Controls on the turnover of fluvial organic carbon in UK rivers – combining experimental, observational and modelling approaches.

Abstract:

In-stream processing of allochthonous dissolved organic carbon (DOC) and particulate organic carbon (POC) within the UK has been shown to be significant flux pathways in the terrestrial carbon cycle with both DOC and POC evolving into carbon dioxide (CO₂).

A mass balance approach based upon long term monitoring records was used to consider the loss of DOM and POM across UK watersheds. The total flux of carbon to UK rivers from the terrestrial biosphere was 21.8 tonnes C/km²/yr and the net catchment loss was 70%. Including the role of fluvial organic nitrogen means that for total nitrogen species UK rivers are gaining 9.6 tonnes N/km²/yr from the terrestrial biosphere but are losing 63% of this nitrogen by the tidal limit.

In a parallel study, in-situ experiments investigating rates of degradation in unfiltered surface water from a headwater, peat-dominated stream. Experiments were conducted on unfiltered samples (DOM and POM could be considered); on fresh samples (experiments were started stream-side); and over 70 hours (similar to in-stream residence time of the UK). The study found that the DOC concentration of samples in the daylight declined by 64%, compared with 6% decline for the samples kept in the dark: the POC loss in the light was 13%. The organic matter was characterised using elemental analysis (CHNOP); uv-vis spectroscopy, pyrolysis GC-MS; thermos-gravimetric analysis; bomb calorimetry and solid-state ¹³C nmr. Initial rate kinetics in the light were as high as 3^rd order, but the study could show that no single rate law could describe the whole diurnal degradation cycle.

Developing a physically-based set of rate laws for the turnover of DOM and POM over the appropriate timescales showed that the derived set of rate laws was able to explain experimental data with a 13% MAPE based on turnover in three types of organic matter (particulate, labile dissolved, refractory dissolved) although the order and rate of reactions did change between sets of observations. The modelling suggests that activation energies are low and that processes are not sensitive to temperature change. Application of the modelling scheme to organic matter turnover in an English river, showed that annual removal of total organic carbon (TOC) was equivalent to a total in-stream loss rate of between 58% over a median in-stream residence time of 35 hours.