From subsidies to stressors: Positively skewed ecological gradiencts alter biological responses to nutrients in streams

Authors: DEVILBISS, STEPHEN - Us Geological Survey (USGS); Taylor, Jason; HICKS, MATTHEW - Us Geological Survey (USGS)

Submitted to: Ecological Applications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/2/2024
Publication Date: 1/17/2025
Citation: Devilbiss, S.E., Taylor, J.M., Hicks, M.B. 2025. From subsidies to stressors: Positively skewed ecological gradiencts alter biological responses to nutrients in streams. Ecological Applications. 35:e3086. https://doi.org/10.1002/eap.3086.
DOI: https://doi.org/10.1002/eap.3086

Interpretive Summary: Rivers that have experienced significant change associated with human activities have less diverse aquatic communities and less variation in quality, with more rivers being degraded. This makes it difficult to develop ecological indicators based on aquatic animals such as fish or insects that are commonly used by agencies and stakeholders to monitor changes in stream health. Federal scientists overcame this obstacle by using new approaches to analyze biological data from altered streams. They found that by identifying organisms that were responsive to changes observed within agricultural watersheds, more sensitive indexes could be developed for the specific purpose of monitoring changes in stream health in degraded systems. This is important because agencies and stakeholders need simple efficient approaches to monitoring improvements in water quality and stream health in response to implementation of best management practices that reduce excess nutrients and other stressors in agricultural or other modified watersheds.

Technical Abstract: Subsidy-stress gradients offer a useful framework for understanding ecological responses to perturbation and may help inform ecological metrics in highly modified systems. For example, ecological baselines that are positively skewed in highly modified ecoregions can lead to typically tolerant organisms that exhibit a subsidy response (increased abundance in response to environmental stressors) shifting to a stress response (declining abundance at higher concentrations). As a result, observed biological tolerance in modified ecosystems may differ from less modified regions, creating significant challenges for detecting biological responses to restoration efforts. Using the agriculturally-dominated Mississippi Alluvial Plain (MAP) ecoregion in Mississippi, USA as a case study, we tested the hypothesis that macroinvertebrate taxa that typically display a subsidy response to nutrient enrichment in less modified ecoregions (i.e., nutrient-tolerance) shift to a stress response to increasing nutrients in highly modified watersheds with elevated baseline nutrient conditions (i.e., nutrient-intolerance). We used Threshold Indicator Taxa Analysis (TITAN) to identify macroinvertebrate taxa that are either tolerant or intolerant to elevated nutrient gradients in the MAP ecoregion. The abundance and diversity of MAP-specific intolerant taxa were either unresponsive or exhibited a subsidy response to increasing nutrients in less modified ecoregions with lower nutrient concentrations, but declined at higher concentrations, providing evidence for a stress response to elevated nutrients in the MAP. Additionally, MAP-specific tolerant and intolerant taxa richness responded to increased nutrients predictably and consistently across space and time. Thus, considering the subsidy-stress concept and establishing regionally-specific biological tolerance can provide meaningful interpretations of biological community data and inform long-term stream health in highly modified ecoregions. Lastly, we demonstrate the efficacy of this approach with sediment bacterial communities characterized with amplicon sequencing, which lack sufficient life history characteristics necessary for the development of multi-metric indices. Both macroinvertebrate and bacterial communities responded similarly to increasing nutrient concentrations, suggesting DNA-based approaches may provide an additional, efficient biological assessment tool for weight of evidence approaches when monitoring water quality improvements in highly modified watersheds. Redefining regional biological tolerance or using less traditional assemblages could be useful for tracking ecological responses to nutrient reduction efforts in highly modified watersheds.