Geochemical characteristics of suspended particulate matter around Piscadera Bay and its influence on near shore ecosystems, Cura&ccedil;ao (Caribbean Sea)

Sánchez Barranco, Virginia; van de Loosdrecht, Nienke C. J.; Mienis, Furu; de Goeij, Jasper M.; Hennekam, Rick; Reichart, Gert-Jan; Stuut, Jan-Berend; de Nooijer, Lennart J.

doi:10.5194/egusphere-2025-4873

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https://doi.org/10.5194/egusphere-2025-4873

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28 Nov 2025

| 28 Nov 2025

Geochemical characteristics of suspended particulate matter around Piscadera Bay and its influence on near shore ecosystems, Curaçao (Caribbean Sea)

Virginia Sánchez Barranco, Nienke C. J. van de Loosdrecht, Furu Mienis, Jasper M. de Goeij, Rick Hennekam, Gert-Jan Reichart, Jan-Berend Stuut, and Lennart J. de Nooijer

Abstract. Caribbean coral reefs face rising pressure from coastal development, yet the pathways by which urban pollution reaches these endangered ecosystems remain poorly understood. Bays act as dynamic channels, trapping, transforming, and releasing materials that can impact adjacent reef systems. We investigated the seasonal and spatial variability of suspended particulate matter (SPM)—a key vector for pollutants and nutrients— coming from an urbanized bay in Curaçao and determined its effect on surrounding coral reefs. Using sediment traps deployed across spatial gradients (bay mouth to nearby reefs in the East and West) during the dry (April–May) and wet (October–November) seasons, we measured mass, carbon, and nitrogen fluxes and associated grain-size and geochemical particle composition. Results were compared to environmental conditions (e.g. rain fall, current speed) and revealed a clear spatial gradient of bay influence: the bay mouth showed the strongest terrestrial signal, followed by the eastern reef (sheltered from currents) with elevated SPM fluxes of fine particles enriched in terrigenous elements (Si, Fe, Al, and Mn), while the western reef (exposed to open-ocean flow) exhibited lower fluxes of coarser particles with elevated Ca/Fe, Pb, Cu and Ni. This indicates diminished bay effect and stronger marine influence mixed with localized pollution. During the dry season, differences in SPM fluxes and composition between reef sites were minimal, but wet season conditions amplified spatial patterns, with rainfall-driven runoff locally increasing dissolved and particular matter delivery. This implies that reef vulnerability to bay-derived pollution locally depends on both proximity to source waters and seasonal hydrodynamic variability, with sheltered reefs experiencing the greatest impacts during periods of enhanced terrestrial runoff.

Received: 02 Oct 2025 – Discussion started: 28 Nov 2025

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Virginia Sánchez Barranco, Nienke C. J. van de Loosdrecht, Furu Mienis, Jasper M. de Goeij, Rick Hennekam, Gert-Jan Reichart, Jan-Berend Stuut, and Lennart J. de Nooijer

Status: final response (author comments only)

RC1:
'Comment on egusphere-2025-4873', Anonymous Referee #1, 24 Dec 2025
Review Comments on “Suspended Particulate Matter (SPM) dynamics around Piscadera Bay”.

This manuscript presents Important information on the spatial gradient of land-sea connection. However, in order to rule out normalisation artefacts, the interpretation of trace metal enrichment at the distal site necessitates thorough verification against absolute concentrations. Accurately estimating the new pollution load entering the reef system also depends on differentiating resuspension from primary flow. Some of the observations for the document are as follows:
The dry and wet seasons are represented by two 30-day deployment periods used in the study. Although this provides an overview, previous research on sediment dynamics (e.g., Storlazzi et al., 2011) highlights that sediment movement is frequently very episodic and event-driven. One high-rainfall event on November 2 has a significant impact on the "wet season" results in this publication. Therefore, a 30-day window might not be adequate to describe the inter-seasonal fluctuation. The clear wet season signal may not be a seasonal baseline, but rather an anomaly. Since the frequency of these pulse episodes is still unknown, care must be taken while comparing these short-term fluxes to yearly benthic degradation rates.

In shallow, high-energy settings, sediment traps were placed 0.7–0.8 meters above the seafloor. The study acknowledges that resuspension likely affects the material collected, but the precise amount of new sediment entering the reef remains unknown due to the lack of techniques to distinguish between primary terrestrial input and resuspended bottom sand (gross vs. net flux).

The Western reef has the highest ratios of anthropogenic trace elements (Pb/Al, Cu/Al, Ni/Al), while being the furthest from the bay and having the lowest terrestrial signal (low Al, Fe). Aluminium (Al) normalisation is commonly used to account for dilution effects and grain size. Aluminium (Al) might falsely inflate these ratios when used to normalise data in carbonate-rich environments with extremely low Al concentrations. However, in carbonate-rich, low-terrigenous environments (like the Western reef), Al concentrations become extremely low. This could result in a false positive for high pollution intensity while absolute concentrations may actually be low. The authors attribute this to marine influence or localised pollution. To ascertain whether this is actual accumulation or a concentration by lack of dilution artefact, a critical examination of the absolute fluxes of these metals, rather than merely ratios is required.

