360
Ecological Classification and Mapping of Aquatic Systems
riparian vegetation corresponded with valley morphology in the Sierra Nevada Mountains of California. Recent studies have identified features of
valley and channel morphology of montane streams
that correlate with lateral flow exchange, upwelling
of hyporheic waters, and the presence of summer
thermal refugia (Ebersole, 1994) and spawning of
bull trout in upwelling-influenced reaches (Baxter
et aI., in press).
24.3.4 Wetland and Estuarine Classification
Wetlands are important aquatic systems that experience seasonally high groundwater tables, which
in tum create anaerobic soil conditions that favor
the dominance of hydrophytic plant communities.
Groundwater hydraulics, soil properties, and hydrophytic plant community composition are currently used in the identification of jurisdictional
wetlands within the United States. However, hierarchical wetland classification systems that integrate these components are currently lacking in
most locales. The most inclusive classification of
wetlands for the United States is that of the USDI
Fish and Wildlife Service; this system's goal is to
create "boundaries on natural ecosystems for the
purpose of inventory, evaluation and management"
(Cowardin et aI., 1979). This classification system
represents a generalized hierarchical taxonomy that
has proved useful in a variety of regional scale assessment efforts; however, mapping of finer-level
taxa within this system is often difficult and expensive.
Estuaries occur along the continental shelf and
represent important mixing zones of fresh and marine waters. Classification of estuarine systems has
relied primarily on the integration of geomorphic,
hydrologic, and climatic criteria. The most simplistic estuarine classification is the Venice System, which uses salinity regime as its sole criterion
(Anon., 1959). This classification system has long
been considered to be overly simplistic, prompting
researchers to develop multivariate statistical classifications of salinity regimes in the description of
biologically relevant estuarine types (Bulger et aI.,
1993). Geomorphic classifications of estuaries
commonly emphasize the importance of ecological
processes, such as climatic inputs and runoff. These
factors, in tum, influence biological responses such
as migration and spawning. The estuarine classification of New South Wales Australia by Roy
(1984) is one useful example of such a geomorphic
process-based classification system.
Aquatic classifications of coastal-marine systems have only recently been developed and are
based in large part on hierarchical watershed classification methodologies (Lotspeich, 1980; Seaber
et aI., 1987). For example, Ray and Hayden (1993)
adopted hierarchical watershed classification concepts in the development of a watershed-seashed
classification that recognized five coastal-zone subdivisions: uplands, coastal plains, tidelands, continental shelf shoreface entrainment zone, and continental shelf offshore entrainment zone. The
correspondence between such classification systems and the regional diversity patterns of reef
fishes has been reviewed by Robins (1991).
24.4 Use of Terrestrial Ecological
Units in Description of
Aquatic Ecosystems
Terrestrial ecological units commonly delineate
geoclimatic environments that serve two primary
purposes in aquatic ecosystem description. They
are used (1) to group (classify) hydrologic units
(e.g., river basins and watersheds) into similar types
based on their structure and function and (2) to predict finer-scale aquatic patterns (e.g., channel units
and stream reaches) within hydrologic units. The
following is a description of how terrestrial ecological units can be used to characterize aquatic
ecosystems.
Hydrologic units can be effectively characterized
based on the geoclimatic settings in which they are
nested (Jensen et aI., 1997). The transport of water, sediment, and solutes is governed by geoclimatic factors such as elevation, relief, slope, landforms, soils, and climate. Because terrestrial
ecological units distinguish important hydrogeomorphic properties of a hydrologic unit, they are
very useful in grouping hydrologic units into types
with similar hydrologic responses. Understanding
the relations that exist between land and aquatic
systems is key to predicting their response to natural or human-induced disturbances. For example,
predicting effects of mass erosion on sediment delivery to streams and sediment routing downstream
to critical fish habitat commonly requires knowledge of the nature and interrelationship of the landtype associations, landtypes, stream networks, and
valley segments that occur in an area.
