Controls on carbon cycling in two contrasting temperate zone estuaries: The Tyne and Tweed, UK

In order to evaluate the respirationephotosynthesis dynamics in two contrasting North Sea estuaries, pH, temperature, alkalinity, chlorophyll-a (chl-a), and isotopic ratios of dissolved inorganic carbon (d13CDIC) and dissolved oxygen (d18ODO) were measured in the Tyne (July 2003) and Tweed (July 2003 and December 2003) estuaries. Using a concentration-dependent isotope mixing line, d13CDIC values in the Tweed (July 2003) demonstrated mostly conservative behaviour across the estuary, reflecting mixing between riverine and marine sources, although some samples were slightly more 13C-enriched than predicted d13CDIC values. Low pCO2 (less than 2 times atmospheric pressure) and 18O-depleted d18ODO signatures below equilibrium with the atmosphere provided further evidence for net autotrophy in the Tweed estuary in summer 2003. Conversely, in the Tyne during the summer and in the Tweed during the winter higher pCO2 (up to 6.5 and 14.4 times atmospheric partial pressure in the Tweed and Tyne, respectively), slightly 13C-depleted d13CDIC and 18O-enriched d18ODO values indicated heterotrophy as the dominant process. The relatively large releases of CO2 observed during these two estuarine surveys can be attributed to significant oxidation of terrigenous organic matter (OM). This study therefore demonstrates the usefulness of combined d18ODO and d13CDIC isotopes in examining the relationship between respirationephotosynthesis dynamics and the fate of terrestrially derived OM during estuarine mixing.