Early diagenesis in sediments of a Mediterranean lagoon (Venice lagoon, Italy) and its relationship to hypoxia

the interaction between sediment biogeochemistry and hypoxia has been largely investigated in ecosystems periodically subjected to the presence of large anoxic zones, and in landlocked stratified environments. less attention has been provided to shallow water systems in which stratification is absent, and the wind is expected to have a major role in water mixing and reoxygenation. the present work focuses on sediments of a shallow water lagoon, located in a densely populated area subjected to multiple stressors (venice lagoon, italy), with the goal of increasing the understanding of the links between diagenetic processes occurring in sediments and the dynamics of dissolved oxygen. sediment data were collected over three consecutive years, from 2015 to 2017, during the spring-summer season, at a total of 5 different stations, located in different areas of the lagoon. measured variables included sediment porosity, grain size and solids composition, porewater microprofiles of o2, ph and h2s, porewater profiles, including dic, alk, nh4, no3, dissolved fe. in addition, long-term water quality monitoring data (years 2005-2017) collected by the venice water authority (magistrato alle acque) were analyzed, in order to assess the tendency of the sampled areas to undergo hypoxia. the latter analysis, which was performed at 4 monitoring stations, pointed out that the lagoon waters experience low oxygen concentrations in summer months, with median values below 50% saturation, and that consecutive days with minimum saturation values below 25% (55 mmol/l) can occur, and last one week or more. the inter-annual consistency of data collected by sediment coring was first performed and, subsequently, sampled stations were divided in two categories based on their diagenetic intensity. at intense and moderate diagenesis stations the average dic flux, estimated trough a simple steady-state model (profile) were respectively of 2.8 and 1.0 mmol m-2 d-1, so4 fluxes of 1.6 and 0.4 mmol m-2 d-1, while diffusive oxygen uptake fluxes, calculated from the sediment microprofiles data, were of 28.5 and 17.5 mmol m-2 d-1. at the organic-rich stations, dissolved sulfide accumulated in porewaters very near the sediment-water interface, reaching values of 0.7 mm at 10 cm. considering the typical physico-chemical summer conditions at these stations, we estimated the time required for a complete o2 depletion of the water column by sediment oxygen demand in absence of mixing, which would be comprised between 4 and 14 days, respectively at intense and moderate diagenesis stations. this estimation highlights that changes in regional climate conditions of these systems such as the increase in the frequency of summer heat waves, currently experienced in the mediterranean region, could extend the duration and increase the severity of hypoxic events. in this respect, an aspect of relevance concerns the consequences that hypoxia can have on water bodies through release of sulfide.

在周期性受到大型无氧区影响的生态系统和内陆分层环境中，沉积物生物地球化学与缺氧之间的相互作用已得到广泛研究。然而，对于分层缺失的浅水系统，尤其是风力在水中混合和再氧化的主要作用所预期的系统，关注相对较少。本研究聚焦于位于人口密集区，并受到多重胁迫（威尼斯潟湖，意大利）的浅水潟湖沉积物，旨在增进对沉积物中发生的成岩过程与溶解氧动态之间联系的理解。沉积物数据收集跨越了连续三年的春季夏季，从2015年至2017年，在潟湖的不同区域共设置了5个不同站点的总测量。测量的变量包括沉积物孔隙率、粒度和固体成分、氧（O2）、pH和硫化氢（H2S）的孔隙水微剖面、孔隙水剖面，包括二价碳（DIC）、碱度（alk）、氨（NH4）、硝酸盐（NO3）和溶解铁。此外，还分析了威尼斯水务局（Magistrato alle Acque）收集的长期水质监测数据（2005-2017年），以评估采样区域发生缺氧的趋势。在4个监测站点的后续分析中，指出潟湖在夏季月份的水体氧气浓度较低，中值低于50%的饱和度，并且连续几天最小饱和度值低于25%（55 mmol/l）的情况可能发生，持续时间可达一周或更长。首先对沉积物钻取收集的数据进行了年际一致性分析，随后根据成岩强度将采样站点分为两类。在强烈和适度成岩站点，通过简单的稳态模型（剖面）估计的平均DIC通量分别为2.8和1.0 mmol m-2 d-1，硫酸盐通量分别为1.6和0.4 mmol m-2 d-1，而扩散氧摄取通量，从沉积物微剖面数据计算得出，分别为28.5和17.5 mmol m-2 d-1。在有机质丰富的站点，溶解硫化物在沉积物-水界面附近的孔隙水中积累，在10 cm处达到0.7 mm的值。考虑到这些站点典型的物理化学夏季条件，我们估计了在没有混合作用的情况下，由沉积物氧需求引起的整个水柱完全氧耗尽所需的时间，分别在强烈和适度成岩站点为4至14天。这一估计突显了区域气候变化条件（如地中海地区目前经历的夏季热浪频率的增加）可能延长缺氧事件的持续时间并加剧其严重性。在这方面，一个相关的问题是缺氧对水体可能产生的影响，即通过释放硫化氢。