Environmental Engineering Reference
In-Depth Information
history of active channels across floodplains produces lakes or sloughs, and heterogeneous,
patchy distributions of sediments in the stream itself, and in places now abandoned by
active channels. Heterogeneity in the amount and rate of water penetration into soils, and of
drainage and soil oxygenation of the patchy substrate, sets up different fluxes of nutrients
as well as communities of microbes and plants within the stream bottom and adjacent flood-
plain. Stream and riparian patchiness sets up the template for nutrients, organic matter, and
biological activity, all of which drive ecosystem metabolism (
Naiman et al. 2005
).
The productivity of stream ecosystems is widely held to be influenced by the existence
and nature of any plant canopy that overhangs the stream channel. Usually the effects are
assumed to be primarily the result of decomposition of leaf litter that falls into the stream.
Experiments have shown that other aspects of heterogeneity can also be important
(
Lagrue et al. 2011
). Although overarching canopies do add leaf litter, they also reduce
stream temperature and input of sunlight. Together, these factors are associated with
reduced decomposition in shaded reaches of streams.
Marine Benthos
The spatial heterogeneity created by organisms in shallow marine and tidal habitats is
well known. However, the attention to the ecosystem implications of such heterogeneity is
of more recent origin. Spatially extensive assessments have documented that decreasing
the heterogeneity in hard substrate marine sessile benthos decreases species richness in
such waterscapes. The net result of patchiness is to increase coexistence regionally
(
Munguia et al. 2011
). Such effects also appear in soft-bottomed areas. A model system in
which species can preferentially select habitat patches alters the local composition of spe-
cies and the distribution of organism density (
Godbold et al. 2011
). At the scale of individ-
ual patches, the intensity of bioturbation or burrowing in the soft substrate decreases, but
across patches, the ecosystem nutrient concentration increases.
FIGURE 10.7
Spatial complexity in a small stream ecosystem (Black Creek in the Hudson Valley of New York
state). In the background is an open canopy area above the stream, in which sunlight and temperature would be
higher than elsewhere, while leaf litter input would be less. In the middle ground are two contrasting habitats, a
pool of flowing water and a gravel bar through which subsurface or hyporheic flow would pass. In the fore-
ground are downed logs, behind the submerged portions of which sediment and organic matter would accumu-
late. Each of these habitats will have different dynamics of material processing.
(Photo copyright S.T.A. Pickett.)
