The biogeochemical reactivity of suspended particulate matter at nested sites in the Dee basin, NE Scotland.

Variation in the organic matter content associated with suspended particulate matter (SPM) is an often overlooked component of carbon cycling within freshwater riverine systems. The potential biogeochemical reactivity of particulate organic carbon (POC) that affect its interactions and fate, i.e. respired and lost to the atmosphere along river continua or ultimately exported to estuarine and oceanic pools was assessed. Eleven contrasting sites draining nested catchments (5-1837 km(2)) in the River Dee basin, NE Scotland were sampled during summer 2008 to evaluate spatio-temporal variations in quantity and quality (biogeochemical reactivity) of SPM during relatively low flow conditions. Mean SPM concentrations increased from 0.21 to 1.22 mg L(-1) between the uppermost and lowest mainstem sites. Individually, POC concentrations ranged from 0.08 to 0.55 mg L(-1) and accounted for ca. 3-15% of total aqueous organic carbon transported. The POC content was partitioned into autotrophic (2.78-73.0 mg C g(-1) SPM) and detrital (119-388 mg C g(-1) SPM) biomass carbon content. The particulate respired CO(2)-C as a % of the total carbon associated with SPM, measured by MicroResp™ over 18 h, varied in recalcitrance from 0.49% at peat-dominated sites to 3.20% at the lowermost mainstem site. Significant (p<0.05) relationships were observed between SPM biogeochemical reactivity measures (% respired CO(2)-C; chlorophyll α; bioavailable-phosphorus) and arable and improved grassland area, associated with increasing biological productivity downstream. Compositional characteristics and in-stream processing of SPM appear to be related to contributory land use pressures, that influence SPM characteristics and biogeochemistry (C:N:P stoichiometry) of its surrounding aqueous environment. As moorland influences declined, nutrient inputs from arable and improved grasslands increasingly affected the biogeochemical content and reactivity of both dissolved and particulate matter. This increases the potential for recycling of the organic matter that is either transported from upstream or entering further along the riverine continuum.