Browsing by Subject "Aquatic habitats"

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    Hierarchical Influences of Channel, Riparian And Landscape Features on Coarse Woody Debris in Low-gradient, Midwestern Streams
    (University of Minnesota Duluth, 1998) Johnson, Lucinda B; Host, George E; Richards, Carl
    Coarse woody debris is an important component of many small to medium streams, directly influencing stream geomorphology as well as many ecosystem properties and processes (reviewed by Harmon, et al. 1984, Gregory and Davis 1992, Gurnell, et al. 1995; Table 1). Woody debris exerts control over the structure of aquatic habitats by impeding flow, thereby increasing flow heterogeneity in the channel, influencing the pool-riffle sequence, erosional processes, channel dimensions, and deposition and retention of sediment and organic matter. Habitats created by CWD are varied, including plunge pools, backwaters and eddies, as well as the interstices of debris dams and individual logs (O’Connor 1991). These habitats are critical for fish as well as invertebrate species, providing flow and predation refugia for fish, oviposition and pupation sites, a feeding platform for invertebrates, and a substrate for biofilm production (Sedell et al. 1988, Shearer and Webster 1988)). The structure and dynamics of physical habitat in streams (Southwood 1977) and potential sources of colonizers (Gore 1982) regulate the composition and function of stream communities. Increased retention of particulate organic matter and production of FPOM from decomposing logs alters nutrient fluxes through the biota and subsequently influences the functional response of the fish and invertebrate communities within the stream (Minshall et al. 1982, Sedell et al. 1988). In response to changes in organic matter storage, functional responses of invertebrate communities, taxa abundance, and production have been reported to vary between erosional and depositional habitats, as well as between reaches with and without debris dams (Molles 1982, Smock et al. 1982, 1989), or logs (Wallace et al. 1995). In regions with unstable substrates, snags support a large proportion of the insect biomass and production (Benke, et al. 1984, Smock, et al. 1985). In many regions of the United States coarse woody debris was historically a prominent feature in streams, such that logjam s stretched for kilometers on both small and larger streams (Swanson, et al. 1976, Triska 1984, Maser and Sedell 1995). Debris removal was initiated to provide unobstructed waterways for navigation and transportation of harvested logs. In 1776 the U.S. Congress appropriated money to clear driftwood from streams and rivers to improve navigation, beginning with the Mississippi River. Removal of woody debris in rivers remains an active role of the U.S. Army Corps of Engineers (Harmon, et al. 1986), and is one of the primary roles of County Drain Commissioners (locally elected officials charged with creation and maintenance of an extensive network of drainage ditches) in the state of Michigan. Geomorphic features, to some degree, have regulated the original vegetation of the landscape (Grimm 1984, Host and Pregitzer 1992), as well as the historical and current land use/land cover patterns within the region. Many land management practices directly and indirectly influence the abundance of coarse woody debris (CWD) in streams. Alteration of the hydrologic regime resulting from stream channelization, wetland filling, or urbanization frequently results in increased bank erosion, one of the primary mechanisms of CWD input to streams in non-mountainous regions (Keller and Swanson 1979; Davis and Gregory 1994). Especially in small to medium-sized streams, forest management alters the species composition, number, and size distribution of trees in the upland, and thus dramatically modifies the potential source and input rates of CWD to streams (Bilby 1984, McDade et al. 1990, Gumell, et al. 1995, Fetherston et al. 1995). In agricultural and suburban regions potential sources of woody debris as well as the stream retention capacity are altered by management practices such as grazing, landscaping, riparian vegetation thinning or removal, dredging and channelization. The riparian zone and the land-water ecotone mediate inputs of sediment, nutrients, and particulate organic matter to streams, in addition to providing other important ecosystem functions (Gregory, et al. 1991). Coarse woody debris produced in the riparian zone by fire, disease, insect damage, ice/snow loading, and wind-throw represents the potential source for the stream (Keller and Swanson 1979). Processes such as mass soil wasting, bank undercutting and erosion, and flooding transport this material into the stream. In some systems beaver may be the primary vector transporting large volumes of CWD to the channel (Naiman, et al. 1986, Maser and Sedell 1995). The processes controlling CWD input to streams are influenced locally by tree species, stand age, soil stability, and human intervention (e.g., forest harvest and riparian zone clearing), and regionally by geology, climate, valley geomorphology and land use patterns. Since CWD fundamentally influences both the structure and function of many streams, identifying the myriad of factors that regulate its abundance and distribution is essential for understanding how many aspects of stream ecosystems are regulated. Many studies have examined the role of coarse woody debris in high- and low-gradient catchments (Table 1). However, few studies have attempted to quantify the relationship between landscape factors and the observed patterns in CWD abundance and distribution in low gradient systems, particularly in landscapes that are not dominated by forests. Landscape-scale factors such as land use patterns and surficial geology influence the abundance of woody debris found in stream channels (Ralph et al. 1994; Richards, et al. 1996) and undoubtedly also play a role in mediating the impact of disturbance events that influence the export of CWD and smaller organic matter fragments. By examining the factors influencing large woody debris at a range of spatial scales, the extent to which local and regional factors regulate the abundance and distribution of CWD can be discriminated. The goals of this paper are to: 1) characterize the abundance, size, and distribution of CWD in low gradient streams in developed landscapes; 2) quantify the relative influence of reach- and catchment-scale factors on the abundance and distribution of CWD.