Destroy nothing, reveal everything: unveiling a century of pollution change in the Venice Lagoon (Italy) by non-invasive X-ray fluorescence biomonitoring of historical macroalgal specimens

The Venice Lagoon is a highly biodiverse yet heavily anthropized coastal ecosystem that has undergone profound environmental transformations over the past century. Between 1930 and 1932, Aristocle Vatova conducted a comprehensive survey of the lagoon's algal flora, collecting specimens from 68 sites. These were later identified, mainly by Victor Schiffner, and are now preserved at the Natural History Museum of Venice. In this study, we compared the elemental composition of Ulva spp. specimens from this historical collection with that of samples collected in 2025 at 27 corresponding sites, using portable X-ray fluorescence (XRF) spectrometry. This fully non-destructive approach enabled a direct century-scale comparison of potentially toxic element (PTE) concentrations without damaging irreplaceable material. Overall, PTE concentrations in Ulva spp. decreased markedly from the 1930s to 2025, with median old/new ratios ranging from 1.2 for Ba to 7.4 for Zn and site-specific maxima exceeding 10 for P (11.6), Cr (10.6), Mn (17.0), Fe (14.5), and Zn (21.5). Localized increases in Cu and Cr were detected near urban (Venice, Chioggia) and industrial areas, suggesting possible point-source inputs. These findings demonstrate the value of historical macroalgal collections as quantitative archives for reconstructing long-term pollution trends and validate XRF as a robust, replicable analytical tool for non-invasive environmental monitoring. The century-scale decline in PTEs reflects a substantial improvement in the environmental quality of the Venice Lagoon and establishes a framework for applying similar retrospective biomonitoring approaches in other coastal ecosystems.