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Title: Periphyton responses to nutrient and atrazine mixtures introduced through agricultural runoff

item Murdock, Justin
item Shields Jr, Fletcher
item Lizotte, Richard

Submitted to: Ecotoxicology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/3/2012
Publication Date: 2/15/2013
Citation: Murdock, J.N., Shields Jr, F.D., Lizotte Jr, R.E. 2013. Periphyton responses to nutrient and atrazine mixtures introduced through agricultural runoff. Ecotoxicology. 22(2):215-230.

Interpretive Summary: Agricultural runoff from fields simultaneously introduces many different pollutants into aquatic ecosystems. It is not clear how aquatic microorganisms respond to mixtures of pollutants, as many pollutants act antagonistically on organism development. When introduced individually, some pollutants (e.g., nutrients) can stimulate growth, and some (e.g., herbicides) can inhibit growth. We simulated a runoff event from an agricultural field into a wetland to assess how periphyton respond to nutrients and atrazine within a complex runoff mixture. We also conducted a laboratory study on periphyton from the wetland to isolate the individual and combined impacts of these two pollutants. Atrazine reduced periphyton growth and nutrients stimulated periphyton growth in laboratory chambers. Mixtures generally had an intermediate influence on periphyton, as one pollutant did not completely override the effect of the other. In the wetland, nutrients and atrazine contributed to periphyton changes, but other pollutants, specifically total suspended solids, were more influential. The direction of wetland periphyton response (i.e., stimulation or inhibition) was inconsistent with distance from the runoff input site and often changed between adjacent sampling sites. Pollutant relationships with periphyton were complex, with atrazine correlated with increased periphyton biomass and nutrients correlated with decreased biomass. Overall, periphyton biomass was more impacted than functional aspects such as primary productivity and nutrient uptake rates. The introduction of multiple pollutants in complex mixtures can change the influence of individual pollutants like nutrients and atrazine on periphyton. Knowing species composition and system hydrology, in addition to runoff chemical characteristics, can enable better predictions of periphyton responses to agricultural runoff.

Technical Abstract: Agricultural runoff often contains pollutants with potential antagonistic impacts on periphyton, such as nutrients and atrazine. The individual influence of these pollutants on periphyton has been extensively studied, but their impact when introduced in a more realistic scenario of multiple agricultural pollutants is less clear. We simulated a field-scale runoff event into a riverine wetland that mimicked pollutant composition typical of field runoff of the Mississippi River Alluvial Plain. Periphyton biomass and functional responses to two antagonistic pollutants (i.e., nutrients and atrazine) within the runoff were measured for two weeks along a 500 m section. Additionally, laboratory chamber assays were used to better identify potential wetland periphyton changes due to nutrients, atrazine, and their interactions within the runoff. Generally, nutrients stimulated, and atrazine reduced periphyton chlorophyll a (Chl a) in chambers. In the wetland, nutrient and atrazine relationships with periphyton were weaker than in chambers, and when found, were often opposite of those in chambers. Total nitrogen (TN) was inversely related to Chl a, and total phosphorus was inversely related to periphyton respiration (R) rates. Atrazine (10 – 20 µg L-1 in the wetland) had a positive relationship with periphyton ash-free dry mass (AFDM), and interacted with TN to weaken the negative relationship between TN and AFDM. Wetland periphyton biomass changes were better correlated to total suspended solids in the runoff than nutrients or atrazine. Periphyton function was resilient to runoff as periphyton gross primary production (GPP)/R ratios were not strongly impacted. However, whole-system GPP and R decreased over the two-week period, suggesting that although periphyton metabolism recovered quickly, other biological components contributing to whole-system metabolism can take longer to recover. The individual and combined impacts of nutrients and atrazine on periphyton in complex pollutant mixtures can vary substantially from their influence when introduced separately.