Spatio-temporal variability in dissolved organic carbon quantity and quality in Scottish headwater streams

Several studies have identified increases in the export of dissolved organic carbon (DOC) across the Northern Hemisphere since the 1980s. The mechanisms associated with this upward trend remain the subject of much debate with reductions in atmospheric deposition of sulphur and nitrogen, and changes in land use proposed as plausible hypotheses. Despite considerable focus on trends in DOC quantity, few studies have also explored changes in the composition or quality of DOC over similar decadal timescales. To date, the focus has been on seasonal and event based compositional dynamics, despite the significant ecological and economic implications of long term shifts to greater contributions of aromatic compounds.

In this paper, we leverage a unique dataset of DOC concentration and absorbance records (Abs. 254 nm and 400 nm) collected from 43 riverine sites and loch outflows across Scotland (latitude: 55N - 57.5 N) between 1981 and 2016. The sites drain small-medium sized rural watersheds (area: ~0.1 km² - 85 km²) across a range of altitudes (median catchment elevation: 106 - 905 m a.s.l.), where land use consisted of predominately conifer forest or open moorland/ montane habitat.

Using non-linear mixed effect models, we identified consistent upward trends in DOC concentrations for most catchments. Breakpoints in the temporal trends in DOC concentration coincided with land cover changes, particularly shifts in forestry practice (i.e. planting - felling cycles). Trends in DOC quality were inferred from relationships between DOC and Abs 250 and exhibited greater inter-site variability than those observed for DOC concentrations. Both increases and decreases in aromatic carbon content of DOC were observed over time, potentially reflecting changes in atmospheric deposition and catchment land cover. In addition, our results reveal the importance of climate variability (wet and dry periods) for watershed hydrological processes and in turn DOC concentrations - superimposing seasonal and event scale variability on longer-term trends. This research highlights the need for sustained long-term monitoring programs to identify watershed-specific controls on water quality trends, particularly given the role the terrestrial and hydrological processes will play in modulating responses to future climatic extremes.