Environmental and Fish Assemblage Contrasts in Blackwater and Clearwater Streams

Abstract

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.

Distribution and Characteristics of Blackwater Rivers and Streams of the Contiguous United States

“…Blackwater sites in the NPL and NAP did tend to have more fines (% Fines). Overall, these findings contradict expectations from prior studies which generally characterize blackwaters as being low-gradient and having a sandy bottom (e.g., Benke & Wallace, 2015;Colvin et al, 2020;Meyer, 1990). This lack of concordance with the literature likely results from sites in those studies being selected based on their unique nature whereas sites in our study were randomly selected from a pool of all possible sites in the CONUS, including smaller order streams.…”

“…Multiple factors likely contribute to this general lack of differences across physical habitat measures within ecoregions. An important first consideration is that the water influencing the prevailing chemistry at a site classified as blackwater is, in most cases, contributed to the wetted channel by off‐channel habitats (Benke & Wallace, 2015; Colvin et al., 2020; Flotemersch, 2023; Meyer, 1990; Smock & Gilinsky, 1992) and not the result of an autochthonous process within the wetted channel. The exception to tannin‐rich water being primarily a product of off‐channel habitats is where instream conditions contribute tannins to the stream (e.g., instream swamps; Todd et al., 2009).…”

“…We identified a subset of sites with Color ≥50 PCU and DOC ≥10 mg L −1 as likely blackwater sites. These two chemical analytes were selected because they are common attributes mentioned in the literature describing blackwaters in the United States (e.g., Benke & Wallace, 2015; Colvin et al., 2020; Flotemersch, 2023; Meyer, 1990; Smock & Gilinsky, 1992). The stated value DOC was selected based on ranges observed in the literature (e.g., DOC 4–74 mg L −1 ) for sites described as blackwaters (e.g., Benke & Parsons, 1990; Meyer, 1986; Smock & Gilinsky, 1992).…”

“…Those in the Atlantic coastal plains are probably the most well‐known and well‐studied. The relatively undisturbed condition of blackwater systems in southern parts of the coastal plains resulted in blackwater rivers and streams in this area being the focus of a flurry of stream ecology research efforts in the late 1970s that continues through to today (Benke & Wallace, 2015; Colvin et al., 2020; Meyer, 1990; Smock & Gilinsky, 1992). Considerable research has also been conducted in the northern extent of the Atlantic coastal plains to better understand and conserve blackwater habitats in the New Jersey Pinelands National Reserve (NJPC, 2022).…”

Unravelling the influence of landscape alteration from flow alteration on benthic macroinvertebrate assemblage response in the Delaware River Basin

“…In addition to the challenges of modelling flows on the coastal plain, these streams also offer challenges in measuring changes in lotic benthic macroinvertebrates assemblages (Colvin et al, 2020; Dail et al, 2013; Davis et al, 2003; USEPA, 1997). Coastal plain streams represent environments that are substantially different from streams in other ecoregions.…”