The effects of drought and nutrients on stream communities

Drought and nutrient pollution can affect the dynamics of stream
ecosystems in diverse ways. While the individual effects of both stressors
are broadly examined in the literature, we still know relatively little
about if and how these stressors interact. Here, we performed a mesocosm
experiment that explores the compounded effects of seasonal drought via
water withdrawals and nutrient pollution (1.0 mg/L of N and 0.1 mg/L of P)
on a subset of Ozark stream community fauna and ecosystem processes. We
observed biological responses to individual stressors as well as both
additive and antagonistic stressor interactions. We found that drying
negatively affected periphyton assemblages, macroinvertebrate
colonization, and leaf litter decomposition in shallow habitats. However,
in deep habitats, drought-based concentration effects caused trophic
cascades that released algal communities from grazing pressures; while
nutrient enrichment caused bottom-up cascades that influenced periphyton
variables and crayfish growth rates. Finally, the combined effects of
drought and nutrient enrichment interacted antagonistically to increase
survival in longear sunfish; and stressors acted synergistically on
grazers causing a trophic cascade that increased periphyton variables.
Because stressors can differentially impact biota—and that the same
stressor pairing can act both additively and antagonistically on different
portions of the community simultaneously—our broad understanding of
individual stressors might not adequately inform our knowledge of
multi-stressor systems.