Mercury concentrations within Peruvian mangrove sediments

Wilson Machado, Breno Rodrigues, Victor de Freitas, Anderson Rocha, Christiane Duyck, Adan Lino, Olaf Malm, Christian J. Sanders, Matthieu Carré, Alexander Pérez

► To cite this version:

Wilson Machado, Breno Rodrigues, Victor de Freitas, Anderson Rocha, Christiane Duyck, et al. Mercury concentrations within Peruvian mangrove sediments. Anthropocene Coasts, 2026, 9, pp.8. 10.1007/s44218-026-00127-y. hal-05502869

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# Mercury concentrations within Peruvian mangrove sediments

Wilson Machado<fnref n="1" />, Breno Rodrigues<fnref n="1" />, Victor de Freitas<fnref n="1" />, Anderson Rocha<fnref n="2" />, Christiane Duyck<fnref n="2" />, Adan Lino<fnref n="1" />,<fnref n="3" />, Olaf Malm<fnref n="3" />, Christian J. Sanders<fnref n="4" />, Matthieu Carré<fnref n="5" /> and Alexander Pérez<fnref n="6" />*

## Abstract

The assessment of mercury (Hg) contamination in Peruvian mangrove sediments was conducted across two contrasting environments: the anthropogenically impacted mangrove forest of "Puerto Pizarro" (PP), affected by shrimp aquaculture, mining activities, and urban expansion, and the relatively pristine Mangrove Sanctuary of Tumbes (MS). In PP, Hg concentrations ranged from 291 to 177 ng g⁻<fnref n="1" />, yielding a Pollution Index (PI) of 1.6 and exceeding the sediment quality guideline defined by the Effect Range Low (ERL; 150 ng g⁻<fnref n="1" />), indicative of potential Hg toxicity. In contrast, Hg concentrations in MS ranged from 135 to 17 ng g⁻<fnref n="1" />, remaining below the ERL and within natural background levels reported for Peruvian marine sediments. Using the estimated mean background concentration in MS sediments (70 ng g⁻<fnref n="1" />), enrichment factors of up to 3.2 were observed in PP sediments. Mercury concentrations in MS exhibited significant positive correlations with fine-grained sediments (silt and clay; r = 0.66) and reactive iron phases (r = 0.70), reflecting natural geochemical controls on Hg accumulation. Conversely, no significant correlations were detected in PP, suggesting that anthropogenic inputs override sedimentological and geochemical processes. This study provides the first assessment of Hg concentrations in mangrove ecosystems located at the eastern South Pacific distributional limit and highlights the role of mangroves as effective biogeochemical barriers that mitigate Hg transfer to adjacent coastal ecosystems.

**Keywords** Mercury contamination, Mangrove sediments, Anthropogenic impact, Enrichment factor, Biogeochemical barrier

## 1 Introduction

Mangroves are highly productive coastal ecosystems of significant ecological and socioeconomic importance. These environments support fisheries and aquaculture activities (Nordlund et al. 2018; Jänes et al. 2020; Jinks et al. 2020) and play a crucial role in regulating the carbon and nutrients cycle in coastal zones (Chmura et al. 2003; Duarte et al. 2013; Pérez et al. 2017). Additionally, mangrove ecosystems function as biogeochemical barriers to pollutants, including the influx of heavy metals resulting from anthropogenic activities (Lacerda et al. 2001; Machado et al. 2016). Mangrove sediments have been identified as natural sinks for heavy metals such as mercury (Hg), which exhibit a strong affinity for organic and sulfidic compounds (Lacerda et al. 2001; Ding et al. 2009; Machado et al. 2016). However, factors such as

<footnote>*Correspondence:
Alexander Pérez
alexander.perez.s@upch.pe</footnote>

<footnote n="1">¹ Programa de Geoquímica, Departamento de Geoquímica, Instituto de Química, Universidade Federal Fluminense, Niterói, Brasil</footnote>

<footnote n="2">² Departamento de Química Analítica, Instituto de Química, Universidade Federal Fluminense, Niterói, Brasil</footnote>

