Influence of carbonates on the riverine carbon cycle in an anthropogenically dominated catchment basin: evidence from major elements and stable carbon isotopes in the Lagan River (N. Ireland)

Investigation of anthropogenic versus natural controls of the carbon cycle in the Lagan River revealed a strong influence of carbonates. This was evident by increasing pH values along the river as well as isotopic compositions of the riverine dissolved inorganic carbon (delta(13)C(DIC)) that approached expected values for carbonate dissolution. This predominant influence of carbonates existed despite their minor abundance of only similar to 5% in this densely populated catchment basin and outlines their capacity of buffering anthropogenic influences and CO2 turnover. These effects should be even more pronounced in other rivers where carbonates occupy a larger proportion of the catchment basin geology. Other controls on the riverine carbon cycle were silicate weathering and respiratory turnover of organic material that originated mainly from anthropogenic inputs and increased the DIC pool by up to 26.6%. Predominant natural controls on the Lagan River carbon cycle changed to anthropogenic ones closer to the mouth. Before discharging into Belfast Lough, a recently installed weir caused stagnant seawater to make up between 53% and 92% of the water mass. Poor vertical mixing caused O-2 decreases and anaerobic sedimentary activity that resulted in methane production. Recently installed aerators at the sediment surface did not prevent ongoing methanogenesis. This was documented by decreased pCO(2) and more C-13-enriched DIC values at the sediment-water interface when compared to those of surface waters from the same sampling sites. Installations of such weirs in estuaries of other rivers may cause similar anoxic effects that influence their biogeochemistry. (C) 2003 Elsevier B.V. All rights reserved.