Estuarine sediments store particulate contaminants including mercury (Hg). We studied Hg sediment dynamics in two intertidal mudflats at Great Bay estuary, NH, over multiple years. Sediments at both mudflats were physically mixed down to ~ 10 cm, as determined by ⁷Be measurements, albeit via different mechanisms. Portsmouth mudflat (PT) sediments were subject to bioturbation by infaunal organisms and Squamscott mudflat (SQ) sediments were subject to erosion and redeposition. The presence of higher concentrations of fresh Fe(III) hydroxide at PT suggested bioirrigation by the polychaetes (Nereis virens). At depths where infaunal bioirrigation was observed, pore-water inorganic Hg (Hg_i) and methylmercury (MeHg) were lower potentially due to their interaction with Fe(III) hydroxide. Methylmercury concentrations increased immediately below this zone in some samples, suggesting that the observed increase in material flux in bioirrigated sediments may initiate from lower depths. Pore water in sediment at PT also had higher fractions of more protein-like and labile DOC than those at SQ that can lead to increased MeHg production in PT, especially at depths where Hg_i is not removed from solution by Fe(III) hydroxide. Where sediment erosion and redeposition were observed at SQ, Hg species distribution was extended deeper into the sediment column. Moreover, methyl coenzyme M reductase (MCR) and mercury reductase (mer-A) genes were higher at SQ than PT suggesting differences in conditions for Hg cycling. Results showed that the near-surface region of high MeHg concentrations commonly observed in unmixed sediments does not exist in physically mixed sediments that are common in many estuarine environments.

河口沉积物存储着包括汞（Hg）在内的颗粒污染物。我们研究了新罕布什尔州大湾河口两个潮间带滩涂中的汞沉积物动力学，历时多年。通过⁷ Be测量确定，两个泥滩的沉积物物理混合至约10 cm ，尽管是通过不同的机制进行的。朴茨茅斯滩涂（PT）沉积物受到了有害生物的生物扰动，而Squamscott滩涂（SQ）沉积物受到了侵蚀和再沉积。在PT处存在较高浓度的新鲜氢氧化Fe（III），表明多毛cha（Nereis virens）进行了生物灌溉。在观察到不充分生物灌溉的深度，孔隙水无机汞（Hg _i）和甲基汞（MeHg）可能由于与氢氧化Fe（III）相互作用而降低。在一些样品中，该区域下方的甲基汞浓度立即增加，这表明所观察到的生物灌溉沉积物中物质通量的增加可能是从较低深度开始的。与SQ相比，PT沉积物中的孔隙水也含有更多的类蛋白质和不稳定的DOC，这会导致PT中MeHg的产量增加，尤其是在Hg _i不能被氢氧化三价铁从溶液中去除的深度。在SQ观察到沉积物侵蚀和再沉积的地方，Hg物种分布更深地延伸到沉积物柱中。此外，甲基辅酶M还原酶（MCR）和汞还原酶（mer-A）基因在SQ处高于PT，表明汞循环的条件有所不同。结果表明，在许多河口环境中常见的物理混合沉积物中不存在通常在未混合沉积物中观察到的高MeHg浓度的近地表区域。