<footnote n="3">³ Instituto de Biofísica, Universidade Federal Do Rio de Janeiro, Rio de Janeiro, Brasil</footnote>

<footnote n="4">⁴ Faculty of Science and Engineering, National Marine Science Centre, Southern Cross University, Coffs Harbour, NSW, Australia</footnote>

<footnote n="5">⁵ Laboratoire des Sciences du Climat et de l'Environnement-IPSL, Unité Mixte CEA-CNRS-UVSQ (UMR 8212), Université Paris-Saclay, Gif-sur-Yvette, France</footnote>

<footnote n="6">⁶ Laboratorios de Investigación y Desarrollo (LID), Facultad de Ciencias e Ingeniería (FACI), Centro de Investigación para el Desarrollo Integral y Sostenible (CIDIS), Universidad Peruana Cayetano Heredia, Lima 15102, Peru</footnote>
anthropogenic influences, hydrological dynamics, geo-
morphological characteristics, and environmental dis-
turbances can alter sediment geochemistry in mangrove
areas (Pérez et al. 2017, 2021), potentially affecting their
capacity to retain specific metals. Studies have reported
that mercury (Hg) concentrations in preserved man-
grove eco-systems are typically below 100 ng g⁻¹, a value
considered representative of background levels for such
environments (Alam et al. 2010; Lei et al. 2019). How-
ever, determining regional background concentrations
of heavy metals presents significant challenges, as areas
classified as pristine may nonetheless be exposed to local
and diffuse pollution over time (Chiappetta et al. 2016;
Lei et al. 2019). Establishing accurate background con-
centrations is essential for defining baseline conditions in
coastal ecosystems, thereby enhancing understanding of
the factors that influence pollution dynamics (De Paula
and Mozeto 2001; Abessa et al. 2018). Further research
on Hg pollution in South American regions is warranted
due to the prevalence of uncontrolled anthropogenic
activities, which may compromise key mangrove eco-
system functions such as metal retention (Abessa et al.
2018; Canham et al. 2021). Notably, mangrove ecosys-
tems in these regions are frequently affected by shrimp
farming, mining operations, and urban expansion, all of
which contribute to anthropogenic Hg influx into coastal
waters (Lacerda et al. 2011; Pérez et al. 2020). Sedimen-
tary mercury (Hg) concentrations along Peruvian coastal
systems were initially examined within a global compara-
tive study of ancient and modern deposition (Shen et al.
2020); however, specific values have yet to be reported.
This study aims to estimate Hg concentrations in surfi-
cial sediments from two contrasting mangrove areas in
the Tumbes region of northern Peru: the impacted man-
groves of “Puerto Pizarro” (PP), which are influenced by
anthropogenic activities, and the Mangrove Sanctuary of
Tumbes (MS) (Pérez et al. 2017, 2020). Additionally, this
study aims to provide the first estimation of Hg back-
ground levels and pollution status for ecosystems located
at the mangrove distribution boundary of the eastern
South Pacific Ocean.

**2 Materials and methods**

**2.1 Study area and sampling**

The study was conducted in two estuarine mangrove sys-
tems (Fig. 1) located within the Tumbes region of north-
ern Peru, separated by approximately 13 km. Both sites
are subject to a semi-diurnal tidal regime with an aver-
age tidal amplitude of ~1.5 m and occur under a semi-
arid climatic setting (Pérez et al. 2017). Both mangrove
forests fringing the tidal creeks are dominated by *Rhiz-
ophora mangle*, *Rhizophora harrisonii*, and *Avicennia
germinans* (Pérez et al. 2017; Dioses et al. 2023). These

