Cattle exclusion using fencing reduces Escherichia coli (E. coli) level in stream sediment reservoirs in northeast Ireland

It is now recognised that freshwater sediments provide favourable conditions for the survival of faecal indicator bacteria and that these reservoirs can potentially recontaminate overlying water. This study investigated bed sediment contamination with Escherichia coli in streams with both restricted and unrestricted livestock access in a catchment with intensive cattle-based agriculture in northeast Ireland. Levels in a catchment dominated by low intensity agriculture (sheep farming) were also assessed. Sediment samples were collected at three key times in the annual management cycle for cattle in Ireland: April, when cattle return to fields after winter housing; July, the middle of the grazing season; and October, prior to the winter period when slurry spreading is prohibited by regulation and when cattle return to housing. The most contaminated bed sediments were in streams with unrestricted cattle access, with maximum values of 1.3 × 106 CFU g dry wt−1 in July. However, even in the stream with access restricted by fencing, sediment E. coli levels were significantly higher on all sampling occasions compared to the low intensity agriculture catchment. In April, in both years, there was no significant difference between E. coli levels in the stream with restricted cattle access and two streams with unrestricted access. In July, the most contaminated sites were found in streams with unrestricted access, however, in October there was again no significant difference between the fenced and unfenced streams. The main factors explaining differences between sites were sediment organic matter, which had a positive relationship to E. coli levels, and flow on the previous day, which had a curvilinear relationship with higher levels at both very low flow and very high flow. Fencing, or alternatively any evidence of recent presence of cattle, were also significant factors in the model. Overall, these results suggested that agricultural management practices such as the application of animal waste on fields could lead to contamination of stream bed sediments even when cattle access is restricted. Nonetheless, they also confirmed that fencing is a useful mitigation option to reduce faecal contamination of stream bed sediments.