Summary: | Elevated fine inorganic sediment supply in streams may impair many biological
functions, however the contribution of exposure duration to altering these functions has
not been previously considered. I evaluated the effects of fine sediment pulse duration
on invertebrate assemblages and rainbow trout growth and mortality. I constructed
streamside flow-through channels at Moffat Creek in the central interior of British
Columbia for this experiment. Fourteen experimental channels, each containing
invertebrates and 10 rainbow trout swim-up fry, received fine inorganic sediment
treatments of a constant concentration but varied in pulse duration, ranging from 0 to 6
hours per pulse. A pulse was initiated every second day for total of ten pulses over 19
days. Benthic invertebrate abundance and family richness declined (both r²=0.77,
P<0.01) as sediment pulse duration increased. Similarly, the family richness of
invertebrate drift declined as pulse duration increased (r²=0.20, P=0.04), however, drift
abundance increased with increasing pulse duration (r²=0.35, P<0.03). Drift samples
were numerically dominated by the family Chironomidae (midges) comprising up to
99% of the drifting insects within the high pulse duration channels. Principal
components analysis (PCA) of invertebrate family presence/absence revealed that the
primary variation in the drift assemblage (PC1, 27%) was attributed to variation in the
presence/absence of Simuliidae; longer pulse duration resulted in the disappearance of
Simuliidae (r²0.44 , P<0.01). The primary variation in the benthic assemblage (PC1,
38.8%) was described by the presence/absence of Elmidae, Nemouridae, Baetidae,
Leptophlebiidae and Heptageniidae; increasing pulse duration resulted in the virtual
disappearance of those families (r²=0.79, P<0.01). Rainbow trout length and mass
linearly declined (r²=O.69, P<0.01; r²=0.70, P<0.01 respectively) with increased
sediment pulse duration. There was no significant relation (P>0.05) between sediment
pulse duration and trout mortality. Analysis of variance revealed that invertebrate, drift,
benthos and trout growth were significantly affected by sediment exposure time.
Relative to the controls, significant negative changes occurred between the three and
five-hour treatments. The hypothesis that trout growth was more affected by direct
effects of sediment (e.g. impaired visual feeding ability, physiological stress), than
indirectly (through modification of the prey abundance or composition) was tested using
path analysis. The results indicate that direct effects were the primary influence of
sediment on trout growth although significant alterations of the invertebrate community
were observed.
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