Can riparian forests mediate impacts of urbanization on stream fish assemblages?
Roy, Allison Hunt
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Streamside riparian forests have been used to minimize degradation of water quality and biotic integrity in watersheds impacted by agricultural and silvicultural activities; however, little is known about the function of riparian forests in urban and suburban areas. This study tested the importance of riparian areas at (a) the local, reach scale; (b) the catchment scale; and (c) given varying levels of hydrologic and sediment disturbance in urbanizing streams in the Etowah River basin, Georgia, USA. In a study with paired open and forest canopies, riparian forests were not necessary for maintaining reach-scale habitat quality or biotic integrity (as measured by aquatic macroinvertebrates, salamanders, and fishes). Open reaches exhibited higher amounts of algae and herbivorous fishes, suggesting that openings in riparian canopies may provide patches of increased productivity leading to shifts in the trophic basis of production. At the catchment scale and based on data from 66 streams, fish species expected to be sensitive to disturbance were reduced in streams with lower % riparian forests along the entire upstream network. Catchmentwide development acted as a filter on fish assemblage integrity, with losses of sensitive species occurring at levels of riparian deforestation exceeding ca. 30 %. Local scale riparian deforestation secondarily affected tolerant species, which consistently had high abundances in streams with < ca. 60 % local riparian forest cover. I then examined relations between hydrologic alteration and fish assemblages, and asked whether the importance of riparian forests was contingent on levels of sediment and hydrologic disturbance. Increased frequency, magnitude, and duration of storm events and prolonged baseflow duration explained 20-66 % of the variation in fish assemblage richness and abundance, and these hydrologic disturbance variables were associated with increased percent impervious surface cover. Hydrologic alteration overwhelmed any influence of riparian forests on stream biota. However, for sensitive fish species, % riparian forest cover was predictive of richness and abundance if an interaction effect between riparian forests and level of sediment disturbance was included. Watershed management strategies must, at a minimum, simultaneously address hydrologic, sediment, and riparian disturbance in order to protect fish assemblage integrity.