River sediments provide a link between catchment pressures and ecological status in a mixed land use Scottish River system

This study evaluates water quality, suspended and bed sediment, ecological and catchment land use data for 13 catchments of the mixed land use River Dee, NE Scotland, where pollution point sources are limited. Samples were collected at key times of biological activity (early and late summers). Mean river water concentrations were smaller in main stem and upland sites and greater in tributaries where agricultural pressures were greater and were 2–41 μgPO4-P l−1, 8–58 μg total dissolved P l−1 and 1–6 mg l−1 of suspended particulate matter (SPM). SPM was 7–372 times enriched in biologically available P (BAP; determined using an FeO paper strip method) and 2–122 times in organic C relative to bed sediments. Ratios in river water concentrations of BAP attributed to the SPM (0.1–1.0 μgP l−1) to PO4-P had the greatest range at baseflow (0.01–0.80) with larger values for low land use intensity catchments. During May chlorophyll a concentrations were related to SPM BAP (p<0.001), but later in summer to PO4-P, and there was a corresponding change in the organic composition of SPM observed by IR spectroscopy. SPM concentrations and SPM BAP were better related to intensive grassland land use (p<0.001) than was PO4-P concentration (p<0.01) and also predicted abundances of filter feeding macroinvertebrates (p<0.001). Within this river system SPM quantity and composition proved to be an indicator of river biogeochemical functioning and requires further investigation as a potentially sensitive monitoring tool and to increase our understanding of chemical ecological links.