The enrichment of ecosystems with nutrients (i.e, eutrophication) is known to be one of the primary causes of water impairment in the U.S. High loads of nitrogen and phosphorus from point sources (wastewater treatment plants) and distributed sources (agriculture and urban runoff) stimulate bacterial metabolism and algal growth, often causing episodes of hypoxia (low level of dissolved oxygen) due to the subsequent demand for oxygen as microorganisms decompose the excess of organic matter. The depleted oxygen levels in turn may lead to fish kills and a range of other effects reducing biodiversity.
Vanessa Garayburu-Caruso, a master's student at the University of New Mexico, in collaboration with her faculty advisor Dr. Ricardo González-Pinzón, is conducting a study about nutrient processing along the Jemez River-Rio Grande continuum which spans eight stream orders. Due to the complexity of the physical, chemical, and biological interactions between the solute and the soil, it is still challenging to describe accurately and predict solute transport and nutrients fate in freshwater ecosystems. Therefore, prior studies have been restricted to less complex headwater streams because they are more tractable and make up to 80 percent of the waterways on our planet. The goal of this study is in that sense rather more ambitious: 1) to characterize spatial and temporal differences in nutrient processing in riverbed sediments along the entire continuum (i.e., from first- to eighth-order streams); 2) to characterize uptake limitations by limiting nutrients; and 3) to understand how natural and anthropogenic influences affect microbial populations and metabolic activity along the Rio Grande continuum.
UNM Grad Student Vanessa Garayburu-Caruso. Photo courtesy of New Mexico Water Resources Research Institute.
The approach taken is to conduct column experiments to understand nutrient changes in shallow sediment-water interactions along representative sites of the Jemez River-Rio Grande continuum in New Mexico. One representative site for each stream order has been chosen, with the exception of the seventh-stream order where two sites were used to investigate the high incidence of anthropogenic influences originating from the Albuquerque metropolitan area (i.e., discharges from wastewater treatment plants and irrigation channels). For each site, plastic columns packed with three selected types of sediment were deployed in the stream for three months (enough time to allow natural biological communities to colonize the sediments). After the incubation period, the columns were taken to the lab to be tested. Several in-lab tracer experiments using nitrate and phosphates were conducted to further analyze nutrient uptake kinetics and metabolism.
Preliminary results indicate that headwater streams have better nutrient uptake capacity than higher stream orders. More generally, it is expected that these experiments will help determine how natural changes in bacterial communities and sediment composition occurring seasonally along the river continuum tend to define nutrient processing, as well as the relationship between biofilm formation and riverbed sediment diversity. This work will also help to support the development of a new generation of field and lab experiments to quantify nutrient processing in large rivers, where only about 10 percent of all reported tracer experiments have been conducted in the past. The research also should provide insight into how best to manage impaired reaches along the Rio Grande basin.
Garayburu-Caruso, recipient of a New Mexico Water Resources Research Institute Student Water Research Grant, will present final results of the project in a report that will be posted on the institute’s website. Click here to view the report.