31
2.4.2 Conceptual Models of Ecological Process and Form
As with physical conceptual models, incorporation of ecological models can
enhance river management by explicitly recognizing the spatial and temporal patterns within river ecosystems and the processes that drive these patterns. Management
incorporating the flood-pulse model, for example, recognizes that periodic inundation of the floodplain is vital to maintaining fish and other river organisms. This
section briefly reviews some of the major conceptual models of river ecosystems.
The most widely used ecological conceptual models of rivers emphasize one of
three characteristics: longitudinal patterns of process and form; lateral connectivity
within river corridors; and changes through time. The river continuum concept
(RCC) first described longitudinal patterns (Vannote et al. 1980). The RCC posits
progressive downstream changes in the relative importance of primary production
versus respiration and in the structure and function of aquatic communities
(Fig. 2.7). Ward and Stanford (1983, 1995) describe a variation in these downstream
trends associated with the presence of dams and reservoirs that reset the downstream patterns, as described in the serial discontinuity model. Other studies emphasize hierarchical patch dynamics, which characterizes river corridors as consisting
of relatively homogeneous patches from the scale of microhabitat up to channel
reaches, with distinct changes in process and form between patches (Pringle et al.
1988; Poole 2002; Thorp et al. 2006).
Longitudinal patterns of process can also be described in the context of nutrient
spiraling. Nutrients such as nitrogen and phosphorus are displaced downstream as
they complete a cycle (Webster and Patten 1979) through the generalized compartments of water, particulates, and consumers (Newbold et al. 1981). Spiraling length
refers to the downstream distance required for one complete cycle or, for organic
carbon, as the distance between its entry into the river corridor and its oxidation
(Fig. 2.8) (Elwood et al. 1980). Spiraling length reflects the utilization of nutrients
relative to the available supply, as well as physical characteristics of the river corridor, such as the ability to at least temporarily retain solutes and particulates in
areas of reduced transport capacity (Fisher et al. 1998; Battin et al. 2008; Baker
et al. 2012). Short spiraling lengths reflect high rates of material cycling, but disturbance can cause the spiraling length to increase (Fisher et al. 1998).
Lateral connectivity within river corridors is the focus of the flood-pulse model
(Junk et al. 1989), which describes the ecological influence of the seasonal flood
pulse on large floodplain rivers such as the Amazon (Fig. 2.9). Water, sediment,
nutrients, and organisms move from the channel onto the floodplain during peak
flood flow and then return to the main channel and secondary channels during the
receding limb and base flow. This repeated movement enhances nutrient availability, as well as habitat and biodiversity within the channel and the floodplain. A
subsequent iteration of this model emphasizes flow pulses, which are smaller-scale
fluctuations in discharge that change the extent of flow and standing water within a
braided or anabranching channel segment, as well as flow levels along the margins
2.4 Conceptual Models
2.4.2 Conceptual Models of Ecological Process and Form
As with physical conceptual models, incorporation of ecological models can
enhance river management by explicitly recognizing the spatial and temporal patterns within river ecosystems and the processes that drive these patterns. Management
incorporating the flood-pulse model, for example, recognizes that periodic inundation of the floodplain is vital to maintaining fish and other river organisms. This
section briefly reviews some of the major conceptual models of river ecosystems.
The most widely used ecological conceptual models of rivers emphasize one of
three characteristics: longitudinal patterns of process and form; lateral connectivity
within river corridors; and changes through time. The river continuum concept
(RCC) first described longitudinal patterns (Vannote et al. 1980). The RCC posits
progressive downstream changes in the relative importance of primary production
versus respiration and in the structure and function of aquatic communities
(Fig. 2.7). Ward and Stanford (1983, 1995) describe a variation in these downstream
trends associated with the presence of dams and reservoirs that reset the downstream patterns, as described in the serial discontinuity model. Other studies emphasize hierarchical patch dynamics, which characterizes river corridors as consisting
of relatively homogeneous patches from the scale of microhabitat up to channel
reaches, with distinct changes in process and form between patches (Pringle et al.
1988; Poole 2002; Thorp et al. 2006).
Longitudinal patterns of process can also be described in the context of nutrient
spiraling. Nutrients such as nitrogen and phosphorus are displaced downstream as
they complete a cycle (Webster and Patten 1979) through the generalized compartments of water, particulates, and consumers (Newbold et al. 1981). Spiraling length
refers to the downstream distance required for one complete cycle or, for organic
carbon, as the distance between its entry into the river corridor and its oxidation
(Fig. 2.8) (Elwood et al. 1980). Spiraling length reflects the utilization of nutrients
relative to the available supply, as well as physical characteristics of the river corridor, such as the ability to at least temporarily retain solutes and particulates in
areas of reduced transport capacity (Fisher et al. 1998; Battin et al. 2008; Baker
et al. 2012). Short spiraling lengths reflect high rates of material cycling, but disturbance can cause the spiraling length to increase (Fisher et al. 1998).
Lateral connectivity within river corridors is the focus of the flood-pulse model
(Junk et al. 1989), which describes the ecological influence of the seasonal flood
pulse on large floodplain rivers such as the Amazon (Fig. 2.9). Water, sediment,
nutrients, and organisms move from the channel onto the floodplain during peak
flood flow and then return to the main channel and secondary channels during the
receding limb and base flow. This repeated movement enhances nutrient availability, as well as habitat and biodiversity within the channel and the floodplain. A
subsequent iteration of this model emphasizes flow pulses, which are smaller-scale
fluctuations in discharge that change the extent of flow and standing water within a
braided or anabranching channel segment, as well as flow levels along the margins
2.4 Conceptual Models