24.3 Review of Classification Systems for Aquatic Biophysical Environments
359
logic position, lake size, groundwater influence,
substrate type, human accessibility for species introductions and harvest, top-down influences of
fishes on trophic structure, and others) complicate
understanding of the mechanisms underlying lake
biotic organization. For many management purposes, however, it may be sufficient to recognize
and accurately map general patterns of covariation
that exist across the landscape, using physical and
geographical variables as correlative proxies of biotic community type. In this spirit, for Aquatic
ECOMAP, Maxwell et al. (1995) propose classifying lakes based on several simple categorical or
easily scaled primary criteria: geology (lake basin
genesis and surrounding landscape physiography),
hydrology (riverine linkage, groundwater linkage,
and water-stage regime), and lake morphometry
(surface area, mean depth, lake orientation and
shoreline complexity, and ratio of lake area to
catchment area).
In their lake classification scheme, Maxwell et
al. (1995) identify a suite of secondary attributes
that are highly correlated with the primary classification criteria, but also use additional information
in some circumstances. These attributes are not so
easily quantified and scaled, and most data are difficult or impossible to obtain without direct field
observations from each lake. They include lake
thermal regime, stratification pattern, hydrologic
retention time, water column color and clarity, water chemistry, trophic status, and composition of
the aquatic biota. In the Maxwell et al. approach,
after lakes are classified into groups or ordinated
according to these criteria, an individual lake can
be decomposed into depth-based zones (littoral,
pelagic, and profundal), and zones can be further
subdivided into lake sites, including special features such as deltas, bays, inlets, outlets, and spring
sources (Table 24.3).
Busch and Sly (1992) present a comprehensive
examination of approaches to lake habitat classification. Presumably based on the work of Busch and
Sly, The Nature Conservancy (1997) has proposed
a simplified lake habitat classification hierarchy directly analogous to their stream habitat types described previously. This scheme includes a macrohabitat type-alliance level that applies to whole
small lakes or basins within larger lakes and a lower
habitat unit type-association level defined by
smaller-scale features, such as substrate variation
and depth or thermal strata. These categories are
nested within larger ecoregional classifications. It
is somewhat unclear how this approach accounts
for other factors known to determine lake biota,
such as location within the drainage network, the
presence of keystone predators, or variation in lake
size or depth spanning several orders of magnitude.
Although many studies have demonstrated the
correspondence of lake chemical and physical parameters with the geological and environmental
context of catchment basins (e.g., Gibson et al.,
1994), relatively few published studies have yet
demonstrated coherence of these relationships with
lake biota across a region. Local assemblage structure of fishes has been shown to reflect the noneqUilibrium, biogeographic differences among
lakes in the suite of colonizing species, sifted
through the local habitat suitability template or filter that is imposed by within-lake biophysical conditions (Tonn et al., 1990; Kelso and Minns, 1996).
24.3.3 Groundwater and Hyporheic
Zone Classification
Aquatic ECOMAP (Maxwell et al., 1995) provides
at least a minimal framework for characterizing regional groundwater resources, but none of the classification systems for streams and stream habitats
published to date recognizes and incorporates the
hyporheic zone, that portion of the valley fill where
stream water and groundwater freely intermix
(Stanford and Ward, 1992, 1993; Brunke and Gonser,
1997; Ward, 1997, and citations therein). The hyporheic zone itself constitutes a unique and important habitat for a specialized fauna, and it plays a
poorly recognized but potentially critical role in
mediating the exchange of nutrients and carbon between stream and riparian or floodplain ecosystems. At least in temperate, nondesert environments, upwelling hyporheic waters are thermally
buffered compared to waters that have remained at
the surface. As a result of such thermal and nutrient fluxes, virtually anywhere hyporheic water returns to the surface it substantially increases aquatic
habitat diversity and supports localized hotspots of
aquatic productivity.
