Department of Aquatic Ecosystem Analysis, Helmholtz Centre for Environmental Research, Brückstraße 3a, 39114 Magdeburg, Germany.
Institute for Environmental Sciences, Universität Koblenz-Landau, Fortstrasse 7, 76829 Landau, Germany.
Sci Total Environ. 2022 Sep 1;837:155689. doi: 10.1016/j.scitotenv.2022.155689. Epub 2022 May 5.
The hyporheic zone underneath stream channels is considered a biogeochemical hotspot reducing nutrient loads being transported downstream due to its high surface-to-volume ratio in combination with the hyporheic exchange. However, the effect of environmental stressors such as high amounts of fine sediment (FS; grain size <0.2 mm) on nutrient cycling in the hyporheic zone are not well understood. Physical clogging caused by fine sediment (FS) decreases the hyporheic exchange, thus, diminishing its potential to reduce nutrient loads despite increasing its surface-to-volume ratio. We determined the effect of physical clogging on nutrient cycling based on net change rates of dissolved inorganic nitrogen (DIN; nitrate-N, ammonium-N), soluble reactive phosphorus (SRP), and dissolved organic carbon (DOC) for a sand and gravel hyporheic zone. We performed three experimental runs in 12 flumes with four-week duration each following a factorial design. First, we determined nutrient cycling in sand and gravel in absence of clogging, and then tested the clogging effect for each sediment type under increasing clogging (0-480 g of FS addition increasing by 60 g per level). Without clogging, gravel acted as a source of nitrate-N; and both sand and gravel released SRP. Regardless of the clogging level and the resulting reduced hyporheic exchange, we found no changes in DOC and nitrate-N dynamics but net-release of ammonium-N and SRP for gravel. In contrast, in sand, physical clogging inhibited DOC release for flumes with the higher FS. We propose that not physical clogging but DOC availability limited the nutrient uptake, as molar ratios of DOC to DIN and SRP ranged 1.2-1.5 and 77-191, respectively, indicating severe C limitation of N-uptake and partial C limitation of P-uptake. Our results suggest an interplay between nutrient molar ratios and physical clogging, which emphasize the interactions between hydrology and the stoichiometry of organic carbon, nitrogen and phosphorus in the hyporheic zone.
河道底部的底栖带被认为是一个生物地球化学热点区域，由于其高的表面积与体积比，再加上底栖带交换，它可以减少下游输送的营养负荷。然而，高浓度细颗粒泥沙（FS；粒径 <0.2 毫米）等环境胁迫因子对底栖带养分循环的影响还不太清楚。细颗粒泥沙（FS）造成的物理堵塞会减少底栖带交换，从而减少其减少营养负荷的潜力，尽管其表面积与体积比会增加。我们基于溶解无机氮（DIN；硝酸盐-N、铵-N）、可溶性反应磷（SRP）和溶解有机碳（DOC）的净变化率，确定了物理堵塞对沙砾质底栖带养分循环的影响。我们在 12 个长 1.2 米、宽 0.1 米、高 0.06 米的水槽中进行了三个为期四周的实验，每个实验都采用了因子设计。首先，我们在没有堵塞的情况下确定了沙砾中的养分循环，然后在每个泥沙类型下测试了堵塞效应，每个泥沙类型的堵塞程度逐渐增加（480g FS，每次增加 60g）。在没有堵塞的情况下，砾石是硝酸盐-N 的源；而沙子和砾石都释放了 SRP。无论堵塞程度和由此导致的底栖带交换减少如何，我们都没有发现 DOC 和硝酸盐-N 动态的变化，但砾石的铵-N 和 SRP 呈净释放。相比之下，在含有较高 FS 的水槽中，物理堵塞抑制了沙子中 DOC 的释放。我们提出，不是物理堵塞而是 DOC 的可利用性限制了养分的吸收，因为 DOC 与 DIN 和 SRP 的摩尔比分别为 1.2-1.5 和 77-191，表明氮的吸收受到严重的碳限制，磷的吸收受到部分碳限制。我们的结果表明，养分摩尔比与物理堵塞之间存在相互作用，这强调了水文学和有机碳、氮和磷在底栖带中的化学计量之间的相互作用。
