Estimating the effects and timing of anthropogenic impacts on the composition of macrobenthic communities is challenging because early 20th 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 sediment 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 20th century were replaced by muds with relicts of flood layers and high content of total organic carbon (TOC) deposited in the late 20th century at the Po prodelta. The 20th 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 20th century is characterized by a decrease in stratigraphic disorder and by an increase in temporal resolution of death 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 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 ~1950 AD. Therefore, the effects of enhanced food supply on macrobenthic communities were overwhelmed by oxygen depletion even when hypoxic conditions are limited to few weeks per year in the northern Adriatic Sea. Preservation of trends in molluscan abundance and flood events in sedimentary sequences was enhanced by eutrophication that reduced bioturbational mixing.

Supplementary Table 1Supplementary Table 1 – 210Pb sediment data (with standard deviations) measures at three stations.Supplementary Table 2Supplementary Table 2 – Amino acid racemization and calibrated age data of Corbula gibba from two Po stations (analyzed in this paper), from the Bay of Panzano (Tomasovych et al. 2017, Geology, and analyzed this paper), and from Piran (Mautner et al. 2018, Marine Pollution Bulletin).Supplementary Table 3Supplementary Table 3 – Abundances of mollusks in two replicate cores at Po 3 (cores M13 and M14), at Po4 (cores M20 and M21), and in the Bay of Panzano (cores M28 and M29), and concentrations of elements, TOC, and total nitrogen.Supplementary codeR language source code for age unmixing procedure and for reproduction of some plots and analyses underlying the manuscript.The excel file "Supplementary Table 2" should be saved as tab-delimited txt file. This file is uploaded by the script.