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2952947 
Journal Article 
VARIABILITY IN NITROGEN STABLE ISOTOPE RATIOS OF MACROALGAE: CONSEQUENCES FOR THE IDENTIFICATION OF NITROGEN SOURCES 
Julia Ochoa-Izaguirre, M; Soto-Jimenez, MF 
2015 
Yes 
Journal of Phycology
ISSN: 0022-3646
EISSN: 1529-8817 
51 
1 
46-65 
WOS:000349400900005 
In our research, we collected and analyzed numerous macroalgal specimens (738) for isotopic analysis sampled over a year at monthly intervals across 20 sites within the Urias lagoon complex, a typical subtropical coastal ecosystem located in the Gulf of California. We quantified and characterized (chemically and isotopically) the N loads received by Urias throughout a year. We studied the spatial-temporal variation of the chemical forms and isotopic signals of the available N in the water column, and we monitored in situ different environmental variables and other hydrodynamic parameters. Multiple N sources (e. g., atmospheric, sewage, seafood processing, agriculture and aquaculture effluents) and biogeochemical reactions related to the N cycle (e. g., ammonia volatilization, nitrification and denitrification) co-occurring across the ecosystem, result in a mixture of chemical species and isotopic compositions of available N in the water column. Increased variability was observed in the delta N-15 values of macroalgae (0.41 parts per thousand-22.67 parts per thousand). Based on our results, the variation in delta N-15 was best explained by spatio-temporal changes in available N and not necessarily related to the N sources. The variability was also explained by the differences in macroalgal biology among functional groups, species and/or individuals. Although the delta N-15-macroalgae technique was a useful tool to identify N sources, its application in coastal ecosystems receiving multiple N sources, with changing environmental conditions influencing biogeochemical processes, and high diversity of ephemeral macroalgal species, could be less sensitive and have less predictive power. 
biogeochemical processes; Eutrophication; isotopic fractionation; macroalgal metabolism; delta N-15-Macroalgal