Riparian and Associated Habitat Characteristics Related to Nutrient Concentrations and Biological Responses of Small Streams in Selected Agricultural Areas, United States, 2003-04

Physical factors, including both in-stream and riparian habitat characteristics that limit biomass or otherwise regulate aquatic biological condition, have been identified by previous studies. However, linking the ecological significance of nutrient enrichment to habitat or landscape factors that could allow for improved management of streams has proved to be a challenge in many regions, including agricultural landscapes, where many ecological stressors are strong and the variability among watersheds typically is large. Riparian and associated habitat characteristics were sampled once during 2003–04 for an intensive ecological and nutrients study of small perennial streams in five contrasting agricultural landscapes across the United States to determine how biological communities and ecosystem processes respond to varying levels of nutrient enrichment. Nutrient concentrations were determined in stream water at two different sampling times per site and biological samples were collected once per site near the time of habitat characterization. Data for 141 sampling sites were compiled, representing five study areas, located in parts of the Delmarva Peninsula (Delaware and Maryland), Georgia, Indiana, Ohio, Nebraska, and Washington. This report examines the available data for riparian and associated habitat characteristics to address questions related to studyunit contrasts, spatial scale-related differences, multivariate correlation structure, and bivariate relations between selected habitat characteristics and either stream nutrient conditions or biological responses. Riparian and associated habitat characteristics were summarized and categorized into 22 groups of habitat variables, with 11 groups representing land-use and landcover characteristics and 11 groups representing other riparian or in-stream habitat characteristics. Principal components analysis was used to identify a reduced set of habitat variables that describe most of the variability among the sampled sites. The habitat characteristics sampled within the five study units were compared statistically. Bivariate correlations between riparian habitat variables and either nutrient-chemistry or biological-response variables were examined for all sites combined, and for sites within each study area. Nutrient concentrations were correlated with the extent of riparian cropland. For nitrogen species, these correlations were more frequently at the basin scale, whereas for phosphorus, they were about equally frequent at the segment and basin scales. Basin-level extents of riparian cropland and reachlevel bank vegetative cover were correlated strongly with both total nitrogen and dissolved inorganic nitrogen (DIN) among multiple study areas, reflecting the importance of agricultural land-management and conservation practices for reducing nitrogen delivery from near-stream sources. When sites lacking segment-level wetlands were excluded, the negative correlation of riparian wetland extent with DIN among 49 sites was strong at the reach and segment levels. Riparian wetland vegetation thus may be removing dissolved nutrients from soil water and shallow groundwater passing through riparian zones. Other habitat variables that correlated strongly with nitrogen and phosphorus species included suspended sediment, light availability, and antecedent water temperature. Chlorophyll concentrations in seston were positively correlated with phosphorus concentrations for all sites combined. Benthic chlorophyll was correlated strongly with nutrient concentrations in only the Delmarva study area and only in fine-grained habitats. Current velocity or hydraulic scour could explain correlation patterns for benthic chlorophyll among Georgia sites, whereas chlorophyll in seston was correlated with antecedent water temperature among Washington and Delmarva sites. The lack of any consistent correlation pattern between habitat characteristics and organic material density (ash-free dry mass) within study areas may indicate that the density of organic matter is not generally sensitive to nutrient enrichment in small agricultural streams. For all sites, and for the Nebraska, Delmarva, and Georgia subsets of sites, the reach-mean areal coverage of aquatic macrophytes and macroalgae was strongly related to channel shading. 