Dredging effects on selected nutrient concentrations and ecoenzymatic activity in two drainage ditch sediments in the lower Mississippi River Valley

Authors: Moore, Matthew; Locke, Martin; Jenkins, Michael; Steinriede, Robert; McChesney, Daniel

Submitted to: International Soil and Water Conservation Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/29/2017
Publication Date: 7/10/2017
Publication URL: http://handle.nal.usda.gov/10113/5735251
Citation: Moore, M.T., Locke, M.A., Jenkins, M., Steinriede Jr, R.W., McChesney, D.S. 2017. Dredging effects on selected nutrient concentrations and ecoenzymatic activity in two drainage ditch sediments in the lower Mississippi River Valley. International Soil and Water Conservation Research. 5:190-195. http://doi.org/10.1016/j.iswcr.2017.06.004.
DOI: https://doi.org/10.1016/j.iswcr.2017.06.004

Interpretive Summary: Over time, agricultural drainage ditches accumulate sediment and must be dredged in order to maintain sufficient water flow. When dredging occurs, sediment nutrient content and microbial activity, both essential to a ditch's natural mitigation capacity to buffer contaminants, changes. Little evidence is available on measurement of those changes and how long recovery to pre-dredge levels will take. Through a study of two ditches in the lower Mississippi River Valley, significant decreases in microbial activity were noted between pre- and post-dredge samples. This is of great concern, since microbial activity is one of the primary drivers in pesticide mitigation within drainage ditches. Although nutrient concentrations also decreased between pre- and post-dredge samples, due to erosion and runoff, nutrient concentrations quickly recovered in ditch sediments. A greater understanding of nutrient dynamics and microbial activity in ditches is necessary to help address water quality issues, such as those plaguing the Gulf of Mexico.

Technical Abstract: Agricultural drainage ditches are conduits between production acreage and receiving aquatic systems. Often overlooked for their mitigation capabilities, agricultural drainage ditches provide an important role for nutrient transformation via microbial metabolism. Variations in ecoenzyme activities have been used to elucidate microbial metabolism and resource demand of microbial communities to better understand the relationship between altered nutrient ratios and microbial activity in aquatic ecosystems. Two agricultural drainage ditches, one in the northeast portion of the Arkansas Delta and the other in the lower Mississippi Delta, were monitored for a year. Sediment samples were collected prior to each ditch being dredged (cleaned), and subsequent post-dredging samples occurred as soon as access was available. Seasonal samples were then collected throughout a year to examine effects of dredging on selected nutrient concentrations and ecoenzymatic activity recovery in drainage ditch sediments. Phosphorus concentrations in sediments after dredging decreased 33-66%, depending on ditch and phosphorus extraction methodology. Additionally, ecoenzymatic activity was significantly decreased in most sediment samples after dredging. Fluorescein diacetate hydrolysis activity, an estimate of total microbial activity, decreased 56-67% after dredging in one of the two ditches. Many sample sites also had significant phosphorus and ecoenzymatic activity differences between the post-dredge samples and the year-long follow-up samples. Results indicate microbial metabolism in dredged drainage ditches may take up to a year or more to recover to pre-dredged levels. Likewise, while sediment nutrient concentrations may be decreased through dredging and removal, runoff and erosion events over time tend to quickly replenish nutrient concentrations in replaced sediments. Understanding nutrient dynamics and microbial metabolism within agricultural drainage ditches is a crucial step toward addressing issues of nutrient enrichment in aquatic receiving systems, especially those contributing to the Gulf of Mexico.