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Headwater streams comprise the majority of the stream network, providing important ecological functions to the downstream network. Although we are beginning to understand how network structure may influence fish, our understanding of how it influences benthic macroinvertebrate dispersal and population connectivity is limited. We also know little about how these patterns and processes may be disrupted as a result of human-driven landscape change such as stream barriers to movement and creation of artificial habitats such as stormwater and farm ponds. In this study, I investigated the effect of stream network position, stream size, and local habitat on benthic macroinvertebrates, and determined to what degree road crossings and impoundments may be degrading benthic macroinvertebrate and fish communities in headwater streams. These mechanisms were explored using Maryland Department of Natural Resources, (MDNR) Maryland Biological Stream Survey (MBSS) benthic macroinvertebrate, fish, and environmental data from first-order streams in the Piedmont region of Maryland. Using an Information Theoretic Approach (ITA), models were developed based on the hypothesized relationships between benthic macroinvertebrate and fish community structure and several network and anthropogenic impact variables. Based on my results, aquatic community structure was dependent on local habitat conditions and stream network structure. Both assemblages responded negatively to roads, which may suggest an isolation effect. These results also suggest that impoundments are acting as sources for benthic macroinvertebrates and fish, including non-native species.
Using a forested headwater stream system as a model, the effects of inter-annual variation in summer discharge regimes on aquatic insect communities were investigated. More specifically, the benthic invertebrate community response to the intensity, minimum discharges, frequency, duration and abruptness of summer low-flow events were examined. We hypothesized that intensification of summer low-flow events, both in duration and magnitude, have some negative impacts on benthic macroinvertebrate communities in riffles. Examples of negative impacts include reduction in their abundance and/or biodiversity. First, the abundance and functional trait data of the benthic macroinvertebrates in the three streams in the Malcolm Knapp Research Forest, British Columbia, Canada, were analyzed with respect to the low-flow events. Second, population models were built to simulate the potential responses of lotic aquatic insect communities to future climate change scenarios that differ in the rate of intensifications in extreme flow events: a low-flow event scenario within the current range versus 10% increase in intensity. The summer low-flow events were found to have a significant relationship with benthic macroinvertebrate communities through three-table ordinations of the empirical data. The community structure was correlated with a major ocean-atmosphere regime shift (Pacific Decadal Oscillation). The intensity and duration of low-flow events explained the observed shift in community structure favouring r-selected traits (e.g. short life cycle, high reproduction rate). The two low-flow severity scenarios showed the significant differential impacts on the aquatic insect community structures when individual populations were modeled according to their traits. Aquatic insects could be separated into three groups according to their sensitivities, measured by extinction rates, toward the two scenarios.
Stream fish communities have been extensively studied, but effects of stream drying on fish communities and fishes' ability to migrate warrants further investigation. Relationships among discontinuity of surface flow, fish communities and fish movements were investigated during 2004 in the Interior Highlands of Arkansas. Stream drying was measured along 2-km study sites within eight headwater streams draining 2,800 ha (+/- 9%) to quantify the amount of habitat available during the summer season. Fish were also sampled during June, August, and October in 30% of the wetted area of a 1-km study reach centered within each 2-km study site. Population metrics were assessed during each sample by three-pass electrofishing depletion. Movement of fish within the 1-km was measured by batch marking every fish in June by the 50 m section in which they were captured. Stream dryness reached as high as 84% in four streams that exhibited extensive dryness; whereas, four streams exhibited little or no dryness. Desiccated fish were observed in dry streams during September, illustrating that mortality is extensive in these ecosystems. Distributions of distances moved by recaptured fishes were weighted by the effort to capture each fish and termed "effort-weighted". These distributions differed for wet and dry streams; dry streams had a Gaussian distribution and wet streams had a bimodal distribution. Overall, downstream movement distances were significantly greater for wet streams and movements were generally shorter in dry streams. Unmarked fish recolonized previously dry sections of stream hundreds of meters from perennial pools after flow resumed in dry streams. Coefficients of variation for fish abundance and species richness were greater in dry streams than wet streams, although the means were similar. Percent of endemic species was greater in wet streams versus dry streams. Cyprinids were the dominant species and predators in dry streams; whereas, Centrarchids favored wet streams. Thus, seasonally discontinuous surface flow influences fish community structure and limits movement in aquatic ecosystems that exhibit seasonal dryness. Therefore, explicit measures of stream dryness in these ecosystems will further the understanding of the persistence of fish communities in steams with seasonally discontinuous surface flow.
Abstract: In-stream habitat structure and water chemistry have significant influence on the structure and composition of stream macroinvertebrate assemblages. Habitat at this local scale can be significantly affected by the geomorphology of a stream or region. Both in-stream habitat and geomorphology are, in turn, influenced by other factors operating at the landscape scale (e.g., land use, connectivity of habitat patches, etc.). It is unclear which of these three scales of habitat has the greatest influence over lotic assemblage structure. Anthropogenic disturbance to a stream ecosystem can occur at all three scales of habitat, and is particularly common in predominantly agricultural systems. The Sugar Creek watershed in northeastern Ohio represents several different types of anthropogenic disturbance, including dairy farming, crop production, urbanization, and industrialization. The South and Middle Forks of the Sugar Creek watershed, dominated by agriculture and a mix of agriculture and industry, respectively, were sampled in early summer 2005 for habitat and macroinvertebrates. Richness, evenness, diversity, familylevel biotic index, percent Diptera Chironomidae, and the number of macroinvertebrates were all similar across the drainages. The percent Ephemeroptera, Plecoptera, and Trichoptera was significantly larger in the Middle Fork than in the South Fork. There were no significant differences in habitat or macroinvertebrate assemblages between the two drainages overall. In-stream habitat structure and water chemistry explained 58.8% of the variation between sites among macroinvertebrate taxa. Geomorphology explained 10.4% and land use 9.4% of the variation. Shared variances between different scales of habitat did not explain substantial amounts of variation among macroinvertebrate taxa. These results have, however, identified several sites in the South Fork with good potential for Best Management Practice implementation and several sites in the Middle Fork for preservation.
Habitat fragmentation and loss threaten global biodiversity, but organism responses to changing habitat availability are mediated by structural properties of their habitats. In particular, organisms inhabiting dendritic landscapes with hierarchically arranged branches of habitat tend to have limited access to some patches even in the absence of fragmentation. Consequently, organisms inhabiting dendritic landscapes such as streams respond strongly to fragmentation. Using a combination of meta-analysis, field observations, and ecological network modeling I show that stream fishes respond to fragmentation in predictable ways. First, I addressed how dams and stream dewatering have created a mosaic of large river fragments throughout the Great Plains. Using a geographic information system and literature accounts of population status (i.e., stable, declining, extirpated) for eight "pelagic-spawning" fishes, I found stream fragment length predicted population status (ANOVA, F2, 21 = 30.14, P