, as measured with a Pitot tube, falls off rather steeply
below the canopy. However, in the regions of fastest flow at the water
surface, there is still a very appreciable pressure (indicating up to
1 m sec-l) when the Pitot tube is held in contact with the rock bottom
(Fig. 9). There are no doubt pockets of relatively still water, as described
by Ambiihl(l959; see also Macan, 1963), between the large boulders towards the northern end of the Rapids. Nevertheless the direction of the
current and the turbulence change with the tide, and few places are
likely to offer permanent shelter. Macroscopic organisms living on the
Sill must be subject to a very considerable velocity gradient in the
boundary layer over the rock surface, as discussed by Crisp (1955).
Other physical and chemical conditions of the water and underlying
substrate are considerably affected by the current. Rocks in the main
stream have little sediment upon them, and those on the Sill are clean,
although there are some broken shells in crevices between them. Rocks
in the quieter areas-Codium Bay, Renouf‘s Bay, Harbour, Eddy
Creek, and off Nita’s Rock - have a distinct deposit of fine sediment
resting on their upper surfaces and among the algae and other organisms which grow upon them. The distribution of sediments on boulders
throughout the Rapids area was investigated by means of a special box
which acted as a miniature diving bell (Kitching et al., 1952). Each
boulder was placed in the box under water and the water in the box
was then displaced by air, to fix the sediment in position, before the
apparatus was brought to the surface. The sediment was then washed
separately from the top and the bottom of the boulder, graded by the
use of wire sieves, dried, and weighed.
The average density of sediment is shown in Fig. 10. On the tops of
boulders the total amount is least on the Sill and continues low to the
209
level with Mark 7 on the quay (Fig. 4); to the north and south of this
point the greatest speed attained falls off, and is less than 1 m sec-l in
mid-channel at each end of the Rapids. The current is weak in places
where the shore recedes from the main channel at either end of
the Rapids - Renouf’s Bay, Eddy Creek, and off Nita’s Rock - and
these places are occupied by eddies when their end of the Rapids
is downstream.
The bottom of the Rapids is covered by a forest of the large brown
algae Saccorhiza polyschides and (over the Sill) Laminaria digitata
(Fig. 7). The fronds of these algae form a canopy held up a little distance
above the bottom and populated by a characteristic but limited fauna
and flora (p. 222). The current speeds at the upper surface of the canopy
at several stations are given in Fig. 8. They are little different from those
at the surface of the water.
The canopy partially protects the bottom from the current. The
current speed, as measured with a Pitot tube, falls off rather steeply
below the canopy. However, in the regions of fastest flow at the water
surface, there is still a very appreciable pressure (indicating up to
1 m sec-l) when the Pitot tube is held in contact with the rock bottom
(Fig. 9). There are no doubt pockets of relatively still water, as described
by Ambiihl(l959; see also Macan, 1963), between the large boulders towards the northern end of the Rapids. Nevertheless the direction of the
current and the turbulence change with the tide, and few places are
likely to offer permanent shelter. Macroscopic organisms living on the
Sill must be subject to a very considerable velocity gradient in the
boundary layer over the rock surface, as discussed by Crisp (1955).
Other physical and chemical conditions of the water and underlying
substrate are considerably affected by the current. Rocks in the main
stream have little sediment upon them, and those on the Sill are clean,
although there are some broken shells in crevices between them. Rocks
in the quieter areas-Codium Bay, Renouf‘s Bay, Harbour, Eddy
Creek, and off Nita’s Rock - have a distinct deposit of fine sediment
resting on their upper surfaces and among the algae and other organisms which grow upon them. The distribution of sediments on boulders
throughout the Rapids area was investigated by means of a special box
which acted as a miniature diving bell (Kitching et al., 1952). Each
boulder was placed in the box under water and the water in the box
was then displaced by air, to fix the sediment in position, before the
apparatus was brought to the surface. The sediment was then washed
separately from the top and the bottom of the boulder, graded by the
use of wire sieves, dried, and weighed.
The average density of sediment is shown in Fig. 10. On the tops of
boulders the total amount is least on the Sill and continues low to the