Management Applications of Optical Remote Sensing in the Active River Channel - Fluvial Remote Sensing for Science and Management - page 32

or the presence of certain species can be damaging to
ecosystem health, water quality, and human structures.
Problems associated with SAV and algal blooms range
from eutrophication to acute toxicity associated with
blue green algae to fouling of engineering works. Remote
mapping of SAV and algae for management applications
should thus seek to provide more than a simple percent
cover map; ideally, it would also provide information on
species composition, biomass, and physiology.
Almost all SAV research has focused on lakes, deltas,
wetlands, estuaries and coastal waters. As with turbidity,
where research also has centered on large water bodies,
focusing on these environments has not significantly
advanced remote sensing of SAV in small stream systems,
where local-scale variations can confound the spectral
signal. Work from large water bodies thus provides a
guide to mapping SAV and algae, but should not be
transferred directly to smaller streams without additional
study and validation.
Silva et al. (2008) review remote sensing and plant
physiology issues to consider in mapping SAV. A central
issue in mapping SAV is the relatively strong absorption of
optical wavelengths by water. An identical plant therefore
looks different to the sensor when it is emergent, just
beneath the surface, or more deeply submerged. Similarly,
changes in plant structure, age, and reflectance confuse
the identification of plants and complicate estimation
of biomass.

Researchers thus use field measurements that incor-
porate plant- and location-specific variations to develop
regressions that use individual bands, band ratios, or
principal components to predict biomass (Silva et al.,
2008). Models of this sort have yielded R 2 values of 0.79
(Armstrong et al., 1993) to 0.85 (Zhang, 1998) for com-
parisons of measured and estimated SAV biomass in large
relatively stationary water bodies. Regression-based esti-
mates reach a plateau, however, beyond which biomass
continues to increase without a corresponding change
in the spectral reflectance - the signal becomes saturated
(Figure 2.4). In addition to biomass, remote sensing has
been used to map SAV community type, chlorophyll
concentration (Penuelas et al. 1993), photosynthetic effi-
ciency (Penuelas et al. 1997, 1993), and foliar chemical
composition (LaCapra et al. 1996).
Hyperspectral data are useful for separating SAV and
algal chlorophyll signals (Williams et al. 2003) and iden-
tifying invasive species (Underwood et al., 2006). Even
the additional spectral information, however, does not
entirely overcome the complex signals generated by
variable turbidity, water depths, and plant physiology
(Hestic et al., 2008). Regardless of sensor type or plat-
form, the variability in results among research projects
and the potential complexities in mapping SAV indicate
that - from a management perspective - this applica-
tion is still in a developmental rather than an opera-
tional phase.

Figure 2.4 Submerged aquatic vegetation (SAV), Browney Brook, County Durham England, and algae along a side channel of the
Tummel River, Scotland. The distinct spectra of chlorophyll relative to water and substrate enables mapping of general locations of
SAV and algae, although separation of species and mapping of parameters such as biomass is more problematic.