Assessing nutrient dynamics in mangrove porewater and adjacent tidal creek using nitrate dual-stable isotopes: A new approach to challenge the Outwelling Hypothesis?

The importance of mangrove-derived material in sustaining coastal food webs (i.e. the Outwelling Hypothesis) is often invoked in support of mangroves conservation. Biogeochemical cycling, particularly nitrogen (N) and phosphorus (P) in mangrove ecosystems, however, is poorly understood because of high spatial heterogeneity and temporal variability of sources, sinks, and transformation pathways. Here we show that the distribution of N and P are intimately related to vegetation distribution, tidal cycles, and seasonality. We examined the dynamics of N and P in sediments and in a tidal creek of the Can Gio Mangrove Forest, Vietnam. Our objectives were to (1) determine the spatial distribution of dissolved inorganic nitrogen and phosphorus in the mangrove forest along a Rhizophora-Avicennia-mudflat transect; and (2) identify the respective inputs and transformation pathways of N and P in the water column via 24-h time series measurements in a tidal creek. Sediment porewater had N-NH₄ ⁺ and N-NO₃ ⁻ concentrations <11 μM, except in the mudflat where N-NH₄ ⁺ was as high as 162 μM. This difference was likely due to N-NH₄ ⁺ uptake by trees in the vegetated areas and suggests that mangrove sediments can be a zone of NH₄ ⁺ production via ammonification of organic nitrogen. In all stands, P-PO₄ ³⁻ concentrations were three-fold higher during the wet season, with a maximum of 34.4 μM. This can be explained by enhanced microbial activity during the rainy season. The phosphorus seasonal trend was also observed in the creek water but with a maximum P-PO₄ ³⁻ value of 4.3 μM only. In the tidal creek, N-NH₄ ⁺ was highly variable (0 to 51 μM), with the higher values measured at low tide and related to porewater discharge from the mudflat. Our data suggest that mangroves act both as a sink of dissolved inorganic nutrients via vegetation uptake and a source of ammonium from unvegetated mudflat porewater towards the tidal creek. The dual stable isotopes approach (δ¹⁵N_NO3 & δ¹⁸O_NO3) revealed that this ammonium was later nitrified within the water column. Moreover, the approach showed that some nitrate originated from the river-estuarine system during rising tides. The export of ammonium from mangrove porewater is presumably entirely consumed before exiting the tidal creek, thereby limiting the spatial extent of mangrove Outwelling. Nevertheless, our multi-isotope approach leads us to conclude that nutrients recycling via mangrove-derived organic matter mineralization may play a fundamental role in sustaining coastal food web.