Chapter 1 Minimum Flows and Levels - Southwest Florida Water ...

equirements. Results of PHABSIM analyses are used to assess flow needs
during periods of low to medium flows.
3.3.4 Woody Habitats
Stream ecosystem theory emphasizes the role of instream habitats in
maintaining ecosystem integrity. These habitats form a mosaic of
geomorphically defined substrate patches (Brussock et al. 1985), each with
characteristic disturbance regimes and macroinvertebrate assemblages (Huryn
and Wallace 1987). For instance, invertebrate community composition and
production in a blackwater river varies greatly among different habitat types,
where the habitats are distinguished by substrates of different stability (e.g.,
sand, mud and woody debris) (Benke et al. 1984, Smock et al. 1985, Smock and
Roeding 1986). Ecosystem dynamics are influenced by the relative abundance
of these different habitat types. Changes in community composition and function
occurring along the river continuum are in part a consequence of the relative
abundance of different habitat patches, which are under the control of channel
geomorphology and flow. For determining MFLs, we identify key habitats and
features that play a significant role in the ecology of a river system using a
habitat-based approach that includes a combination of best available data and
site-specific field work.
Among the various instream habitats that can be influenced by different flow
conditions, woody habitats (snags and exposed roots) are especially important.
In low-gradient streams of the southeastern U.S.A. coastal plain, wood is
recognized as important habitat (Cudney and Wallace 1980; Benke et al. 1984,
Wallace and Benke 1984; Thorp et al. 1990; Benke and Wallace 1990). Wood
habitats harbor the most biologically diverse instream fauna and are the most
productive habitat on a per unit area basis (Benke et al. 1985). Comparisons of
different instream habitats in a southeastern stream indicates that production on
snags is at least twice as high as that found in any other habitat (Smock et al.
1985).
Wood provides advantages as habitat, as it is relatively stable and long lived
compared to sand substrata, which constantly shift (Edwards and Meyer 1987).
Even bedrock substrates, though the most stable of all, are susceptible to
smothering by shifting sand and silt. Wood is a complex structural habitat with
microhabitats (such as interstices that increase surface area) that provide cover
for a variety of invertebrates. As an organic substrate, wood is also a food
resource for utilization by microbial food chains, which in turn supports
colonization and production of macroinvertebrates. As physical impediments to
flow, woody structures enhance the formation of leaf packs and larger debris
dams. These resulting habitats provide the same functions as woody substrata
in addition to enhancing habitat diversity instream. Organisms in higher trophic
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