the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 19 Sep 2025
River floods in the Anthropocene impact sea-floor geochemistry, pollutants and bacterial communities in coastal systems
Abstract. This study examines the sedimentary and microbial responses offshore the Marche Region (Italy) to the September 2022 flood, one of the most severe recent hydrological events, which delivered large amounts of sediment and anthropogenic contaminants to the Adriatic Sea. We employed a multidisciplinary approach integrating sedimentology, geochemistry, organic matter analysis, pollutant assessments (Polycyclic Aromatic Hydrocarbons, PAHs and Poly- and Perfluorinated alkyl substances, PFASs), and benthic microbial community structure. Sediments collected five days post-event offshore six river mouths reveal that flood deposits, ranging from fine sand to coarse silt, were largely confined within the nearshore zone down to the 15 m isobath. This distribution reflects intense riverine inputs and a brief windstorm-enhanced coastal circulation that generated patchy, temporary sediment accumulations in the prodelta sector. Heavy metal concentrations remained below regulatory thresholds, whereas organic pollutants were heterogeneously distributed, with peaks offshore urban and industrial zones. PAH signatures indicate mixed pyrogenic and petrogenic sources, while next-generation PFASs (6:2FTS) showed localized but severe contamination linked to upstream industrial activities. Simultaneously, the flood introduced strong spatial heterogeneity in benthic bacterial communities, with sediment texture and organic matter content driving compositional shifts. Freshwater-associated taxa became prominent in offshore deposits, highlighting riverine sedimentary imprints. Despite the flood's magnitude onshore, its offshore sedimentary and ecological signatures were spatially limited and ephemeral. These findings underscore the ecological significance of episodic sediment and contaminant inputs, while highlighting the challenges in detecting such transient events in the marine stratigraphic record.
- Preprint
(2519 KB) - Metadata XML
-
Supplement
(586 KB) - BibTeX
- EndNote
Status: open (until 06 Jan 2026)
- RC1: 'Comment on egusphere-2025-4423', Anonymous Referee #1, 08 Dec 2025 reply
Viewed
| HTML | XML | Total | Supplement | BibTeX | EndNote | |
|---|---|---|---|---|---|---|
| 1,110 | 97 | 15 | 1,222 | 20 | 13 | 14 |
- HTML: 1,110
- PDF: 97
- XML: 15
- Total: 1,222
- Supplement: 20
- BibTeX: 13
- EndNote: 14
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1
The conceptual foundation is solid. However, the manuscript is overly long, making the key findings difficult to extract. Several sections would benefit from re-organization, and some contextual or secondary material, such as methodological background in section 3.6, should be moved to the Supplementary Information to streamline the main narrative.
Methods require clearer definitions and traceability to meet reproducibility standards. Additionally, the title overstates the interpretive breadth of the manuscript and should be revised to better reflect the scope of the results.
Despite the criticisms, the manuscript has strong potential to analyze an interesting short-term impact of river floods on delta regions. A more concise presentation, clearer methods, improved figures, and better integration of modeling outputs as context for the observations would substantially strengthen the paper and its contribution to understanding episodic sediment and contaminant delivery in coastal systems.
I therefore recommend major revisions, focusing on the aspects stated above.
Specific comments:
Figure 1 lacks a legend for geological formations and does not name formations in the figure. Sampling sites use "yellow dots" without station names or codes, hindering cross-referencing. The inset of "geochemical provinces" is redundant or requires a full explanation in a separate figure, as its color scheme duplicates that of the main map.
Figure 2 is overloaded and unclear. Label panels explicitly (e.g., a, b, c) and ensure captions describe each panel. Define key elements such as chlorophyll a concentrations, site codes, and color scales. Label the flood event on the x-axis in hours for clarity.
The criteria distinguishing “flood deposits” from “pre-flood deposits” are not clearly defined and need clarification.
Clarify how many sediment samples were analyzed for pollutants and how this differs from geochemical analyses. Summarizing this in a supplementary table would aid reader comprehension.
Section 3.6 does not make it clear how the meteorological and oceanographic datasets were actually used. At present, the authors appear to have extracted outputs from existing models without performing any substantive processing or analysis of their own. This relates directly to my general comment: why not integrate these datasets with the observed stratigraphy to build even a simple process-based model of the event? Doing so would substantially strengthen the manuscript and make the overall interpretation more robust. This integration is attempted in Fig. 12, but it comes too late in the manuscript. If this is out of the scope, then I would move this data to the SI.
Results. The results bring together all the different factors, which, as far as I know, have not been assembled in this way before. In this sense, this study is new in its multidisciplinary scope.
Figure 4: Clarify where satellite-derived chlorophyll a shading is shown, and include it in the legends if relevant.
The Discussion repeats several results rather than synthesizing them, which further contributes to the sense of fragmentation. A more integrative, interpretation-focused discussion would significantly improve the manuscript’s reading pace.
In the discussion, the disciplinary components remain mostly detached. The manuscript is strong in its geochemical, microbiological, and pollutant analyses. Yet, the meteo-oceanographic component is comparatively weak and does not provide the mechanistic support needed for some of the arguments you make. As it stands, the modeling should probably be removed from the main text and treated as supplementary contextual information, useful for the Discussion but not framed as a core analytical component.
Move Figure 12 earlier, as it is essential for following the narrative.