286
P. 9. MEADOWS AND J. I. ULLMPBELL
and to fish farming, perhaps these are research fields that might interest
applied institutions such as fisheries laboratories.
C. Preshwater animals
Fresh waters may range from temporary ponds of under a metre in
depth that dry out in hot weather, to deep and stable oligotrophic and
eutrophic lakes, and from steep fast mountain streams to flat slow rivers.
The water may be saturated with oxygen, as in a stream, or may be
anaerobic in the hypolimnion of a rich eutrophic lake during summer.
Temperatures may vary widely especially in fairly stagnant waters,
and here marked annual and diurnal fluctuations are well known.
Once the thermocline is established in eutrophic lakes during the
spring, temperatures may change 10°C or more within a few metres
vertically. Wide variations also occur in the nature of the bottom:
boulders and gravel in fast-moving streams, sand and mud banks in
slower rivers, peat bottoms in acid bog pools, and rich muds with
decaying organic matter, algae, and higher plants in lakes and ponds.
The concentrations of inorganic ions and of organic materials vary
with the type of water ; they are low in the soft waters of some glacial
lakes and high in the richer eutrophic ones ; their relative concentrations also differ from place to place and throughout the year
(Mortimer, 1942).
Conditions in fresh waters therefore vary widely, and so it follows
that invertebrates living in fresh waters will have to respond to many
of these variations in order to find and remain in their preferred habitat.
It is likely that animals will use information from water movements,
anaerobic conditions, salt and organic concentrations and so on, as
well as variables that fresh water shares with the sea, such as
temperature, pressure, light, and the nature of the bottom. Let us
consider these.
Many animals that live at the bottom of streams and rivers move
into the current-are rheopositive-and have clear preferences for
particular current velocities when tested experimentally. The isopod
Asellus communis Say is common in small streams with rapids and is also
found less frequently in larger streams and in lakes (Allee, 1912, 1914) ;
in all these environments it must have abundant places to cling to
(Allee, 1912). In laboratory tests it swims into the current at low to
medium current speeds ; however as current speed is increased it clings
to the substrate a t a speed less than the speed common in the parts of
the river where the species is normally found. From these observations
Allee (1914) concludes that the distribution of Asellus communis
in streams cannot be accounted for by its rheotactic response alone,
P. 9. MEADOWS AND J. I. ULLMPBELL
and to fish farming, perhaps these are research fields that might interest
applied institutions such as fisheries laboratories.
C. Preshwater animals
Fresh waters may range from temporary ponds of under a metre in
depth that dry out in hot weather, to deep and stable oligotrophic and
eutrophic lakes, and from steep fast mountain streams to flat slow rivers.
The water may be saturated with oxygen, as in a stream, or may be
anaerobic in the hypolimnion of a rich eutrophic lake during summer.
Temperatures may vary widely especially in fairly stagnant waters,
and here marked annual and diurnal fluctuations are well known.
Once the thermocline is established in eutrophic lakes during the
spring, temperatures may change 10°C or more within a few metres
vertically. Wide variations also occur in the nature of the bottom:
boulders and gravel in fast-moving streams, sand and mud banks in
slower rivers, peat bottoms in acid bog pools, and rich muds with
decaying organic matter, algae, and higher plants in lakes and ponds.
The concentrations of inorganic ions and of organic materials vary
with the type of water ; they are low in the soft waters of some glacial
lakes and high in the richer eutrophic ones ; their relative concentrations also differ from place to place and throughout the year
(Mortimer, 1942).
Conditions in fresh waters therefore vary widely, and so it follows
that invertebrates living in fresh waters will have to respond to many
of these variations in order to find and remain in their preferred habitat.
It is likely that animals will use information from water movements,
anaerobic conditions, salt and organic concentrations and so on, as
well as variables that fresh water shares with the sea, such as
temperature, pressure, light, and the nature of the bottom. Let us
consider these.
Many animals that live at the bottom of streams and rivers move
into the current-are rheopositive-and have clear preferences for
particular current velocities when tested experimentally. The isopod
Asellus communis Say is common in small streams with rapids and is also
found less frequently in larger streams and in lakes (Allee, 1912, 1914) ;
in all these environments it must have abundant places to cling to
(Allee, 1912). In laboratory tests it swims into the current at low to
medium current speeds ; however as current speed is increased it clings
to the substrate a t a speed less than the speed common in the parts of
the river where the species is normally found. From these observations
Allee (1914) concludes that the distribution of Asellus communis
in streams cannot be accounted for by its rheotactic response alone,