Longitudinal variation and response to anthropogenic stress in diatom assemblages of the Lower Mississippi River, USA

We evaluated diatom assemblages as indicators of anthropogenic stress from fi fty-four sites, distributed across 1,350 river kms of the Lower Mississippi River (LMR), as part of the National Rivers and Streams Assess- ment. We used multivariate analyses, as well as European- and regionally-derived stressor models, to examine vari- ation in assemblages relative to land-cover (e.g., agriculture) and water quality (e.g., ions, nutrients) stressors. Longitudinal changes in both periphytic and planktonic diatom assemblage structures, as species composition and assemblage diversity, were signifi cantly different for sites with greater watershed alterations, notably agriculture and development, and total ion (metals and anions) concentrations. The scale of land-cover delineation (e.g., sub- or entire watershed) was important in relation to diatom assemblage structure as land-cover classes were unrelated to assemblage structure at too fi ne or coarse a scale. European indices, from the OMNIDIA program, of trophic class and ecological condition classifi ed the LMR as eutrophic and polluted, but indices were not well correlated with land-cover or water quality variables. A diatom-based stressor response model developed for the Upper Mississippi River (UMR) basin corroborated the LMR as being highly eutrophic and infl uenced by agriculture and develop- ment. However, condition scores were constrained relative to the UMR suggesting few reaches of low anthropo- genic impact exist on the LMR. Our study showed that diatom assemblages are effective indicators of physico- chemical stress in a great river.