The estuarine mixing behaviour of peatland derived dissolved organic carbon and its relationship to chromophoric dissolved organic matter in two North Sea estuaries (U.K.)

Abstract

The mixing behaviour of dissolved organic carbon (DOC) was investigated in two U.K. estuaries with peatland derived river inputs (Tyne and Tweed), under varying discharge regimes and in different seasons. DOC removal (i.e. non-conservative) was common to both estuaries at low salinities, but DOC addition also occurred in the Tyne at higher salinities due to inputs from a large sewage treatment works and polluted urban rivers. DOC in freshwater endmembers was in the range 608.1–2326.5 μM (mean 1229.6 μM, n = 7) for the Tyne and was 226.1 and 668.2 μM on two occasions in the Tweed; the high DOC concentrations reflecting the peatland nature of the catchments and high river discharge. DOC removal in the Tyne estuary was in the range 0–58% ( n = 7) (one transect exhibits 0% removal as pollution sources of DOC dominate medium to high salinity samples so it was not possible to extrapolate a conservative mixing line back to low salinities). Highest percent removals were at highest river flows and correspondingly high turbidity and DOC concentrations in the Tyne estuary. In contrast, DOC removal in the Tweed estuary was consistently 6% ( n = 2), and appeared independent of river flow, turbidity and DOC concentration on the two transects conducted. Both estuaries exhibited a strong correlation between the chromophoric dissolved organic matter (CDOM) absorption coefficient at 350 nm ( a 350) and DOC which was seasonally robust; Tweed ( r 2 = 0.98, p < 0.001, n = 20), Tyne ( r 2 = 0.85, p < 0.001, n = 87) and Tyne without potentially anthropogenic or sedimentary derived DOM source samples ( r 2 = 0.93, p < 0.001, n = 74). The robust nature of these relationships implies that remotely sensed CDOM characteristics have the potential to improve the spatio-temporal resolution of estuarine DOC. This study highlights the variation in estuarine removal of peatland derived DOC between the two estuaries and in the Tyne estuary it shows how differences in river regime can result in significant changes in DOC removal at low salinities. Our findings demonstrate the need to conduct DOM sampling under varying seasons and hydrological regimes to accurately quantify the removal of terrigenous DOM in estuaries and its subsequent flux to the oceans.