Macroalgae δ 15 N values in well-mixed estuaries: indicator of anthropogenic nitrogen input or macroalgae metabolism?

Abstract : Although nitrogen stable isotope ratio (d15N) in macroalgae is widely used as a bioindicator of anthropogenic nitrogen inputs to the coastal zone, recent studies suggest the possible role of macroalgae metabolism in d15N variability. Simultaneous determinations of d15N of dissolved inorganic nitrogen (DIN) along the landesea continuum, inter-species variability of d15N and its sensitivity to environmental factors are necessary to confirm the efficiency of macroalgae d15N in monitoring nitrogen origin in mixed-use watersheds. In this study, d15N of annual and perennial macroalgae (Ulva sp., Enteromorpha sp., Fucus vesiculosus and Fucus serratus) are compared to d15N-DIN along the Charente Estuary, after characterizing d15N of the three main DIN sources (i.e. cultivated area, pasture, sewage treatment plant outlet). During late winter and spring, when human activities produce high DIN inputs, DIN sources exhibit distinct d15N signals in nitrate (NO3-) and ammonium (NH4+): cultivated area (+6.5 ± 0.6 ‰ and +9.0 ± 11.0 ‰), pasture (+9.2 ± 1.8 ‰ and +12.4 ‰) and sewage treatment plant discharge (+16.9 ± 8.7 ‰ and +25.4 ± 5.9 ‰). While sources show distinct d15N - NO3- in this multiple source catchment, the overall mixture of NO3- sources - generally >95% DIN - leads to low variations of d15N - NO3- at the mouth of the estuary (+7.7 to +8.4 ‰). Even if estuarine d15N - NO3- values are not significantly different from pristine continental and oceanic site (+7.3 ‰ and +7.4 ‰), macroalgae d15N values are generally higher at the mouth of the estuary. This highlights high anthropogenic DIN inputs in the estuary, and enhanced contribution of 15N-depleted NH4+ in oceanic waters. Although seasonal variations in d15N - NO3- are low, the same temporal trends in macroalgae d15N values at estuarine and oceanic sites, and inter-species differences in d15N values, suggest that macroalgae d15N values might be modified by the metabolic response of macroalgae to environmental parameters (e.g., temperature, light, DIN concentrations). Differences between annual and perennial macroalgae indicate both a higher integration time of perennial compared to annual macroalgae and the possible role of passive versus active uptake mechanisms. Further studies are required to characterize the sensitivity of macroalgae fractionation to variable environmental conditions and uptake mechanisms.
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Mélanie Raimonet, Gaël Guillou, Françoise Mornet, Pierre Richard. Macroalgae δ 15 N values in well-mixed estuaries: indicator of anthropogenic nitrogen input or macroalgae metabolism?. Estuarine, Coastal and Shelf Science, Elsevier, 2013, 119, pp.126-138. ⟨10.1016/j.ecss.2013.01.011⟩. ⟨hal-01081534⟩

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