Terrestrial ecological unit settings such as subsections and landtype associations (Table 24.2) are
useful criteria for grouping hydrologic units at different scales. The mix of specific criteria used to
stratify hydrologic units, however, must be gov-
Ecological Classification and Mapping of Aquatic Systems
riparian vegetation corresponded with valley morphology in the Sierra Nevada Mountains of California. Recent studies have identified features of
valley and channel morphology of montane streams
that correlate with lateral flow exchange, upwelling
of hyporheic waters, and the presence of summer
thermal refugia (Ebersole, 1994) and spawning of
bull trout in upwelling-influenced reaches (Baxter
et aI., in press).
24.3.4 Wetland and Estuarine Classification
Wetlands are important aquatic systems that experience seasonally high groundwater tables, which
in tum create anaerobic soil conditions that favor
the dominance of hydrophytic plant communities.
Groundwater hydraulics, soil properties, and hydrophytic plant community composition are currently used in the identification of jurisdictional
wetlands within the United States. However, hierarchical wetland classification systems that integrate these components are currently lacking in
most locales. The most inclusive classification of
wetlands for the United States is that of the USDI
Fish and Wildlife Service; this system's goal is to
create "boundaries on natural ecosystems for the
purpose of inventory, evaluation and management"
(Cowardin et aI., 1979). This classification system
represents a generalized hierarchical taxonomy that
has proved useful in a variety of regional scale assessment efforts; however, mapping of finer-level
taxa within this system is often difficult and expensive.
Estuaries occur along the continental shelf and
represent important mixing zones of fresh and marine waters. Classification of estuarine systems has
relied primarily on the integration of geomorphic,
hydrologic, and climatic criteria. The most simplistic estuarine classification is the Venice System, which uses salinity regime as its sole criterion
(Anon., 1959). This classification system has long
been considered to be overly simplistic, prompting
researchers to develop multivariate statistical classifications of salinity regimes in the description of
biologically relevant estuarine types (Bulger et aI.,
1993). Geomorphic classifications of estuaries
commonly emphasize the importance of ecological
processes, such as climatic inputs and runoff. These
factors, in tum, influence biological responses such
as migration and spawning. The estuarine classification of New South Wales Australia by Roy
(1984) is one useful example of such a geomorphic
process-based classification system.
Aquatic classifications of coastal-marine systems have only recently been developed and are
based in large part on hierarchical watershed classification methodologies (Lotspeich, 1980; Seaber
et aI., 1987). For example, Ray and Hayden (1993)
adopted hierarchical watershed classification concepts in the development of a watershed-seashed
classification that recognized five coastal-zone subdivisions: uplands, coastal plains, tidelands, continental shelf shoreface entrainment zone, and continental shelf offshore entrainment zone. The
correspondence between such classification systems and the regional diversity patterns of reef
fishes has been reviewed by Robins (1991).
24.4 Use of Terrestrial Ecological
Units in Description of
Aquatic Ecosystems
Terrestrial ecological units commonly delineate
geoclimatic environments that serve two primary
purposes in aquatic ecosystem description. They
are used (1) to group (classify) hydrologic units
(e.g., river basins and watersheds) into similar types
based on their structure and function and (2) to predict finer-scale aquatic patterns (e.g., channel units
and stream reaches) within hydrologic units. The
following is a description of how terrestrial ecological units can be used to characterize aquatic
ecosystems.
Hydrologic units can be effectively characterized
based on the geoclimatic settings in which they are
nested (Jensen et aI., 1997). The transport of water, sediment, and solutes is governed by geoclimatic factors such as elevation, relief, slope, landforms, soils, and climate. Because terrestrial
ecological units distinguish important hydrogeomorphic properties of a hydrologic unit, they are
very useful in grouping hydrologic units into types
with similar hydrologic responses. Understanding
the relations that exist between land and aquatic
systems is key to predicting their response to natural or human-induced disturbances. For example,
predicting effects of mass erosion on sediment delivery to streams and sediment routing downstream
to critical fish habitat commonly requires knowledge of the nature and interrelationship of the landtype associations, landtypes, stream networks, and
valley segments that occur in an area.
Terrestrial ecological unit settings such as subsections and landtype associations (Table 24.2) are
useful criteria for grouping hydrologic units at different scales. The mix of specific criteria used to
stratify hydrologic units, however, must be gov-