Seasonal benthic organic matter mineralisation measured by oxygen uptake and denitrification along a transect of the inner and outer River Thames estuary, UK

Seasonal measurements of organic matter mineralisation by oxygen uptake and denitrification were carried out from July 1996 to March 1998 along a ∼200 km transect of the River Thames estuary, UK. There was a distinct gradient of decreasing rates of organic matter mineralisation seaward, which was related to the concentration of suspended solids and sedimentary organic carbon (C) at each site. There was clear seasonality and highest rates of oxygen uptake (10 056 μmol O2 m–2 h–1) at the muddy sites, but lower rates and non-temperature-dependent oxygen uptake at the sandier sites. Denitrification, both that driven by nitrate from the overlying water (Dw) and that coupled to nitrification in the sediment (Dn), followed a similar trend to oxygen uptake, from negligible rates of approximately 1 μmol N m–2 h–1 for both Dw and Dn at the furthest offshore site, Site 12, to 11 407 and 8209 μmol N m–2 h–1, respectively, at the inner muddy Site 1. The Thames estuary is heterotrophic and a very efficient organic C filter, trapping and remineralising 77% of its organic C input. Attenuation of fluvial nitrate loads was regulated by freshwater flow. Minimal attenuation (3%) occurred during peak flows (i.e. during periods of shortest freshwater flushing time) and >100% attenuation during periods of lowest freshwater flow (longest flushing times). Including the sewage treatment works (STWs) nitrate load in this calculation reduced the degree of attenuation of the nitrate load to, on average, 11%. Annual rates of Dw and Dn for an inner area of 125 km2 were 112 and 85 Mmol N yr–1, respectively, with a total rate of 196 Mmol N yr–1 (2744 t), which was equivalent to 9% of the total dissolved inorganic nitrogen (DIN) load for 1995-96. A mean denitrification rate (Dw) of 0.64 mol N m–2 yr–1, based on measurements in 4 east coast estuaries, was used to estimate a total rate of denitrification for the entire area of UK east coast estuaries. The total rate of 0.81 Gmol N yr–1 represented 16% attenuation of the total fluvial discharge of nitrate (∼6 Gmol N yr–1) to the UK's east coast estuaries (1995-96) and hence a 16% reduction in the UK nitrate load to the North Sea.