Hyporheic zones and other near-channel groundwater bodies need urgent attention in future research. However, enough is already known that
identification and protection of the ecosystem elements that create and maintain hyporheic exchange
should be among the highest priorities of any ecological assessment that encompasses areas of sandor coarser-grained alluvial or glacial valley fill materials. Within a region, the spatial extent and
influences of hyporheic processes are likely to be
correlated with valley and channel geomorphic descriptors that are already in common use. Harris
(1988) inferred that subsurface hydrology and thus
359
logic position, lake size, groundwater influence,
substrate type, human accessibility for species introductions and harvest, top-down influences of
fishes on trophic structure, and others) complicate
understanding of the mechanisms underlying lake
biotic organization. For many management purposes, however, it may be sufficient to recognize
and accurately map general patterns of covariation
that exist across the landscape, using physical and
geographical variables as correlative proxies of biotic community type. In this spirit, for Aquatic
ECOMAP, Maxwell et al. (1995) propose classifying lakes based on several simple categorical or
easily scaled primary criteria: geology (lake basin
genesis and surrounding landscape physiography),
hydrology (riverine linkage, groundwater linkage,
and water-stage regime), and lake morphometry
(surface area, mean depth, lake orientation and
shoreline complexity, and ratio of lake area to
catchment area).
In their lake classification scheme, Maxwell et
al. (1995) identify a suite of secondary attributes
that are highly correlated with the primary classification criteria, but also use additional information
in some circumstances. These attributes are not so
easily quantified and scaled, and most data are difficult or impossible to obtain without direct field
observations from each lake. They include lake
thermal regime, stratification pattern, hydrologic
retention time, water column color and clarity, water chemistry, trophic status, and composition of
the aquatic biota. In the Maxwell et al. approach,
after lakes are classified into groups or ordinated
according to these criteria, an individual lake can
be decomposed into depth-based zones (littoral,
pelagic, and profundal), and zones can be further
subdivided into lake sites, including special features such as deltas, bays, inlets, outlets, and spring
sources (Table 24.3).
Busch and Sly (1992) present a comprehensive
examination of approaches to lake habitat classification. Presumably based on the work of Busch and
Sly, The Nature Conservancy (1997) has proposed
a simplified lake habitat classification hierarchy directly analogous to their stream habitat types described previously. This scheme includes a macrohabitat type-alliance level that applies to whole
small lakes or basins within larger lakes and a lower
habitat unit type-association level defined by
smaller-scale features, such as substrate variation
and depth or thermal strata. These categories are
nested within larger ecoregional classifications. It
is somewhat unclear how this approach accounts
for other factors known to determine lake biota,
such as location within the drainage network, the
presence of keystone predators, or variation in lake
size or depth spanning several orders of magnitude.
Although many studies have demonstrated the
correspondence of lake chemical and physical parameters with the geological and environmental
context of catchment basins (e.g., Gibson et al.,
1994), relatively few published studies have yet
demonstrated coherence of these relationships with
lake biota across a region. Local assemblage structure of fishes has been shown to reflect the noneqUilibrium, biogeographic differences among
lakes in the suite of colonizing species, sifted
through the local habitat suitability template or filter that is imposed by within-lake biophysical conditions (Tonn et al., 1990; Kelso and Minns, 1996).
24.3.3 Groundwater and Hyporheic
Zone Classification
Aquatic ECOMAP (Maxwell et al., 1995) provides
at least a minimal framework for characterizing regional groundwater resources, but none of the classification systems for streams and stream habitats
published to date recognizes and incorporates the
hyporheic zone, that portion of the valley fill where
stream water and groundwater freely intermix
(Stanford and Ward, 1992, 1993; Brunke and Gonser,
1997; Ward, 1997, and citations therein). The hyporheic zone itself constitutes a unique and important habitat for a specialized fauna, and it plays a
poorly recognized but potentially critical role in
mediating the exchange of nutrients and carbon between stream and riparian or floodplain ecosystems. At least in temperate, nondesert environments, upwelling hyporheic waters are thermally
buffered compared to waters that have remained at
the surface. As a result of such thermal and nutrient fluxes, virtually anywhere hyporheic water returns to the surface it substantially increases aquatic
habitat diversity and supports localized hotspots of
aquatic productivity.
Hyporheic zones and other near-channel groundwater bodies need urgent attention in future research. However, enough is already known that
identification and protection of the ecosystem elements that create and maintain hyporheic exchange
should be among the highest priorities of any ecological assessment that encompasses areas of sandor coarser-grained alluvial or glacial valley fill materials. Within a region, the spatial extent and
influences of hyporheic processes are likely to be
correlated with valley and channel geomorphic descriptors that are already in common use. Harris
(1988) inferred that subsurface hydrology and thus