ecosystems support predominantly subsistence-based
activities, including shellfish harvesting, artisanal fish-
ing, small-scale agriculture, and wood extraction (Pérez
et al. 2017, 2020; Martínez Cabrera 2022). Despite these
similarities, the two mangrove systems differ markedly
in their degree of anthropogenic pressure. The “Puerto
Pizarro” mangroves (PP), located adjacent to a small har-
bor, are exposed to largely uncontrolled human activities,
including shrimp aquaculture, mining, and urban expan-
sion. In contrast, the Mangrove Sanctuary of Tumbes
(MS) is subject to environmental protection and man-
agement policies that regulate and restrict such activities
(Montero 2019; Zapata-Cruz et al. 2024). Mining activi-
ties occur upstream of both study sites along the Tumbes
River (PP) and the Zarumilla River (MS), respectively
(Fig. 1). However, in the case of MS, the presence of the
International Channel located north of the sanctuary acts
as a natural hydrodynamic barrier, limiting the transport
of mining-derived contaminants into the mangrove eco-
system (Pérez et al. 2017). The sampling campaign was
carried out on 15 April 2018 during daylight hours under
low-tide conditions. Surficial subtidal sediments (0–2 cm
depth) were collected in duplicate from 14 stations
within the Mangrove Sanctuary (MS; T1–T14) (Fig. 1A)
and from 8 stations within “Puerto Pizarro” mangroves
(PP; P1–P8) (Fig. 1B) using a hand-operated sediment
corer (Model 77258, WILDCO). Samples were immedi-
ately sealed in plastic bags, stored on ice in the field, and
transported to the laboratory, where they were thawed
and lyophilized prior to chemical analyses.

**2.2 Experimental approach**

For textural composition analysis, 2 g of sediment were
disaggregated following a 16-h agitation in 40 mL of a
4% sodium hexametaphosphate solution (Loring and
Rantala 1992). The silt and clay content was subse-
quently determined using a CILAS 1064 grain size ana-
lyzer to assess the relationship between fine particles
and Hg concentration. Reactive Fe was quantified by
determining 1 M HCl-extractable Fe concentrations
(Huerta-Diaz and Morse 1990) using inductively cou-
pled plasma optical emission spectrometry (ICP-OES,
PerkinElmer Optima 7300) to evaluate the association
between total Hg and Fe oxyhydroxides in coastal sedi-
ments (Harbison 1986; Gagnon et al. 1997; Alongi et al.
2004). This analysis includes Fe oxyhydroxides with
some contribution from non-crystalline Fe sulfides and
Fe carbonates (Huerta-Diaz and Morse 1990; Leventhal
and Taylor 1990). Finally, total Hg concentration in the
sediment was determined using the digestion method
(Malm et al. 1989). For this analysis, duplicate sediment
subsamples (0.5 g dry weight) were digested using a 1:3
HCl:HNO₃ acid mixture. Mercury concentrations were
<figure><img src="image_4.png" /><figcaption>**Fig. 1** Sampling stations across the two study areas. A The Mangrove Sanctuary (MS), encompassing stations T1 through T14. B "Puerto Pizarro" mangroves (PP), comprising stations P1 through P8. Shrimp farming areas are delineated in red</figcaption></figure>

subsequently determined by cold vapor atomic absorption spectrometry (CVAAS) coupled to a flow injection system (PerkinElmer FIMS 400), following established procedures (Bastos et al. 1998; Machado et al. 2016). The obtained Method Detection Limits (MDL) was 3 ng g⁻¹, calculated according to the formula: (3×Sb)/Xb×(V/M), where Sb is the standard deviation of 4 measurements

of the procedural blanks, Xb is the mean of the angular coefficient of the calibration curve, V is the final volume of the sample solution and M is the sample mass (Lino et al. 2016). The recovery of 3.10±0.01 µg g⁻¹ (n=2) was obtained for a certified reference material (PACS-3 marine sediment) that has a certified value of 2.98±0.36 µg g⁻¹ for total Hg. Analytical precision,
assessed through replicate analyses, was better than 5%, expressed as relative standard deviation (RSD), which is consistent with previous applications of this technique in mangrove and coastal sediments (e.g. Machado et al. 2016).

