Can Diatom Assemblages Identify Important Stressor-Response Relationships Necessary to Establish Nutrient Management Goals for Mississippi Alluvial Plain Streams?

Author(s): Hicks, M.; Taylor, J.

Anthropogenic alterations to large river floodplains like the Mississippi Delta disrupt natural disturbance regimes that typically maintain the ecological integrity of lowland stream ecosystems. Anthropogenic activities can also cause shifts in water quality, such as conversion of forested floodplains to intensive agriculture that leads to potential excess nitrogen and phosphorus in runoff to streams. As a result, streams within the Delta are generally habitat limited, exposed to alterations of natural temporal and acute geomorphological and hydrologic regimes, and often experience widespread nutrient enrichment. All of these factors limit development of field-derived stressor-response relationships to establish nutrient reduction goals as one mitigation effort to improve ecological integrity. To address this limitation, in 2015, the U.S. Geological Survey sampled diatom assemblages from 25 streams that were located within the Mississippi Alluvial Plain (MAP) ecoregion in Mississippi but drained portions of upstream ecoregions with greater variation in land management and represented a measurable gradient in total phosphorus (TP) and total nitrogen (TN). We collected epidendric diatom assemblage samples from instream woody debris as this was the primary stable habitat for diatom colonization found within our study systems. Ordination analysis identified a gradient in species composition associated with increasing TP and decreasing dissolved oxygen. Additional variation in assemblage structure was correlated with increasing alkalinity. Our results indicate that diatom assemblages are responsive to phosphorus enrichment and show promise for deriving stressor-response relationships and identifying nutrient reduction targets within Delta streams. However, additional work is needed to better quantify stressor-response relationships. Specifically, using standardized artificial substrates for diatom collection could improve precision, increasing the range of field gradients by adding more sites at low and high nutrient concentrations, and conducting controlled experiments to verify field-derived results will improve future efforts to establish defensible stressor-response relationships for nutrients within Delta streams.

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