Seasonal study of dissolved CH 4, CO 2 and N 2O in a shallow tidal system of the bay of Cádiz (SW Spain)

Abstract

During 2004, 10 samplings were performed in order to measure dissolved methane (CH 4), carbon dioxide (CO 2) and nitrous oxide (N 2O) in the surface waters of Río San Pedro, a tidal creek in the salt marsh area of the Bay of Cádiz (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 CH 4, CO 2 and N 2O concentrations ranged from 11 to 88 nM, 36 to 108 μM 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 CH 4, 1240% for CO 2 and 840% for N 2O. Dissolved CH 4, CO 2 and N 2O 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 CH 4, CO 2 and N 2O 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-Ménard (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. Jähne 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 CH 4, CO 2 and N 2O from the creek to the atmosphere ranged between 34 and 150 μmol CH 4 m − 2 day − 1 , 73 and 177 mmol CO 2 m − 2 day − 1 and 24 and 62 μmol N 2O m −2 day −1, respectively.