U. S. Geological Survey, Woods Hole Coastal and Marine Science Center, Woods Hole, Massachusetts, USA.
Marine Biological Laboratory, Woods Hole, Massachusetts, USA.
Glob Chang Biol. 2022 Aug;28(15):4539-4557. doi: 10.1111/gcb.16217. Epub 2022 May 26.
Saline tidal wetlands are important sites of carbon sequestration and produce negligible methane (CH ) emissions due to regular inundation with sulfate-rich seawater. Yet, widespread management of coastal hydrology has restricted tidal exchange in vast areas of coastal wetlands. These ecosystems often undergo impoundment and freshening, which in turn cause vegetation shifts like invasion by Phragmites, that affect ecosystem carbon balance. Understanding controls and scaling of carbon exchange in these understudied ecosystems is critical for informing climate consequences of blue carbon restoration and/or management interventions. Here, we (1) examine how carbon fluxes vary across a salinity gradient (4-25 psu) in impounded and natural, tidally unrestricted Phragmites wetlands using static chambers and (2) probe drivers of carbon fluxes within an impounded coastal wetland using eddy covariance at the Herring River in Wellfleet, MA, United States. Freshening across the salinity gradient led to a 50-fold increase in CH emissions, but effects on carbon dioxide (CO ) were less pronounced with uptake generally enhanced in the fresher, impounded sites. The impounded wetland experienced little variation in water-table depth or salinity during the growing season and was a strong CO sink of -352 g CO -C m year offset by CH emission of 11.4 g CH -C m year . Growing season CH flux was driven primarily by temperature. Methane flux exhibited a diurnal cycle with a night-time minimum that was not reflected in opaque chamber measurements. Therefore, we suggest accounting for the diurnal cycle of CH in Phragmites, for example by applying a scaling factor developed here of ~0.6 to mid-day chamber measurements. Taken together, these results suggest that although freshened, impounded wetlands can be strong carbon sinks, enhanced CH emission with freshening reduces net radiative balance. Restoration of tidal flow to impounded ecosystems could limit CH production and enhance their climate regulating benefits.
盐水潮汐湿地是碳固存的重要场所，由于经常被富含硫酸盐的海水淹没，因此产生的甲烷（CH ）排放量可以忽略不计。然而，广泛的沿海水文管理已经限制了大片沿海湿地的潮汐交换。这些生态系统经常经历蓄水和变清，这反过来又会导致植被发生变化，例如入侵的芦苇，从而影响生态系统的碳平衡。了解这些研究不足的生态系统中碳交换的控制因素和规模对于了解蓝碳恢复和/或管理干预的气候后果至关重要。在这里，我们 (1) 使用静态室研究了盐分梯度（4-25 psu）在受限制和自然潮汐不受限制的芦苇湿地中的碳通量如何变化；(2) 通过在美国马萨诸塞州韦尔弗利特的赫林河使用涡度相关法研究了受限制的沿海湿地中的碳通量驱动因素。盐分梯度的变清导致 CH 排放量增加了 50 倍，但对二氧化碳（CO ）的影响则不那么明显，在较清新的蓄水地，吸收作用通常会增强。在生长季节，受限制的湿地的地下水位深度或盐度变化不大，并且是一个强大的 CO 汇，每年吸收-352 g CO -C m ，而 CH 排放量为 11.4 g CH -C m 。生长季节 CH 通量主要受温度驱动。CH 通量表现出昼夜循环，夜间达到最小值，而不透明室测量则无法反映这一点。因此，我们建议考虑芦苇中的 CH 昼夜循环，例如，应用这里开发的~0.6 的缩放因子来修正中午的腔室测量值。总之，这些结果表明，尽管变清了，受限制的湿地可以成为强大的碳汇，但随着变清而增强的 CH 排放会降低净辐射平衡。恢复到受限制生态系统的潮汐流动可以限制 CH 的产生并增强其气候调节效益。
