As dull as ditch water? Elevating the status of the undervalued drainage ditch

Background/Question/Methods It is widely recognised that agriculture can have major implications for stream ecosystems (for a review see Allen, 2004), particularly through the input of nutrients and sediment. Due to the continuous nature of river systems, effective management must operate at catchment scales, rather than tributary/reach levels. In lowland agricultural areas, the upper headwaters consist of field drainage ditches, yet these are greatly understudied. It is important to fully understand the ecology of these habitats and the dynamic role they play at a catchment level. Small, low-discharge drainage ditches may provide population refugia for many lotic organisms, and can also act as hydraulically benign nursery habitats. Ditches can also provide important ecosystem services including sediment retention, flood control, water purification and nutrient cycling and may be important in the supply of energy downstream (Peterman et al., 2008). The perimeter to cross-sectional area ratio of a river system, and the corresponding relative importance of allochthonous inputs, declines with increasing distance downstream (Polis et al., 1997, Vannote et al., 1980), such that the effect of agricultural inputs is likely greater in ditches than in downstream reaches. In this study, headwater habitats and communities were characterised along a longitudinal gradient, from the upper drainage channels to downstream reaches of a lowland agricultural catchment, in the south of Ireland.

Results/Conclusions

Despite the high level of anthropogenic disturbance, we found that agricultural drainage ditches support a wide and varied biodiversity and could contribute significantly to the gamma diversity at a whole-catchment level. The life-history strategies and phenology of these communities were investigated. Small changes in the physico-chemical environment were accompanied by large variations in community assemblages over very short distances, resulting in distinct longitudinal faunal zonation. Preliminary results also suggest a threshold exists where the biological community changes from that of an agricultural ditch to that of a stream. This poses important questions regarding biomonitoring of river systems. Are the biomonitoring tools used to assess streams and rivers suitable to measure the health of the upper dendritic drainage ditches? The final results will be discussed in relation to developing biomonitoring tools and indicator species for ditch habitats.