# 3 Results and discussions

## 3.1 Mercury concentrations and toxicity risk

The results of sedimentary Hg concentrations, together with the percentages of silt and clay and the concentrations of reactive Fe for both study areas, are presented in Fig. 2. In the Mangrove Sanctuary (MS), Hg concentrations ranged from 135 to 17 ng g⁻¹, with a mean value of 70±31 ng g⁻¹ (Fig. 2A). In contrast, sediments from “Puerto Pizarro” mangroves (PP) exhibited substantially higher Hg concentrations, ranging from 291 to 177 ng g⁻¹, with an average of 237±42 ng g⁻¹ (Fig. 2A). Mercury concentrations in MS are comparable to those reported

for protected or minimally disturbed mangrove ecosystems, where values typically remain below 100 ng g⁻¹ (Chiappetta et al. 2016; Lei et al. 2019). In such environments, Hg accumulation is largely governed by natural sedimentological and geochemical processes, particularly the preferential association of Hg with fine-grained particles and reactive iron phases (Gagnon et al. 1997; Liu et al., 2016). Consistent with this framework, Hg concentrations in MS exhibited significant positive correlations with the silt and clay fraction (r=0.66) and with reactive Fe concentrations (r=0.70) (Fig. 3), indicating that Hg retention is controlled by adsorption onto fine sediments and Fe (oxyhydr)oxides, which are abundant in mangrove substrates (Liu et al., 2016; Lei et al. 2019). Although previous studies have shown that nutrient inputs derived from shrimp aquaculture can reach MS sediments (Pérez et al. 2018, 2020), and that aquaculture facilities may act as potential sources of Hg in certain coastal settings (Lacerda et al. 2011; Ma et al. 2020), the preservation of these

Mangrove Sanctuary

<figure><img src="image_5.png" /></figure>

“Puerto Pizarro” mangroves

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /><figcaption>**Fig. 2** A Mercury (Hg) concentrations. B Silt and clay content. C Percentage concentrations of reactive iron (Fe) across both study areas</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Fig. 3** A Correlation between mercury (Hg) concentration and silt and clay content. B Correlation between Hg concentration and the percentage concentration of reactive iron (Fe)</figcaption></figure>

natural geochemical associations suggests that external Hg inputs to MS are minimal or efficiently buffered. As a result, Hg concentrations in MS remain within, or below, the natural background range reported for Peruvian coastal sediments (30–144 ng g⁻¹) (Shen et al. 2020). In contrast, Hg concentrations in PP not only exceeded background levels but also displayed no significant correlation with either fine sediment content or reactive Fe (Fig. 3). This decoupling indicates that Hg accumulation in PP is not primarily controlled by natural sedimentary or mineralogical factors but rather reflects the dominance of anthropogenic Hg inputs that override intrinsic geochemical controls. Similar disruptions of Hg–sediment–Fe relationships have been reported in mangrove systems subjected to intense urbanization, mining, and aquaculture pressures (Lei et al. 2019). The potential toxicity risk associated with sedimentary Hg was further evaluated using established sediment quality guidelines. Sedimentary Hg concentrations in PP consistently exceeded the Effects Range Low (ERL; 150 ng g⁻¹) but remained below the Effects Range Median (ERM; 710 ng g⁻¹) (Long et al. 1995; Morse 1994; García-Ordiales et al. 2023), indicating

moderate contamination with potential ecological implications. By contrast, Hg concentrations in MS remained below both ERL and ERM thresholds, further confirming that Hg retention in this protected mangrove system is governed by natural biogeochemical processes rather than by pollution-driven accumulation (Fig. 2A).