X-ray fluorescence (XRF) intensities expressed as centred log-ratios (CLR) are the basis for the elemental analysis. Absolute concentrations (such as mg/kg) are not provided by this semi-quantitative approach. As a result, it is impossible to ascertain whether the elevated metal levels are simply high in comparison to the background or truly above established toxicity criteria.

The study uses spatial patterns rather than conclusive source tracking (e.g., stable isotope analysis) to attribute trace metals (Pb, Ni) to anthropogenic sources like sewage or maritime activity. Pb and Ni may be scavenging onto marine organic matter rather than terrestrial clays, based on their clustering with marine/organic vectors. Organic matter is a powerful scavenger of metals in the water column, according to earlier research (e.g., Salomons and Ostner). The difference between bio-accumulation and clay-bound movement is crucial for deciphering anthropogenic or terrestrial source.

In this study, SPM is attributed to resuspension during the dry season at the Western Reef. Both primary settling flux and resuspended bottom sediment are collected by sediment traps in high-energy shallow reefs (current speeds 0.01–0.07 m/s). It is challenging to differentiate new terrestrial input from old resuspended sediment in the absence of differential settling velocity data or bottom-stress modelling. Because resuspension events are temporary and may not indicate permanent burial, the measured mass fluxes (>10 g m^-2 d^-1) may exceed the true sedimentation burden on the corals.

The study connects the observed dominance of cyanobacteria and turf algae, especially in the Eastern reef, to the significant SPM fluxes. The typical 10 g m^-2 d^-1 stress threshold suggested by Rogers (1990) is greatly exceeded by the reported fluxes (18–40 g m^-2 d^-1) during the wet season. Nonetheless, the analysis suggests a clear causal relationship between the long-term benthic shift and the bay's present flow. It is crucial to consider whether the present SPM regime is responsible for the transition or for maintaining the current algal-dominated state given the 40-year history of degradation stated. The manuscript mentions a feedback loop that might be further investigated because turf algae are better at capturing sediments than corals.

Based on historical data and general literature, the study deduces ecological effects (such as algal dominance and coral damage). To directly connect the reported SPM flows to the current state of reef health, no concomitant biological health measures (such as tissue analysis, photosynthetic efficiency, or coral bleaching surveys) were taken during the investigation.

For the dry season, current speed and direction (ADCP) data were gathered for very brief periods of time (e.g., just 7 days at the mouth and 4 days at the eastern reef). The flushing theory cannot be fully validated, nor can sediment transport pathways be robustly modelled due to the lack of extensive dry-season hydrodynamic data.

The study was carried out in 2023, an El Niño year, and the authors report increased rainfall and runoff. As a result, the indicated fluxes and terrestrial inputs can be unusually high and not indicative of Curaçao's regular climatological circumstances.

The Piscadera Bay is the only subject of the investigation in the present study. Since differing bay geometries and orientations would alter the hydrodynamic shadow effects observed here, the authors acknowledge that it is challenging to extrapolate these particular spatial gradients to other bays in the absence of comparable data.

In order to explain the strong pollution signal despite low terrestrial flow, the study suggests that fluxes and deposits are decoupled at the Western reef. The precise mechanism whether it results from biofilm scavenging or reef structure trapping remains speculative and has not been investigated experimentally.

Before this manuscript can be taken into consideration, the authors must address the aforementioned issues. The manuscript has to be significantly revised.
Citation: https://doi.org/10.5194/egusphere-2025-4873-RC1
RC2:
'Comment on egusphere-2025-4873', Anonymous Referee #2, 06 Apr 2026
Review Comments on “Geochemical characteristics of suspended particulate matter around Piscadera Bay and its influence on near shore ecosystems, Curaçao (Caribbean Sea)”.
This manuscript investigates SPM magnitude and composition around Piscadera Bay on the southern coast of Curaçao. This study investigates the potential sources of SPM inputs based on environmental data and the elemental composition of SPM at the bay's mouth and at two reef locations east and west of the mouth during the dry and wet seasons of 2023. The manuscript is well-written, clear, and concise, but the figures need significant work.
The use of two 30-day periods from a single year (2023) is insufficient to characterize the true "seasonal variability", which typically requires observations spanning multiple years. Please revise the text to avoid broad terms like "seasonal variation" or "seasonal patterns". Instead, specifically refer to the "2023 wet and dry season conditions" or equivalent. If the term "seasonal" must be used, it should be explicitly qualified as the variability specific to the 2023 study period. I recommend adding a comparison of the key environmental forcings (e.g., wind, precipitation, currents) from 2023 against long-term historical records. This will help evaluate how representative 2023 was compared to a "typical" year. If the environmental conditions in 2023 are shown to be equivalent to long-term averages, the authors may then extend their conclusions and speculate on broader seasonal variability within the Discussion section.

