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.
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Status: open (until 18 Jan 2026)
- RC1: 'Comment on egusphere-2025-4423', Anonymous Referee #1, 08 Dec 2025 reply
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RC2: 'Comment on egusphere-2025-4423', Anonymous Referee #2, 29 Dec 2025
reply
This manuscript by Pellegrini et al. presents a comprehensive and multidisciplinary investigation of sedimentary, geochemical, and microbial responses to a recent flood event in a coastal system. The topic is timely and highly relevant in the context of rapid environmental and climatic change, and the dataset is potentially valuable. However, several major issues need to be addressed before the manuscript can be considered for publication.
Major comments：
- I suggest the authors to reconsider the title. The study focuses on a single flood event within one coastal system, and therefore the current framing is overly broad.
- The manuscript lacks some temporal and spatial extension to fully emphasize the broader significance of the findings. For example, based on the presented results, what might be the potential impacts of major flood events on coastal systems globally over decadal timescales? Alternatively, how might the historical recurrence of flood events in the studied region have shaped the local biogeochemical environment and microbial assemblages over longer timescales?
- The authors point out the flood event has an ephemeral impact, which is reasonable for a single event. However, I am curious whether repeated flood events could exert cumulative or long-term effects on sedimentary biogeochemistry, contaminant retention, and microbial-mediated processes.
- I expect to see more discussion about grain-size controls on offshore enrichment of PAHs, microbial communities, and organic matter. The source, transport pathways, and ultimate fate of flood-derived sediments are likewise strongly grain-size dependent, and this relationship is currently underdeveloped in the manuscript.
- Last but perhaps most important, the overall narrative remains broad but fragmented. The interactions among the components discussed (seafloor geochemistry, contaminants, and bacterial communities), and their combined implications for coastal biogeochemical functioning, are insufficiently integrated.
Minor comments：
L75: In addition to low reactivity and primary productivity, could organic carbon preservation also be influenced by microbial priming effects and sediment resuspension?
L97: I did not find a clear discussion later in the manuscript on how contaminants influence microbial community development, despite this being implied here.
L146: Was grain-size distribution explicitly considered during sampling? Upper and lower portions of event deposits often differ markedly in grain size, organic matter content, and contaminant load. Using a single sample to represent the entire event deposit may therefore be insufficient.
L174: TOC measurements appear to be insufficiently described.
L364: The description of these patterns would be more appropriate in the Discussion section, where underlying mechanisms can be better explored.
L480: If flood-transported material accumulates preferentially at the 10–15 m isobath, does this imply the existence of distinct pollutant sinks or biogeochemical environments? If so, what are the broader implications for material cycling and pollution management?
L516: Overall, the discussion of bacterial communities remains weakly connected to the other results. While spatial differences in bacterial assemblages are expected given variable sediment inputs, do these differences imply mechanistic changes in biogeochemical cycling?
Figure comments:
Figures 1 & 2: The maps and overlapping information is visually overwhelming. Perhaps consider integrating key elements into a single figure, or presenting multiple panels with a consistent and simplified layout.
Figure 3: Sampling depth and vertical resolution are not clearly indicated. Additionally, lithological information from other sites should be shown for comparison. Panel labels (a, b, c) are missing.
Figure 12: Indicating sedimentation hotspots directly on the figure would help readers grasp the main spatial patterns.
Citation: https://doi.org/10.5194/egusphere-2025-4423-RC2
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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.