The Diversity of Burrowing Benthic Invertebrates and their Impact on Phosphorus Dynamics in Agricultural Drainage Ditches
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Agriculture remains the most widespread cause of impairment of freshwater habitats, but farm lands with artificial drainage structures such as ditches have specific locations where natural physical and biogeochemical processes can be used to reduce nutrients delivered to local watersheds. Agricultural drainage ditches can also be sources of biodiversity, serving as patches of uncropped aquatic habitat that may provide a significant amount of diversity to agricultural landscapes. Macroinvertebrate communities play important roles in nutrient cycling in natural aquatic ecosystems, but to this date no information exists on the role of invertebrate communities in biogeochemical processes occurring in ditches. The overall goal of my dissertation was to determine the structure of the aquatic macroinvertebrate community of agricultural drainage ditches, and to determine the functions these species play in nutrient cycling.
First, I performed a broad survey of aquatic macroinvertebrates in drainage ditches and related the community composition to environmental conditions. Ditches support different communities of macroinvertebrates, and community composition is correlated with physical habitat characteristics such as flow velocity (r2=0.58) and ditch size (r2=0.56), rather than water quality. I then measured the burrowing community of small (field) and large (collection) ditches over a year to determine how size class and seasonality affect taxonomic and functional group composition. I found small and large ditches support different taxa due to the intermittent water condition of small ditches, but both types of ditches support similar functional groups. There is limited diversity among functional feeding groups in ditches, but the majority of macroinvertebrates (101 of 140 taxa) are benthic taxa that may cause bioturbation of ditch sediments. I used microcosms to measure the effect that different burrowing species (Ilyodrilus templetoni, Limnodrilus hoffmeisteri, Crangonyx sp., Chironomus decorus S.G.) have on phosphorus dynamics between ditch sediments and water. Results show different species can increase (0.28 to 2.05 mg/L) or decrease (0.08 to 0.41 mg/L) soluble, reactive phosphorus concentrations in surface water, depending on the type of burrowing and environmental conditions. Different types of burrowers likely alter phosphorus dynamics through different mechanisms, and increasing diversity of burrowers could have non-additive effects on phosphorus uptake by ditch sediments.