Water quality, habitat, and fish assemblage relationships in middle-order agriculture and forest streams of the Mississippi Alluvial Plain

Agriculture has greatly influenced water quality, habitats, and fish assemblages in streams of the Mississippi Alluvial Plain (MAP) ecoregion. However, MAP streams have historically been understudied compared to streams in other agricultural regions of the USA. In this study, water quality, habitat, and fish assemblage composition were assessed seasonally (spring, summer, and fall) in eight representative MAP streams located across three U.S. states. The study design included four streams containing highly agricultural watersheds (herein termed “agriculture” streams) and four streams containing mostly forested watersheds (herein termed “forest” streams), which were intended to represent reference conditions for MAP streams. In general, forest streams contained significantly better instream and riparian habitats than agriculture streams (P = 0.010–0.040) whereas agriculture streams contained significantly greater levels of primary nutrients (P < 0.001–0.010). Differences between agriculture and forest streams with respect to other physical and chemical variables were intermittent and season dependent. Fish assemblages in agriculture and forest streams were structured primarily along an environmental gradient reflecting instream habitat conditions, water nutrient concentrations, and benthic chlorophyll-a production. Structurally, fish assemblages in both stream types contained many regionally common species, though some species appeared to exhibit affinities for a particular stream type. Functionally, fish assemblages in agriculture streams contained more tolerant species, more omnivores, and fewer insectivores compared to forest stream assemblages, which were nearly all insectivores. Overall, one-third of the fish specimens collected in forest streams classified as intolerant species. Our results suggested that stream water quality, habitat, and fish assemblages differed between agriculture and forest streams in the MAP, with fish assemblages exhibiting both structural and functional differences. Results were consistent with a larger body of literature from smaller, headwater streams whereby land-use changes (e.g., row-crop agriculture) impacted the physical, chemical, and biological characteristics of stream ecosystems. Results further highlight the importance of land use management and its effects on habitat diversity in stream ecosystems, and that protecting the few remaining undisturbed or less-disturbed streams should be a priority.