Environmental Engineering Reference
In-Depth Information
refer to this review. Research on the effects of flow fluctuations on aquatic inver-
tebrates in the Pacific Northwest is limited, although more information is avail-
able elsewhere in North America. The studies that have been done suggest that
aquatic invertebrates can be severely impacted by flow fluctuations. Fluctuations
substantially reduce invertebrate diversity and total biomass and change the species
composition under most circumstances. One study from the Skagit River found that
flow fluctuations had a greater adverse impact on the aquatic invertebrate commu-
nity than a substantial reduction in average flow (Gislason, 1985). The reduction
in aquatic invertebrate production can impact salmonid production as a result of
reduced feeding (Cushman, 1985).
Additional research is needed on the effects of flow fluctuations on aquatic inver-
tebrates in the Pacific Northwest; however, a thorough study would be a formidable
task. It would involve many species with different life cycles, behavioral responses,
lethal responses, and contributions as prey to salmonids. Populations of some spe-
cies may change rapidly under normal conditions, thus it may be difficult to associ-
ate cause and effect. Flow fluctuations can impact the aquatic invertebrates in the
following ways:
•
Stranding
—Flow fluctuations can strand many species of aquatic inverte-
brates, much in the same way fish can become stranded (Gislason, 1985;
Phillips 1969). Death may result from suffocation, desiccation, temperature
shock, or predation.
•
Increased drift
—Many aquatic invertebrates are sensitive to reductions
in flow and respond by leaving the substrate and floating downstream.
This floating behavior is drift. Nighttime drift is normal; however, drift
becomes highly elevated under unnatural fluctuations in flow (McPhee
and Brusven, 1975; Cushman, 1985). This elevated drift may be an emer-
gency response to avoid stranding, a response to overcrowding of the
inter-gravel habitat, or a response by aquatic species that are adapted to a
narrow range of water velocity. This response may temporarily increase
fish food supply (McPhee and Brusven, 1975), but when repeated fluc-
tuations occur many species are flushed out of the river and the aquatic
invertebrate biomass usually declines, often substantially (Cushman,
1985; Gislason, 1985). Elevated drift also occurs in response to sudden
increases in flow, capturing terrestrial insects from the river banks and
scouring some aquatic invertebrates from the river substrate (Mundie and
Mounce, 1976).
•
Detritus feeders
—Under stable flow conditions, floating detritus (leaves,
woody debris) accumulates on the shores of the river as a result of cur-
rent and wind action on sand or gravel substrate. This detritus remains
close to the river margin and often remains damp for days or weeks at a
time. Under fluctuating flows, this organic detritus becomes suspended
(Mundie and Mounce, 1976) and is flushed out of the river or redeposited
at the high waterline where it desiccates during low low periods. As a
result the invertebrate detritus community is less capable of exploiting
this resource.
