Net daily metabolism in agricultural drainage ditches

Over one-third of farmland in the North Central Region of the United States has been drained to create conditions suitable for crop and livestock production. In Michigan, up to 80% of stream miles are headwater streams which, in farming communities, are converted into agricultural drainage ditches with a conventional trapezoidal shape. Recently, two-stage ditches have attracted increasing attention as a means of maintaining the ecological function of headwater streams while still satisfying the need for arable land for farming. To evaluate the degree to which these modified headwater streams function as natural stream ecosystems, we compared net daily metabolism in trapezoidal, two-stage, and natural (reference) streams. Results from diel oxygen curves indicate a dramatic difference between two-stage ditches and trapezoidal or natural streams. Rates of both gross primary production and community respiration were very high in two-stage ditches. Respiration rates exceeded gross primary productivity, indicating negative net daily metabolism. Oxygen flux was minimal in trapezoidal ditches and natural streams, and gross primary productivity nearly equaled respiration in most cases. These results may aid in watershed management in areas of intensive agriculture by providing insight into the ecological significance and biological potential of agricultural drainage ditches.