Abstract

OCHOA-IZAGUIRRE, María Julia  and  SOTO-JIMENEZ, Martín Federico. Evaluation of nitrogen sources in the Urías lagoon system, Gulf of California, based on stable isotopes in macroalgae. Cienc. mar [online]. 2013, vol.39, n.4, pp.413-430. ISSN 0185-3880.

The concentrations of chemical forms of nitrogen (N) and their isotopic composition in the water column and the δ¹⁵N signals in 738 samples of macroalgae collected during one year across the Urías lagoon system (Mazatlán, Sinaloa, Mexico) were analyzed in order to study the N sources. The δ¹⁵N-macroalgae premise is that they register and integrate the variability of N and may provide time-integrated information about N pollution. To validate this premise, we applied two models to predict the isotopic variability of N available in the water column (δ¹⁵N-DIN): one is based on a physical mixing balance of N sources (simple model), and the second includes the effects of the biogeochemical processes on the signals (coupled model, CM). Both models showed significant R², but CM better explained the δ¹⁵N-DIN variability. The relation between δ¹⁵N-DIN_CM and δ¹⁵N-macroalgae also showed a good fit, although lower than expected and with higher dispersion. The multiple N sources and co-occurring biogeochemical reactions, which produce a mixture of chemical species of N and of their isotopic compositions, as well as the high diversity of species (45) explained the elevated variability and lack of fit. Even so, δ¹⁵N in macroalgae is the most useful available tool to quantify the relative contributions of N to ecosystems from different sources. Based on Bayesian isotopic mixing models, the main anthropogenic sources of N to the Urías lagoonal system were domestic effluents (50%), atmospheric deposition (24%), and seafood processing plant effluents (21%), and to a lesser extent, aquaculture effluents (3%), agriculture runoff (1%), and oceanic N (1%).

Keywords : δ¹⁵N; eutrophication; biogeochemical cycle; NLOAD model; MixSIR.

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