HR: 1400h
AN: GS23A-04    [Abstracts]
TI: Anthropogenic nitrogen in a subtropical coastal lagoon (SE Gulf of California): sources and ecological consequences using isotopes stables
AU: * SOTO-JIMENEZ, M
EM: martin@ola.icmyl.unam.mx
AF: Universidad Nacional Autonoma de Mexico, Av. Joel Montes s/n, Mazatlan, SIN 82040, Mexico
AU: PAEZ-OSUNA, F
EM: paezos@servidor.unam.mx
AF: Universidad Nacional Autonoma de Mexico, Av. Joel Montes s/n, Mazatlan, SIN 82040, Mexico
AB: Chemical and isotopic compositions of N were studied in anthropogenic effluents received by Altata-Ensenada- Pabellón lagoon (AEP), a typical coastal ecosystem along the SE GC. Human activities mobilize from 33,000 to 55,000 tons of N per year to the AEP, which the agriculture (70%), livestock (10%), and atmospheric deposition (10%) are the largest N sources. Other N sources including sewage, shrimp farming, poultry, and livestock effluents constituted less than 10% of N inputs. The N is distributed as inorganic N that accounts for an input of 72% (NO3- 65% and NH4+ 7%) and organic N for 28% (>90% of the organic N occurring as DON). The N isotopic composition in the different effluents vary 6.1-16.1‰ for 15N-NH4+ and 2.1-15.9‰ for 15N-NO3. The 15N-NO3- values in agriculture effluents are related to fertilizer, while the enriched values of 15N-NH4+ in sewage and animal effluents and 15N-NO3- in groundwater result of the high ammonification and denitrification during the transport to the lagoon. Overall the C/N ratios (5.3-21.1) and isotopic signatures (13C -30.6 to -21.4‰ and 15N 6.1-12.5‰) in the POM for the diverse effluents had measurable differences and consistent with their origins. The 15N-DIN values measured at the selected stations recording inputs into AEP lagoon waters oscillated from 4.4 to 6.5‰ for 15N- NO3-, 12.9 to 20.9‰ 15N-NH4+, and from 4.4 to 10‰ 15N-POM. Overall the 15N values predicted by mass balances are in the range of the measured isotopic composition. The N requirement in Pacific Mexican lagoons is estimated to be 155 to 400 kg N ha-1 y-1, however, the current level is 1480 kg N ha-1 y-1, with anthropogenic N inputs (1200 kg N ha-1 y-1) of 80%. This excess of primary productivity is evidenced by the phytoplankton and macroalgae blooms that occur year-round, mainly in the areas close to the nutrient discharges from AEP lagoon.
DE: 4870 Stable isotopes (0454, 1041)
SC: Geochemical Society [GS]
MN: 2007 Joint Assembly