Assessing nutrient dynamics in mangrove porewater and adjacent tidal creek using nitrate dual-stable isotopes: A new approach to challenge the Outwelling Hypothesis?
Assessing nutrient dynamics in mangrove porewater and adjacent tidal creek using nitrate dual-stable isotopes: A new approach to challenge the Outwelling Hypothesis?
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DOI：
10.1016/j.marchem.2019.103662
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发表时间：
2019-08
期刊：
影响因子：
3
通讯作者：
Taillardat Pierre;D. Alan;A. Daniel;Widory David;David Frank;Ohte Nobuhito;Nakamura Takashi;J. Evaristo;Thanh-nho Nguyen;T. Vinh;Marchand Cyril
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作者：
Taillardat Pierre;D. Alan;A. Daniel;Widory David;David Frank;Ohte Nobuhito;Nakamura Takashi;J. Evaristo;Thanh-nho Nguyen;T. Vinh;Marchand Cyril
红树林衍生的物质在维持沿海食物网方面的重要性（即向外扩张假说）经常被用来支持红树林的保护。然而，由于源、汇和转化途径的高度空间异质性和时间变异性，对红树林生态系统中的生物地球化学循环，特别是氮（N）和磷（P）的认识还很有限。在这里，我们表明，N和P的分布是密切相关的植被分布，潮汐周期和季节性。我们研究了沉积物中的N和P的动态，并在潮沟的越南的Can Gio红树林。本研究的目的是（1）确定沿着Rhizophora-Avicennia-mudflat样带红树林中溶解态无机氮和磷的空间分布;（2）通过24 h时间序列测量确定潮沟水体中N和P的输入和转化途径。沉积物孔隙水的N-NH 4+和N-NO3−浓度<11 μM，但泥滩的N-NH 4+浓度高达162 μM。这种差异可能是由于N-NH 4+的树木在植被区的吸收，并表明，红树林沉积物可以通过氨化有机氮的NH 4+生产区。在所有林分中，P-PO 43 −浓度在雨季高出三倍，最高值为34.4 μM。这可以用雨季期间微生物活动增强来解释。在小溪水中也观察到磷的季节性趋势，但最大P-PO 43 −值仅为4.3 μM。在潮沟中，N-NH 4+变化很大（0 - 51 μM），在低潮时测得的数值较高，与泥滩的孔隙水排放有关。我们的数据表明，红树林作为一个汇的溶解无机养分通过植被吸收和来源的铵从无植被的泥滩孔隙水对潮沟。双稳定同位素方法（δ 15 NNO 3和δ 18 ONO 3）表明，这种铵后来在水柱中被硝化。此外，该方法表明，一些硝酸盐来源于河流河口系统在涨潮。铵从红树林孔隙水的出口大概是完全消耗之前退出潮沟，从而限制了空间范围的红树林Outwelling。尽管如此，我们的多同位素方法使我们得出结论，营养物质循环通过红树林衍生的有机物矿化可能在维持沿海食物网中发挥着重要作用。
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 aRhizophora-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-PO43−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-PO43−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 (δ15NNO3& δ18ONO3) 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.