Seasonal study of dissolved CH4CO2 and N2O in a shallow tidal system of the bay of Cadiz (SW Spain)

During 2004, 10 samplings were performed in order to measure dissolved methane (CH4), carbon dioxide (CO2) and nitrous oxide (N2O) in the surface waters of Rio San Pedro, a tidal creek in the salt marsh area of the Bay of Cadiz (SW Spain). The inner partvs of the creek is affected by the inputs coming from an intensive fish farm and the drainage of an extensive salt marsh area. Dissolved CH4, CO2 and N2O concentrations ranged from 11 to 88 nM, 36 to 108 mu M and 14 to 50 nM, respectively. Surface waters were in all cases oversaturated with respect to the atmosphere, reaching values of up to 5000% for CH4, 1240% for CO2 and 840% for N2O. Dissolved CH4, CO2 and N2O showed a significant tidal and seasonal variability. Over a tidal cycle, concentrations were always highest during low tide, which points to the influence of the inputs from the fish farm effluent and the drainage of the adjacent salt marsh area, as well as in situ production within the system. Dissolved CH4, CO2 and N2O seasonal patterns were similar and showed maximum concentrations in summer conditions. Using four different parameterizations to calculate the gas transfer coefficients [Liss, P.S. and Merlivat, L., 1986. Air-sea exchange rates: introduction and synthesis. In P. Buat-Menard (Ed.), The Role of Air-Sea Exchanges in Geochemical Cycling. Reidel, Dordrecht, The Netherlands, p. 113-127.; Clark, J.F., Schlosser, P., Simpson, H.J., Stute, M., Wanninkhof, R., and Ho, D.T., 1995. Relationship between gas transfer velocities and wind speeds in the tidal Hudson River determined by the dual tracer technique. In: B. Jahne and E. Monahan (Eds.), Air-Water Gas Transfer: AEON Verlag and Studio, Hanau, Germany, pp. 785-800.; Carini, S., Weston, N., Hopkinson, G., Tucker, J., Giblin, A. and Vallino, J., 1996. Gas exchanges rates in the Parker River estuary, Massachusetts. Biol. Bull., 191: 333-334.; Kremer, J.N., Reischauer, A. and D'Avanzo, C., 2003. Estuary-specific variation in the air-water gas exchange coefficient for oxygen. Estuaries, 26: 829-836.], the averaged air-water fluxes of CH4, CO2 and N2O from the creek to the atmosphere ranged between 34 and 150 mu mol CH4 m(-2) day(-1), 73 and 177 mmol CO2 m(-2) day(-1) and 24 and 62 pmol N2O m(-2). day, respectively. (c) 2006 Elsevier B.V. All rights reserved.