Spatial and temporal water quality in the River Esk in relation to freshwater pearl mussels

Abstract

Riverine systems provide networks of habitats, resources and biodiversity. Globally, riverine biodiversity is under threat due to a variety of human activities; diffuse pollution, particularly in agricultural catchments, raises challenges to river environments. This work addresses the water quality in the River Esk (North York Moors National Park) and its impact on biodiversity, namely the rare, declining population of freshwater pearl mussels (Margaritifera margaritifera). Water quality parameters were monitored both spatially and temporally and the drivers of water quality were investigated. Monthly sampling was undertaken at twenty sites within the Esk catchment. High-resolution monitoring was enabled by three autosamplers and two pressure transducers, which allowed for assessment of the water quality at both baseflow and stormflow. Anion and cation analysis were conducted on all samples and field-based characterisation furthered by use of a YSI multi-parameter probe.

Results revealed a number of concentration hotspots with values of nitrate that are thought unsuitable for freshwater pearl mussels. Other water quality variables were all within acceptable limits. Concentrations of nitrate in sub-catchments with smaller upstream areas proved to be more variable than in larger catchments. Land cover was found to be a key driver of concentration: high upstream percentage of improved pasture resulted in high nitrate concentration; high upstream percentage of moorland resulted in low nitrate concentration. During storm events, concentrations of key parameters were greater than limits suggested for pearl mussels (nitrate up to approximately 3.0 mg l-1 as opposed to limit of 1.0 mg l-1 proposed by Skinner et al. (2003)); this raised the fundamental question of exposure time. The process of connectivity was considered by the application of the risk-based hydrological model SCIMAP. This highlighted a number of areas that could adversely affect the pearl mussel population; these results will require further validation. Empirical work provided a foundation for future management recommendations. A case is made for the importance of expansion or addition of riparian buffer zones. This study demonstrates the importance of obtaining high-resolution data sets to understand habitat quality. The worth of these data is demonstrated in planning interventions in catchments to enable the Water Framework Directive (WFD) and UK Biodiversity Action Plan (BAP) standards to be met.