In 1997, the U.S. Geological Survey conducted a large study in the midwestern United States to determine how nutrients, herbicides, and organic enrichment from nonpoint agricultural sources impact algae and macroinvertebrates compared to natural factors such as riparian vegetation, soil-drainage characteristics, and hydrology. Researchers collected water chemistry (e.g., total and dissolved nutrients, dissolved herbicides, and suspended and organic carbon) and biological samples (e.g., phytoplankton, periphyton) during low-flow conditions in 70 streams in southern Minnesota, eastern Iowa, and western Illinois, an area with intensive agricultural land use. They also calculated stream productivity and respiration from DO concentrations and pH and measured water clarity.
Nutrient concentrations are high in the area, but researchers found that, during low-flow conditions, nitrate+nitrite-nitrogen corresponded to stream flow and total organic nitrogen concentrations were correlated with abundance of phytoplankton as indicated by chlorophyll a concentrations. Researchers also found that, although nutrient concentrations varied in relation to basin soil-drainage properties and riparian zone conditions, nutrient conditions were more closely related to algal abundance and productivity than to physical factors. An examination of algal species showed that periphyton communities were dominated by eutrophic microalgae and that filamentous red algae were abundant in streams with above-average water clarity, velocity, and riparian-tree density. Filamentous green algae, however, were relatively uncommon on submerged woody debris. The algal species present might account for the relatively low chlorophyll a values identified in the study. The research team concluded that algal-nutrient relations were more closely related to landscape characteristics (e.g., riparian zones, soil properties), hydrology (ground and surface water relations), and rainfall-runoff characteristics than to agricultural land use. Findings suggested that, for streams and rivers of the upper Midwest, abundance and composition of phytoplankton was recommended as reflecting trophic conditions (USEPA 2000b).
Reference:
USEPA. 2000b. Nutrient Criteria Technical Guidance Manual: Rivers and Streams. EPA-822-B-00-002. U.S. Environmental Protection Agency, Office of Water, Office of Science and Technology, Washington, DC. Accessed October 2016. https://nepis.epa.gov/Exe/ZyPDF.cgi/20003CVP.PDF?Dockey=20003CVP.PDF.