Environmental and fish assemblage contrasts in blackwater and clearwater streams

We compared 15 blackwater and clearwater streams in coastal drainages of Alabama by quantifying fish assemblages and associated environmental conditions across multiple seasons and characterizing potential relationships using multivariate statistical approaches. Clearwater stream channels tended to be deeper, to have sandier substrates, and to have higher dissolved oxygen concentrations than blackwater streams. In contrast, blackwater streams showed higher dissolved organic carbon levels and higher benthic macroinvertebrate densities. Discriminant function analysis based on environmental and invertebrate data resulted in 90% discrimination between the two stream types. Nonmetric multidimensional scaling (NMDS) of seasonal fish assemblages indicated separation by stream type, with no overlap in fish assemblages of blackwater and clearwater streams in any season, and 12 (39%) of the 31 species captured were indicative of stream type. A suite of environmental and biological variables representing invertebrate density, discharge, stream size, water chemistry, and land use was correlated with NMDS axes separating fish assemblages by stream type. Fish traits were somewhat distinctive between stream types, as there were unique lie-in-wait predators and short-lived species in each stream type. A fourth-corner analysis indicated potential linkages between fishes preferring woody substrate and stream type, further suggesting that stream type may explain fish functional structure. These results emphasize the need to consider these coastal systems as biotically unique ecosystem types in conservation plans as well as to focus more attention on better understanding the diversity patterns in headwater streams within the Coastal Plains region.