2 Riparian and Associated Habitat Characteristics in Selected Agricultural Areas, United States, 2003–04 Data reduction techniques were applied to select a subset of 29 variables, representing 20 categories of habitat characteristics, for multivariate analysis. Factor analysis was used to identify and interpret three leading modes of variation (principal factors) in two data subsets—one for the Georgia sites and one for all other sites combined. The factor analysis for Georgia sites indicated that riparian land use and land cover (LULC) (wetland extent in particular) and channel shading correspond to dominant modes of variability in the habitat data set. The variables that best characterize variation in riparian habitat for the other four study areas included midchannel measures of canopy shading, riparian cropland extent in the 15-meter buffer and 150-meter buffer, and measures of the patchiness of woodland cover in the 15-meter buffer (patch length and gap frequency). LULC metrics calculated for riparian buffers, particularly at the segment scale, were more correlated with the principal modes of variation in the overall habitat data set than was LULC extent for the total basin drained by each site. Correlations of woodland extent within 15 to 50 meters of the channel (reachand segment-level data) with woodland extent in a series of longitudinal bands of the riparian buffer that were located at increasing distance from the channel showed decreased strength as the compared band shifted beyond the first 50 meters from the channel, becoming negligible for areas beyond 100 meters from the channel. For many of the studied agricultural streams, the riparian buffer includes a heterogeneous mix of riparian and upland land covers when the summarized buffer area extends more than about 50 to 100 meters from the streambank, depending upon basin (or stream) size. Comparisons between the extent of reachand segment-level median values of woodland and other cover types within the riparian buffer extending 50 meters from the stream suggest that the reach length used for this study generally is not long enough to accurately represent both the overall composition and patch structure that characterizes the riparian areas along small, agricultural streams. The mean extent of forest plus woody wetland ranged from 5.4 to 76 percent of the riparian buffer area. For the Georgia sites, where riparian woody wetlands were more extensive than for any other study area, canopy closure over the channel was greatest, whereas it was least for sites in Washington and Nebraska. To the extent that riparian woodland is the most important LULC type affecting algal-nutrient relations, correlations indicated that basin characteristics might be effective surrogate predictors of riparian effects at the drainage-network scale. But the results also indicated that basin-level cropland was not an accurate surrogate for riparian cropland extent. Introduction Effective stream management depends on a comprehensive understanding of the complex interactions among riparian and stream habitat, water chemistry, and biological communities. The importance of nutrient enrichment as a stressor on aquatic communities has been widely recognized (Mosisch and others, 2001; Dodds and others, 2002; Mulholland and others, 2004; Alexander and Smith, 2006; Scott and others, 2007; Munn and others, in press). Relations between algal biomass and nutrient concentrations in stream environments typically have been weak (Dodds and others, 2002; Munn and others, in press) because of the interaction of physical and biological factors. These interactions include direct and indirect effects of riparian habitat on aquatic biota, such as the direct effects of riparian woodland shading or the indirect effects of retaining eroded upland sediment by riparian ground cover. Naiman and others (1993) defined the riparian corridor as the area that includes the stream channel and adjacent overbank terrestrial zone, where vegetation is affected by a shallow water table and (or) regime of frequent flooding. Channel banks clearly are key components of riparian corridors, and bank habitat and functions are to some degree inseparable from the function of the larger riparian system (Florsheim and others, 2008). The factors governing biota-habitat relations include chemical and physical characteristics of the habitat. Riparian zone functions are related to stream chemistry through retention and cycling of nutrients and other contaminants (Florsheim and others, 2008). Some of the physical factors that also have commonly been identified as controlling algal biomass or biodiversity include light limitation from canopy shading (Mosisch and others, 2001; Kiffney and others, 2004) and turbidity (Munn and others, 1989), water temperature (Kilkus and others, 1975; Munn and others, 1989), and hydraulic disturbances (Powers, 1992; Biggs, 1995) including floods, fluvial erosion, and mass wasting of streambanks. Present-day understanding of biota-habitat relations in streams is based primarily on comparative studies that described statistical relations between habitat variables and measures of aquatic community structure or function (Hawkins and others, 1993; Kiffney and others, 2004) or, more recently, between habitat-related stressors and ecological condition (Van Sickle and others, 2006; Munn and others, in press). Results from comparison studies may be confounded if the relative importance of various habitat factors varies with habitat type; thus, it may be important to study the effects of individual habitat features (for example, cover or stream shading) while holding o