Quantifying the hidden biodiversity, conservation value and effectiveness of fine sediment detention ponds in agri-environment schemes

Excessive fine sediment (typically particles <2mm) is one of the principal reasons for the failure of waterbodies to achieve good ecological status under the EU Water Framework Directive with widely acknowledged ecological effects for the entire trophic food web (Mathers et al., 2017). Although the input, transport, and storage of fine sediment is a natural component of river function, anthropogenic modifications have resulted in contemporary inputs far exceeding historic levels (Collins and Zhang, 2016). To combat this, a number of UK initiatives have been implemented to reduce fine sediment inputs to waterbodies including the English Catchment Sensitive Farming Delivery Initiative (£29M invested) and the 'Interreg project source to tap' in Northern Ireland (£4.9M). More recently, the Agricultural bill in 2020, will reward farmers and land managers in England for 'public goods' such as enhancing biodiversity, improving water quality or reducing flooding under the Environmental Land Management Scheme (ELM: Gov.UK, 2020). One means of tackling riverine fine sediment whilst also enhancing biodiversity is the installation of sediment traps and buffer ponds (e.g. see Catchment Based Approach, 2021).

Despite their wide occurrence in agri-environment schemes, little is known about the contribution of sediment mitigation methods to landscape biodiversity or their efficacy for reducing fine sediment pollution for riverine health. Some research has suggested that small water bodies including artificial and unique lentic waterbodies (Biggs et al., 2017) may support high numbers of aquatic organisms in addition to those with high conservation value (Cerini et al., 2020). This project will work with leading stakeholders and project partners engaged in agricultural management and monitoring to provide essential information that will feed directly into policy and real-world environmental management. The project will seek to: i) quantify the current uptake and distribution of the various aquatic-based sediment mitigation methods; ii) quantify the biodiversity and conservation value of sediment retention ponds; and iii) assess the effectiveness of sediment retention methods in mitigating fine sediment pressures for riverine systems.