title: The carbon dioxide system on the Mississippi River-dominated continental shelf in the northern Gulf of Mexico: 1. Distribution and air-sea CO2flux.
abstract: River-dominated continental shelf environments are active sites of air-sea CO2exchange. We conducted 13 cruises in the northern Gulf of Mexico, a region strongly influenced by fresh water and nutrients delivered from the Mississippi and Atchafalaya River system. The sea surface partial pressure of carbon dioxide (pCO2) was measured, and the air-sea CO2flux was calculated. Results show that CO2exchange exhibited a distinct seasonality: the study area was a net sink of atmospheric CO2during spring and early summer, and it was neutral or a weak source of CO2to the atmosphere during midsummer, fall, and winter. Along the salinity gradient, across the shelf, the sea surface shifted from a source of CO2in low-salinity zones (0≤S<17) to a strong CO2sink in the middle-to-high-salinity zones (17≤S<33), and finally was a near-neutral state in the high-salinity areas (33≤S<35) and in the open gulf (S≥35). HighpCO2values were only observed in narrow regions near freshwater sources, and the distribution of undersaturatedpCO2generally reflected the influence of freshwater inputs along the shelf. Systematic analyses ofpCO2variation demonstrated the importance of riverine nitrogen export; that is, riverine nitrogen-enhanced biological removal, along with mixing processes, dominatedpCO2variation along the salinity gradient. In addition, extreme or unusual weather events were observed to alter the alongshorepCO2distribution and to affect regional air-sea CO2flux estimates. Overall, the study region acted as a net CO2sink of 0.96 ± 3.7 mol m-2yr-1(1.15 ± 4.4 Tg C yr-1).
authors.affiliation: School of Marine Science and Policy University of Delaware Newark Delaware USA; Department of Marine Sciences University of Georgia Athens Georgia USA.
authors.forename: Wei-Jen
authors.initials: WJ
authors.lastname: Huang
authors.affiliation: School of Marine Science and Policy University of Delaware Newark Delaware USA.
authors.forename: Wei-Jun
authors.initials: WJ
authors.lastname: Cai
authors.affiliation: Department of Marine SciencesUniversity of GeorgiaAthensGeorgiaUSA; Deceased 9 July 2013.
authors.forename: Yongchen
authors.initials: Y
authors.lastname: Wang
authors.affiliation: Department of Fisheries Oceanography School for Marine Science and Technology, University of Massachusetts Dartmouth Massachusetts USA.
authors.forename: Steven E
authors.initials: SE
authors.lastname: Lohrenz
authors.affiliation: U.S. EPA, NHEERL Gulf Ecology Division Gulf Breeze Florida USA.
authors.forename: Michael C
authors.initials: MC
authors.lastname: Murrell
keywords: carbon cycle; gas exchange; river plume