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Diatom Responses to Watershed Development and Potential Moderating Effects of Near-Stream Forest and Wetland Cover
SMUCKER, N., N. E. DETENBECK, AND A. C. MORRISON. Diatom Responses to Watershed Development and Potential Moderating Effects of Near-Stream Forest and Wetland Cover. Freshwater Science. North American Benthological Society, Lawrence, KS, 32(1):230-249, (2013).
Urbanization increasingly affects the biology, habitat, and ecosystem processes of streams and rivers in negative ways, primarily due to stormwater runoff, failing infrastructure, and impervious surfaces. Improving “symptoms of the urban stream syndrome” requires solutions at the catchment scale, but needs further documentation of biological responses to impervious cover (IC) and management strategies. Despite being important to nutrient cycling and energy flow as the foundation of primary production in streams, diatom responses to watershed land use, especially impervious cover, and management practices have been greatly understudied. We characterized relationships of diatom species, communities, and metrics with the percent IC in upstream watersheds and percent green infrastructure (GI) in 120 m riparian buffers on each side of streams. Metrics and community structure had threshold relationships with increasing % IC, with sensitive taxa declining at <1% IC, followed by decreases in low nutrient diatoms and increases in high nutrient, high conductivity, and motile diatoms from 1-5% IC, which culminated in considerably altered diatom community structure beyond 5% IC. Maintaining riparian buffers with >65%, and ideally >80%, GI could substantially reduce watershed % IC effects on diatom metrics and community structure. Our results emphasize the importance of implementing protective measures in the future, especially in watersheds near these IC thresholds, and the potential to reduce urban effects on streams by restoring and conserving near-stream green infrastructure. Using diatoms and other ecological endpoints in monitoring efforts will be crucial to future assessments because the effectiveness of management practices can be observed in the ecological responses despite the amount of impervious area being unlikely to decrease notably. Multiple analytical approaches quantified changes in metrics and diatom community structure that could be used to inform management practices that reduce IC effects and conserve sensitive species, reduce high nutrient diatoms, or protect other valued ecological attributes deemed important by stakeholders.
Watershed development alters hydrology and delivers anthropogenic stressors to streams via pathways affected by impervious cover. We characterized relationships of diatom communities and metrics with upstream watershed % impervious cover (IC) and with riparian % forest and wetland cover in 120-m buffers along each side of upstream networks. Threshold Indicator Taxa ANalysis (TITAN) identified potential threshold responses of diatom communities at 0.6 and 2.9% IC. Boosted regression trees (BRTs) indicated potential thresholds between 0.7 and 4.5% IC at which relative abundances of lownutrient diatoms decreased and those of high-nutrient, prostrate, and motile diatoms increased. These individual thresholds indicated that multiple stressors or magnitudes of stressors related to increasing watershed % IC differentially affected relative abundances of taxa, and these differential effects probably contributed to a more gradual, but still substantial, change in overall community structure. BRTs showed that near-stream buffers with .65% and ideally .80% forest and wetland cover were associated with a 13 to 34% reduction in the effects of watershed % IC on diatom metrics and community structure and with a 61 to 68% reduction in the effects of watershed % pasture on motile and high-P diatom relative abundances. Watershed % IC and riparian % forest and wetland cover probably affect hydrologic, nutrient, and sediment regimes, which then affect diatom community physiognomy and taxa sensitive to nutrients and conductivity. Our results emphasize the importance of implementing mindful development and protective measures, especially in watersheds near watershed % IC thresholds. Effects of development potentially could be reduced by restoring and conserving near-stream forests and wetlands, but management and restoration strategies that extend beyond near-stream buffers are needed.