Citation: Ross, J.A.; Infante, D.M.; Cooper, A.R.; Whittier, J.B.; Daniel, W.M. Assessing Impacts of Human Stressors on Stream Fish Habitats across the Mississippi River Basin. Water 2023, 15, 2400. https:// doi.org/10.3390/w15132400 Academic Editors: Heiko L. Schoenfuss and Jan Kubeˇ cka Received: 7 April 2023 Revised: 2 June 2023 Accepted: 6 June 2023 Published: 29 June 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). water Article Assessing Impacts of Human Stressors on Stream Fish Habitats across the Mississippi River Basin Jared A. Ross 1, * , Dana M. Infante 1 , Arthur R. Cooper 1 , Joanna B. Whittier 2 and Wesley M. Daniel 3 1 Department of Fisheries and Wildlife, College of Agriculture & Natural Resources, Michigan State University, East Lansing, MI 48824, USA 2 School of Natural Resources, University of Missouri, Columbia, MO 65211, USA 3 U.S. Geological Survey, Wetland and Aquatic Research Center, Gainesville, FL 32653, USA * Correspondence: rossjare@msu.edu Abstract: Effective conservation of stream fishes and their habitats is complicated by the fact that human stressors alter the way in which natural factors such as stream size, catchment geology, and regional climate influence stream ecosystems. Consequently, efforts to assess the condition of stream fishes and their habitats must not only attempt to characterize the effects of human stressors but must account for the effects of natural influences as well. This study is an assessment of all stream fish habitats in the Mississippi River basin, USA. The basin supports over 400 stream fish species, drains a land area of 3.2 M km 2 , and includes a myriad of human stressors such as intensive agriculture, urbanization, nutrient loading, and habitat fragmentation by dams and road/stream crossings. To effectively characterize types and levels of human stressors specifically impacting the basin’s stream fish species, our assessment approach first accounted for the influence of natural landscape conditions on species abundances with multiple steps, including stratifying our analyses by region and stream size and quantitatively modeling the influences of natural factors on stream fishes. We next quantified individual fish species responses to explicit human stressors for different measures of land use, fragmentation, and water quality, including summaries of measures in local vs. catchment extents. Results showed that many species had negative threshold responses to human stressors and that impacts varied by species, by region, and by the spatial extents in which stressors were summarized. Our spatially explicit results indicated the degree of stream reach impairment for specific stressor categories, for individual species, and for entire assemblages, all of which are types of information that can aid decision makers in achieving specific conservation goals in the region. Keywords: streams; rivers; habitats; catchments; natural influences; human stressors; ecological assessments; water quality; stream fragmentation; land use 1. Introduction Human-induced stressors within watersheds have degraded stream habitats world- wide, contributing to declines in biodiversity and increased numbers of threatened fish species globally [1–3]. Differing types of landscape-scale stressors within catchments— including agriculture, urban landscapes, impervious surfaces, and dams and other barriers— can affect fishes by degrading stream habitats via multiple direct and indirect modes of habitat alteration. For example, buildings, roads, and other paved surfaces in urban environ- ments indirectly affect stream habitats by altering flow and thermal regimes [4]. Similarly, increased nutrient loading to streams can occur from widespread application of fertilizers on agricultural lands or through other sources of pollution from urban landscapes [5,6]. Further, receiving waterbodies and coastal areas downstream of catchments with heavy nutrient loading are increasingly becoming eutrophic, leading to degraded habitat and altered biological communities [7,8]. Widespread construction of road culverts and dams has substantially decreased both lateral and longitudinal connectivity throughout stream Water 2023, 15, 2400. https://doi.org/10.3390/w15132400 https://www.mdpi.com/journal/water