188
H. Hoang' F. Recknagel J. Marshall . S. Choy
The occurrence of Dytiscidae is suggested to be highly abundant in the
southeastern parts of Australia and most common in littoral areas (Lawrence and
Britton 1991). However the sensitivity curves in Figs. 9c and d showed that they
were only observed in North Queensland at low-order streams.The findings from
these sensitivity results, which were apparently contradictory to previous
knowledge may suggest further research in this area.
9.4.3
Limitations of the Method
The present sensitivity analysis investigated only relationships between one single
input and a specific output. However the occurrence of macroinvertebrates is
always the result of multivariate non linear patterns of habitat conditions. Most
habitat parameters do not occur in isolation but are closely interrelated.
The interrelationship between stream current, water temperature and oxygen
demand of macroinvertebrates is an example. The current continually replenishes
water and hence also oxygen in the immediate vicinity of the respiratory surfaces
of the animals, and quite low levels can be tolerated in strong currents that renew
oxygen at a high rate. Generally, metabolic rates and oxygen demand are higher of
stream invertebrates than of still water forms at a given temperature. Respiration is
temperature-related and rates can increase by 10% or more per 1°C temperature
rise. Thus increased temperature does not only reduce oxygen availability but it
also increases oxygen demand that can add to the physiological stress of
organisms (Giller and Malmqvist 1998).
The most important hydraulic characteristic for individual organism is the
prevailing current velocity striking the organism head-on (Statzner et al. 1988).
Macroinvertebrate species react differently to current velocity and show
differential preferences. As a consequence different flow conditions lead to
divergent assemblages of organisms. In a detailed survey by Quinn and Hickey
(1994), boundary layer Re (Renolds number) was the most strongly correlated
individual variable with invertebrate distribution and taxa richness in two New
Zealand streams. However a combination of mean velocity, substrate size, and
depth gave stronger correlation than any single variable. It appears that the
interaction between current velocity and stream substrate size is particularly
important in determining invertebrate distributions.
Orth and Maughan (1983) identified optimum velocity, depth, and substrate as
determining factors for major taxa of benthic macroinvertebrates of warm-water
woodland stream. The combination of current velocity of 60cmlsec, a depth of 34
cm and rubble-boulder substrate resulted in optimal diversity of benthic
assemblages. Taking into account that habitat selection by benthos may be based
on factor combinations, the investigators derived "joint preference factors" using
the product of the individual preference factors.
H. Hoang' F. Recknagel J. Marshall . S. Choy
The occurrence of Dytiscidae is suggested to be highly abundant in the
southeastern parts of Australia and most common in littoral areas (Lawrence and
Britton 1991). However the sensitivity curves in Figs. 9c and d showed that they
were only observed in North Queensland at low-order streams.The findings from
these sensitivity results, which were apparently contradictory to previous
knowledge may suggest further research in this area.
9.4.3
Limitations of the Method
The present sensitivity analysis investigated only relationships between one single
input and a specific output. However the occurrence of macroinvertebrates is
always the result of multivariate non linear patterns of habitat conditions. Most
habitat parameters do not occur in isolation but are closely interrelated.
The interrelationship between stream current, water temperature and oxygen
demand of macroinvertebrates is an example. The current continually replenishes
water and hence also oxygen in the immediate vicinity of the respiratory surfaces
of the animals, and quite low levels can be tolerated in strong currents that renew
oxygen at a high rate. Generally, metabolic rates and oxygen demand are higher of
stream invertebrates than of still water forms at a given temperature. Respiration is
temperature-related and rates can increase by 10% or more per 1°C temperature
rise. Thus increased temperature does not only reduce oxygen availability but it
also increases oxygen demand that can add to the physiological stress of
organisms (Giller and Malmqvist 1998).
The most important hydraulic characteristic for individual organism is the
prevailing current velocity striking the organism head-on (Statzner et al. 1988).
Macroinvertebrate species react differently to current velocity and show
differential preferences. As a consequence different flow conditions lead to
divergent assemblages of organisms. In a detailed survey by Quinn and Hickey
(1994), boundary layer Re (Renolds number) was the most strongly correlated
individual variable with invertebrate distribution and taxa richness in two New
Zealand streams. However a combination of mean velocity, substrate size, and
depth gave stronger correlation than any single variable. It appears that the
interaction between current velocity and stream substrate size is particularly
important in determining invertebrate distributions.
Orth and Maughan (1983) identified optimum velocity, depth, and substrate as
determining factors for major taxa of benthic macroinvertebrates of warm-water
woodland stream. The combination of current velocity of 60cmlsec, a depth of 34
cm and rubble-boulder substrate resulted in optimal diversity of benthic
assemblages. Taking into account that habitat selection by benthos may be based
on factor combinations, the investigators derived "joint preference factors" using
the product of the individual preference factors.