## 3.2 Mercury associations in mangrove sediments

The percentages of silt and clay (Fig. 2B) and the concentrations of reactive Fe (Fig. 2C) did not display a clear spatial gradient between the two study areas. Nevertheless, in the Mangrove Sanctuary (MS), both variables exhibited significant and positive correlations with Hg concentrations (Fig. 3), whereas no such relationships were observed in “Puerto Pizarro” mangroves (PP) (Fig. 3). This contrast reflects fundamental differences in the dominant controls governing Hg accumulation between the two mangrove systems. In MS, the observed correlations are consistent with the well-established natural affinity of Hg for fine-grained sediments and reactive iron phases in relatively pristine estuarine and mangrove environments (Harbison 1986; Gagnon et al. 1997; Alongi et al. 2004). These relationships indicate that Hg retention in MS is largely controlled by sedimentological and geochemical processes. Moreover, the strength of these correlations suggests that local geomorphological and hydrological factors, such as sediment provenance, tidal exchange, and low-energy depositional conditions, play a key role in regulating the spatial variability of Hg within the sanctuary (Pérez et al. 2017, 2018). By contrast, the absence of significant correlations between Hg and either fine sediment fractions or reactive Fe in PP indicates a decoupling between Hg accumulation and natural sedimentary controls. “Puerto Pizarro” mangroves are subjected to multiple, largely unregulated anthropogenic pressures, including fishing activities, boat traffic, urban expansion, and aquaculture (Montero 2019; Zapata-Cruz et al. 2024). Under such conditions, elevated Hg concentrations are more likely driven by direct or diffuse anthropogenic inputs that override intrinsic geochemical associations, a pattern commonly reported in impacted estuarine systems (Harbison 1986; Gagnon et al. 1997). Importantly, both study areas exhibited comparable silt and clay contents, with values ranging from 69–100% in MS and 74–100% in PP (Fig. 2B), indicating that differences in sediment texture alone cannot account for the markedly higher Hg concentrations observed in PP (Fig. 2A). Instead, the combination of similar fine-grained sediment composition and contrasting Hg levels further supports the interpretation that Hg enrichment in PP is predominantly anthropogenic in origin. Mercury concentrations in MS (17–135 ng g⁻¹) were comparable to natural background levels reported for Peruvian coastal
sediments (30–144 ng g⁻¹; Shen et al. 2020) and other oceanic systems (50–80 ng g⁻¹; Gworek et al. 2016). These values are therefore interpreted as representative of background Hg conditions, supporting their suitability for use in enrichment factor (EF) calculations, as discussed in the following subsection. Accordingly, the average Hg concentration measured in MS (~70 ng g⁻¹) may serve as a robust regional baseline for assessing Hg contamination in Peruvian mangrove ecosystems. Notably, despite the presence of uncontrolled mining activities upstream of the Mangrove Sanctuary, no evidence of Hg inflow or sedimentary contamination was detected within MS (Pérez et al. 2020; Martínez Cabrera 2022). This observation further emphasizes the role of local hydrodynamic and geomorphological barriers in limiting contaminant transport and reinforces the value of MS as a reference site for evaluating anthropogenic Hg enrichment in adjacent mangrove systems (Pérez et al. 2017).

## 3.3 Anthropogenic mercury pollution in sediments

The degree of anthropogenic Hg contamination in sediments is commonly evaluated using the enrichment factor (EF), which quantifies Hg concentrations relative to a defined background level (Selvaraj et al. 2004; Selvaggi et al. 2020). In this study, EF values were calculated using the expression $EF = [(C_{sed} - C_{back})/C_{back}]$, where $C_{sed}$ represents the measured Hg concentration in sediments and $C_{back}$ corresponds to the estimated background concentration (~70 ng g⁻¹) derived from the Mangrove Sanctuary (MS). An EF value greater than 1 indicates the presence of anthropogenic Hg inputs (Adamo et al. 2005; Buruaem et al. 2012). In parallel, the potential ecological risk associated with sedimentary Hg contamination was assessed using the Pollution Index (PI), defined as $PI = [Ci/Si]$, where Ci denotes the Hg concentration in sediments and Si corresponds to the sediment quality guideline, specifically the Effect Range Low (ERL; ~150 ng g⁻¹). According to established criteria, PI values ≤1 indicate unpolluted conditions, 1≤PI<3 reflect low to moderate pollution, 3≤PI<5 indicate a mid-level pollution risk, and PI values >5 correspond to heavy pollution (Long et al. 1995; Davaulter and Rognerud 2001; Chen et al. 2012). Based on these indices, sediments from “Puerto Pizarro” mangroves (PP) exhibited EF values ranging from 1.5 to 3.2 (Fig. 4A), clearly indicating anthropogenic Hg enrichment, with concentrations up to approximately 350% higher than those observed under protected conditions in MS. Likewise, PI values in PP ranged from 1.2 to 1.9 (Fig. 4B), suggesting low to moderate pollution levels with potential ecological implications. In contrast, sediments from MS consistently showed EF values close to or below unity and PI values <1, reinforcing

