Dissolved methane concentration and flux in the coastal zone of the Southern California Bight-Mexican sector: Possible influence of wastewater Karel Castro-Morales a, b, * , J. Vinicio Macías-Zamora b , S. Raúl Canino-Herrera a , Roger A. Burke c a Facultad de Ciencias Marinas, Universidad Autónoma de Baja California, Carr. Tijuana-Ensenada, Ensenada, Baja California 22800, Mexico b Instituto de Investigaciones Oceanológicas, Carr. Tijuana-Ensenada, Ensenada, Baja California 22800, Mexico c Ecosystems Research Division, National Exposure Research Laboratory, U.S. Environmental Protection Agency, 960 College Station Road, Athens, GA 30605, USA article info Article history: Received 14 February 2013 Accepted 19 April 2014 Available online 28 April 2014 Keywords: methane coastal zone outfalls air-water exchanges México abstract We measured dissolved methane concentrations ([CH 4 ]) in the coastal zone of the Southern California Bight-Mexican sector (SCBMex) during two cruises: S1 in the USAeMexico Border Area (BA) during a short rainstorm and S2 in the entire SCBMex during a drier period a few days later. High spatial vari- ability in surface mixed layer (ML) [CH 4 ] was observed, ranging from 2.2 nmol L 1 to 17.8 nmol L 1 . ML- [CH 4 ] was supersaturated at all BA stations during both cruises. The highest [CH 4 ] was 72.4 nmol L 1 (2819 % supersaturated) measured at 10 m depth during S2, about 3 km southwest of the discharge point of the South Bay Ocean Outfall (SBOO). Our results show an apparent connection between wastewater treatment discharges and [CH 4 ]. Application of a sewer CH 4 production model suggests that the SBOO may be a large source of CH 4 to the BA and points to the need to consider point sources in developing coastal marine CH 4 budgets for highly populated areas. Based on our data, the SCBMex appears to be a relatively strong source of CH 4 to the atmosphere compared to other Pacific Basin areas. The average BA sea-to-air CH 4 flux (F) during S1 was (15.5  8.6)  10 2 nmol m 2 s 1 , about 1.5 times higher than F during S2, which had a flux of (9.5  6.9)  10 2 nmol m 2 s 1 mainly due to the higher wind speed during S1. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction The global atmospheric CH 4 concentration increased from about 715 ppb in 1750 to about 1774 ppb in 2005, and contributed an estimated 18 % of the total anthropogenic radiative forcing from long-lived greenhouse gases during that time (Forster et al., 2007). Although the total global CH 4 source is relatively well known, the strength and trends of the individual sources are uncertain. Due to its role in the greenhouse effect and in ozone layer chemistry, a better characterization of CH 4 sources and sinks is needed. The ocean is considered a minor atmospheric CH 4 source, although spatial and temporal variations of oceanic CH 4 concen- trations are uncertain. Typically, oligotrophic waters are only slightly supersaturated (by about 5 %) in CH 4 with respect to at- mospheric equilibrium (Scranton and Brewer, 1977; Owens et al., 1991; Bates et al., 1996; Bange et al., 1998; Karl et al., 2008) resulting in low calculated CH 4 fluxes. High sea-air CH 4 emissions can occur in biologically productive regions such as estuaries, coastal and upwelling areas which contribute about 75 % of the oceanic CH 4 emissions (Bange et al., 1994). The coastal CH 4 source is highly uncertain, however, primarily due to the relative lack of CH 4 flux measurements and the complexity of processes and factors affecting coastal CH 4 fluxes. Potential sources of CH 4 to coastal waters are: rivers and estuaries, in situ water column production, and sediments (Martens and Klump, 1980; Kelley et al., 1990; Hornafius et al., 1999; Mau et al., 2007; Canet et al., 2010). Because CH 4 is produced during several of the treatment stages (e.g. in grit tanks, secondary aeration and sludge holding tanks) in * Corresponding author. Present address: Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Am Handelshafen 12, Bremerhaven 27570, Germany. E-mail addresses: karelcm@gmail.com, karel.castro-morales@awi.de (K. Castro- Morales). Contents lists available at ScienceDirect Estuarine, Coastal and Shelf Science journal homepage: www.elsevier.com/locate/ecss http://dx.doi.org/10.1016/j.ecss.2014.04.017 0272-7714/Ó 2014 Elsevier Ltd. All rights reserved. Estuarine, Coastal and Shelf Science 144 (2014) 65e74