Net Daily Metabolism in Agricultural Drainage Ditches

Abstract

Headwater streams in areas of intensive agriculture are frequently modified through surface and subsurface drainage to increase the transport of water to downstream water bodies and improve cropland in poorly drained areas. Stream modification and subsequent maintenance practices are beneficial to farmers but likely impair stream ecosystem function. Fluvial processes in these overly-widened headwater streams naturally form sediment benches that are stabilized through the establishment of grasses. Because these grassed benches have the potential to take up nutrients and decrease sediment export, they may improve the ecological functioning of modified headwater streams. To evaluate this, I determined whole stream metabolism for three types of headwater streams; reference streams, traditional trapezoidal-shaped drainage ditches, and drainage ditches that had developed grassed benches. I tested whether trapezoidal-shaped drainage ditches and benched drainage ditches functioned similarly to natural (reference) headwater streams in terms of net daily metabolism (oxygen production and consumption), and examined environmental and water quality variables that could influence the rates of net daily metabolism in each type of headwater stream. I found that rates of net daily metabolism were most similar in trapezoidal ditches and reference streams, but that trapezoidal ditches and benched ditches were more directly comparable in terms of overall ecosystem function due to the impacts of agricultural activities on these two systems. My study suggests that the benched system had a greater capacity for nutrient uptake and suspended sediment reduction and therefore provided more opportunities for improved water quality and ecological functioning in agricultural drainage ditches.