its classification as a reference site with respect to Hg contamination. It is important to recognize that Hg accumulation in these mangrove systems is modulated by dynamic hydrodynamic and climatic processes. Both study areas are influenced by semidiurnal tidal currents and seasonal variations in freshwater discharge, which control sediment transport, deposition, and redistribution (Pérez et al. 2017, 2018, 2020). In addition, the region is periodically affected by El Niño events, which are associated with anomalous rainfall and enhanced riverine sediment fluxes (Lagos et al. 2008; Lavado and Espinoza 2014). Such episodic events may intensify sediment erosion, resuspension, and lateral redistribution, altering grain-size composition and redox-sensitive geochemical conditions, thereby exerting a strong but intermittent control on Hg burial and spatial variability (Machado et al. 2002; Ding et al. 2009). While EF and PI provide robust metrics for identifying anthropogenic Hg enrichment and assessing potential ecological risk, they are based exclusively on total Hg

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>**Fig. 4** Mercury (Hg) Pollution in Puerto Pizarro (PP). **A** Hg enrichment factor (EF) relative to the estimated regional background level. **B** Hg pollution index (PI) based on the Effect Range Low (ERL) threshold for Hg concentrations in marine and estuarine sediments (Long et al. 1995; Gworek et al. 2016). Values surpassing the red dotted line in both EF and PI indicate areas affected by anthropogenic Hg sources and potentially hazardous Hg levels for marine biota</figcaption></figure>
concentrations and therefore do not capture informa-
tion on Hg speciation, bioavailability, or transforma-
tion pathways. In sulfidic and organic-rich mangrove
sediments, Hg can undergo complex biogeochemi-
cal transformations, including microbial methylation,
leading to the formation of methylmercury, the most
toxic and bioaccumulative Hg species. Consequently,
elevated total Hg concentrations do not necessarily
translate directly into increased ecological risk, nor do
low concentrations preclude the potential for methyl-
mercury production (Hsu-Kim et al. 2013; Benoit et al.
2001). Future research should therefore prioritize the
characterization of Hg speciation and the assessment
of methylation potential in these mangrove systems,
with particular emphasis on the role of redox condi-
tions, sulfate-reducing microbial communities, organic
matter quality, and iron–sulfur interactions. Integrat-
ing total Hg measurements with methylmercury quan-
tification and process-based indicators would provide
a more comprehensive evaluation of Hg bioavailability
and ecological risk in Peruvian mangrove ecosystems,
especially under scenarios of increasing anthropogenic
pressure and climate-driven hydrological variability.

3.4 Regional comparison of mercury concentrations

Sedimentary mercury (Hg) concentrations in Peru-
vian mangroves were evaluated within a global context
(Fig. 5). In the present study, the maximum Hg con-
centration measured in Puerto Pizarro (PP) reached
291 ng g⁻¹, a value comparable to peak concentrations
reported for mangrove systems in Tanzania (190 ng g⁻¹)
and Mexico (233 ng g⁻¹) (Lacerda 1998; Lei et al. 2019).
In contrast, Hg concentrations in PP were approxi-
mately one order of magnitude lower than those docu-
mented in heavily polluted mangrove environments in
China (1031 ng g⁻¹) and the United States (1020 ng g⁻¹)
(Shi et al. 2020; Duan et al. 2021). Moreover, mangroves
impacted by historical chloralkali industry activities in
Brazil exhibit extreme Hg enrichment, with concen-
trations reaching up to 14,400 ng g⁻¹, exceeding values
observed in PP by two orders of magnitude (Lacerda et al.
2020). These comparisons indicate that Hg concentra-
tions in PP are characteristic of coastal ecosystems sub-
jected to moderate anthropogenic pressure, rather than
severe industrial contamination (Lacerda 1998; Lei et al.
2019). Consequently, Hg levels in Peruvian mangrove
sediments fall within the range reported for naturally to
moderately contaminated mangrove systems worldwide

