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AQUATIC HABITAT CLASSIFICATION ON THE ST. CROIX NATIONAL SCENIC RIVERWAY
Haibo Wan, Department of Fisheries, Wildlife and Conservation Biology, University of Minnesota
This project designed a classification system of the aquatic habitats within the St. Croix National Scenic Riverway. The system can serve as the framework for future monitoring and management of the Riverway.
We developed the classification at two levels: segment and reach. Segments are approximately 15+ km (10+ miles) in distance. Reaches are nested within segments and are approximately 1.5+ km (1+ mile). Segments were demarcated using physical properties, the most notable of which were tributary outlets and channel slopes; they also incorporated information such as ecoregion, land use/land cover, riparian vegetation, and channel substrate. Segments integrate environmental impacts on the aquatic communities through hydrologic processes and sediment dynamics that occur at three spatial scales: basin, riparian zone, and channel. Reaches were demarcated on the basis of channel substrate composition (i.e., frequency of different particle sizes, including boulders, cobbles, gravels, sands, silt, and clay). We took videos of the substrate at the thalweg every 400 m along the Riverway during summer 2004. Proximal sites with similar substrate composition were grouped into reaches. We selected substrate as the principal classification variable for two reasons: (1) diversity and abundance of benthos are related to size and heterogeneity of substrate materials and (2) fine materials (i.e., sand, silt, clay) are a part of the sediment dynamics within the entire basin and could indicate land use change. Also, loading from sediment and nutrients is currently the primary management concern in the St. Croix Basin.
We tested that physical classification framework with mussel community data (from Doolittle 1988), using TWINSPAN cluster analysis. Five clusters were identified; all five are spatially consistent with our segments, suggesting that segment is an effective unit for mussel resource management. No mussel community pattern was found at a scale finer than segment. Mussels are substrate selective; some species require coarse substrate (i.e., boulder, cobble, pebble, gravel). Almost all sites with fine substrate (i.e., sand, silt, clay, muck, mud) had the lowest mussel species richness (<3 species) and abundance (<5 individuals). It is apparent that siltation, a common result of aggressive land use, has the potential to make significant, negative changes in the mussel community on the Riverway, but those changes would be more likely in some reaches than others.
Wan H, Perry J, Ferrin R, Lafrancois B. Aquatic habitat classification on the St. Croix National Scenic Riverway (draft). St. Croix Falls (WI): National Park Service, St. Croix National Scenic Riverway.