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

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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.