Estimating the effects and timing of anthropogenic impacts on the composition of macrobenthic communities is challenging, because early twentieth-century surveys are sparse and the corresponding intervals in sedimentary sequences are mixed by bioturbation. Here, to assess the effects of eutrophication on macrobenthic communities in the northern Adriatic Sea, we account for mixing with dating of the bivalve Corbula gibba at two stations with high accumulation (Po prodelta) and one station with moderate accumulation (Isonzo prodelta). We find that, first, pervasively bioturbated muds typical of highstand conditions deposited in the early twentieth century were replaced by muds with relicts of flood layers and high content of total organic carbon (TOC) deposited in the late twentieth century at the Po prodelta. The twentieth century shelly muds at the Isonzo prodelta are amalgamated but also show an upward increase in TOC. Second, dating of C. gibba shells shows that the shift from the early to the late twentieth century is characterized by a decrease in stratigraphic disorder and by an increase in temporal resolution of assemblages from ~25–50 years to ~10–20 years in both regions. This shift reflects a decline in the depth of the fully mixed layer from more than 20 cm to a few centimeters. Third, the increase in abundance of the opportunistic species C. gibba and the loss of formerly abundant, hypoxia-sensitive species coincided with the decline in bioturbation, higher preservation of organic matter, and higher frequency of seasonal hypoxia in both regions. This depositional and ecosystem regime shift occurred in ca. a.d. 1950. Therefore, the effects of enhanced food supply on macrobenthic communities were overwhelmed by oxygen depletion, even when hypoxic conditions were limited to few weeks per year in the northern Adriatic Sea. Preservation of trends in molluscan abundance and flood events in cores was enhanced by higher frequency of hypoxia that reduced bioturbation in the late twentieth century.

From October 2020, the MOSE system went into operation in the three inlets to protect Venice and its lagoon from flooding caused by high tide events in the Adriatic Sea. While the MOSE increases the prospect of physical defense, how will the new status of a regulated lagoon affect the functioning of the waterbody? In particular, the dissolved oxygen balance in the water column can be affected by the expected increase in water residence times. Sediments play an important role in oxygen consumption in the water column and for this reason sediment oxygen demand (SOD) was studied in situ for the first time, in 16 sites at four lagoon areas. SOD rate, measured throughout 2021 with specially made benthic chambers, varied in the different areas and with seasons (from 0.63 to 24.00 g m⁻² d⁻¹), with higher values at the more confined sites and significantly lower values in those belonging to the open lagoon. Based on SOD rate, consumption times of dissolved oxygen in the water column were estimated at the MOSE closures, from saturation values to hypoxia and in the temperature range of 11.7–33.3°C. Especially in the confined sites, sediments can exert an important depleting action on dissolved oxygen already at water temperatures above 20°C, which are not particularly high for the lagoon.