<figure><img src="image_14.png" /><figcaption>**Fig. 5** Comparison of mercury (Hg) concentrations in Peruvian mangrove sediments with those reported in other intertropical mangrove regions worldwide. Data sourced from Lacerda (1998) and Lei et al. (2019) with additional comparisons including findings from China (Shi et al. 2020), the United States (Duan et al. 2021; Li et al. 2022) and Brazil (Araujo et al., 2021)</figcaption></figure>
(Fig. 5). Nevertheless, despite being comparatively lower than those in heavily polluted sites, Hg concentrations of this magnitude may still pose ecological risks, particularly in dynamic mangrove environments where biogeochemical conditions can favor Hg mobilization and transformation (Lei et al. 2019; Li et al. 2022). In this context, site-specific hydrological processes are likely to exert a strong control on Hg transport, deposition, and potential bioavailability. For instance, prolonged rainy periods in semi-arid mangrove systems may enhance the transport of dissolved Hg from upstream sources (Lacerda et al. 2020). Conversely, during dry periods, Hg accumulation may be intensified by altered flood regimes and increased organic matter deposition, conditions that can promote Hg retention and methylation in sediments affecting the benthic community and mangrove vegetation (Lei et al. 2019; Li et al. 2022). Importantly, mangrove vegetation cover plays a key mitigating role by stabilizing sediments, enhancing particle trapping, and reducing Hg remobilization, as previously documented in other intertropical mangrove systems subject to strong hydrological variability (Pérez et al. 2017, 2018). Further research is therefore required to better constrain Hg export pathways from sediments in this semi-arid mangrove ecosystem, particularly with respect to Hg interactions with iron (Fe) phases and organic matter. These interactions are known to regulate not only Hg sequestration and mobility, but also its transformation into more toxic and bioavailable forms, with potential ecological consequences at regional scales such as bioaccumulation in benthic organisms, disruption of benthic community structure, and physiological stress on mangrove vegetation (Lacerda et al. 2001; Shen et al. 2020). A refined understanding of these processes is essential to assess the long-term ecological risks associated with Hg in dynamic mangrove environments (Lacerda et al. 2001, 2020; Sanders et al. 2012).

## 4 Conclusions

This study provides the first quantitative assessment of mercury (Hg) concentrations in mangrove ecosystems located at the distribution boundary of the eastern South Pacific. Beyond establishing baseline Hg levels, the results highlight the critical role of Peruvian mangroves as effective biogeochemical barriers that can retain Hg within sediments and thereby limit its export to adjacent coastal environments. This function is particularly relevant in semi-arid coastal settings, where hydrological variability and increasing anthropogenic pressures may enhance contaminant mobility and ecological exposure. Further research is therefore required to better resolve the temporal and spatial dynamics of Hg concentrations, especially in response to changing hydrological regimes, land-use pressures, and biogeochemical interactions

within mangrove sediments. Importantly, the findings presented here have direct implications for environmental management, as they provide a scientific basis for incorporating mangrove conservation and restoration into local and regional pollution mitigation strategies. By acting as natural sinks for Hg, these ecosystems play a key role in reducing ecological risks to benthic communities and coastal food webs, underscoring their importance for safeguarding the environmental quality of Peru's highly vulnerable coastal regions.

## Acknowledgements

AP was supported by the Programa Nacional de Investigación Científica y Estudios Avanzados (PROCIENCIA – PERU) through the project "Humedales Altoandinos: Evaluación del cambio climático y capacidad de mitigación" (Grant No. PE501078570-2022-PROCIENCIA). WM was supported by the Brazilian Research Council (CNPq) and CAPES/PRINT/UFF (Grant No. 88881.310184/2018-01), while CJS was supported by the Australian Research Council (Grants DE160100443, DP150103286, and LE140100083).

## Author's contributions

WM and AP: conceptualization, writing and original draft preparation. WM, CJS and MC: conceptualization, review and editing. BR, VF, AR, CD, AL and OM: methodology, analyses and validation. All authors have read and agreed to the published version of the manuscript.

## Data availability

The dataset generated during the current study is available from the corresponding author on reasonable request.

## Declarations

### Ethics approval and consent to participate

N/A.

### Competing interests

No competing interest.

Received: 10 November 2025 Revised: 14 January 2026 Accepted: 20

January 2026

Published online: 30 January 2026

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