I suggest adding current direction to Figure 2 (see comment below) to evaluate which side (east or west) of the mouth will be more impacted by sediment deposition. If the dominant currents are westward, this could explain why more pollutants settle on the western reef. Moreover, the shape of the coastline likely impacts the hydrodynamic profiles of each reef differently, especially because the dominant current flows northeast. The first mention of the current direction I found is line 676, even though lines 585-586 suggest that “differences in current speeds and direction … may influence the transport and resuspension”. Having the current direction in Figure 2 will already tip the reader off that the current is an important factor for the observed differences between the eastern and the western reefs. I suggest reorganizing the discussion accordingly.

Is there any data on wave energy and swell direction? While tidal currents increase the extraction of SPM from the bay, once deposited outside the bay, I would expect the wave and swell energy to be important factors for the resuspension of already deposited materials. The wind is mostly coming from the east-southeast, which would suggest the eastern reef is, on average, more sheltered than the western reef if the swell is also coming from the same direction. This difference may also explain the differences between sites and perhaps between the dry and wet seasons.

Can the presence of higher Pb and Ni on the western reef also be markers of pollution carried northwest by current coming from the Willemstad and Schottegat more than the Piscadera Bay, which has less industrial infrastructure? In the open ocean, higher concentrations of trace metals can be detected several hundreds of kilometers (if not more) from their source of input. Some of the measured signals might not only come from the Piscadera Bay but could also come from the nearby port of Schottegat, which is much larger, is much more industrialized, and has its mouth located “upstream” of the dominant currents. The authors should discuss this hypothesis more in depth, especially because it is difficult to trace the source of the pollutants with the set of measurements used. I suggest revising the interpretation of the results to consider this hypothesis.

Temperature and salinity data would have been a good markers for runoff to help link the high fluxes measured with increased runoff, and to highlight the differences between the elements coming from the Piscadera Bay versus those coming from Willemstad and Schottegat. A map of these parameters would have helped visualize the potential spatial extent of the sediment deposition affecting the reef without too much sampling effort.

Figure 2: Wind speed and direction panel: It is difficult to see the changes in wind speed via color change alone. I suggest using fixed-length and shorter vectors to represent wind direction only. Position the origin (starting point) of each vector along the Y-axis to represent the wind speed magnitude. Keep the current colormap for the vectors. This will provide an additional visual cue that, when combined with the Y-axis position, will significantly improve the readability of wind speed variations. Widen the colorbar of the temperature and pressure panel to improve readability. It would be useful to plot the direction of currents on the panel of current speed using the same plot style as the suggested plot of wind speed and direction (see above comment).

Figure 6: This figure should be improved. The PCA is not very informative, besides telling us that the terrigenous markers, the marine elements, and the others, cluster together, which is to be expected. I suggest re-doing this analysis and showing where the mouth, and east, and west reef sampling sites (dry and wet seasons separated) are located in the PCA space and plotting the variables measured (the elements) as vectors on the same space. That way, at a glance, the reader can compare the sampling locations (mouth, east reef, west reef) and the wet and dry seasons with respect to all the elements analyzed. Regarding the bottom panels, it took me some time to realize that the box plots do not represent concentrations of the elements but represent the distribution of the sampling sites in PCA space. I thought the results did not make any sense because I thought the differences in concentration between wet and dry seasons were opposite to what I would expect with increased runoff (and opposite to the results in the text). I find it very confusing and counter intuitive. I go back to my first suggestion to plot all the samples on the PCA space directly with different marker styles/colors for the dry and wet seasons, the west and east sides, and depths, and adding the vectors of the variables scaled to the PC1 and PC2 axes’ scales. Box plots could still be informative (if the new PCA is not just by itself already) but should represent concentrations of elements and not positions in PCA space. I also suggest renaming the cluster currently named “Trace metals” into something else (maybe related to pollution such as described in the text) because several elements in the other clusters are also trace metals.

Figure 9: The box for the eastern reef hides the part of the coast that I imagine plays an important role in the hydrodynamic differences between the eastern and western reefs under northwest current influence. I suggest moving these large boxes away from important spatial features to be shown on the map and using arrows to point to where these sampling sites are precisely located. The map coverage should be extended further south to show the shape of the coast and the point on the western side of the Piscadera neighborhood that likely impacts the hydrodynamics of the Piscadera Bay mouth significantly. I suggest adding a vector representing the dominant current (for both wet and dry seasons if different).

Lines 655-657: Please define “this material”, is it the pollutants, SPM or something else?

Please review the manuscript and figures for typos (see one example in the Figure 9 pie chart: “Cyanibacteria”).
Citation: https://doi.org/10.5194/egusphere-2025-4873-RC2

Virginia Sánchez Barranco, Nienke C. J. van de Loosdrecht, Furu Mienis, Jasper M. de Goeij, Rick Hennekam, Gert-Jan Reichart, Jan-Berend Stuut, and Lennart J. de Nooijer

Supplement

https://doi.org/10.5194/egusphere-2025-4873-supplement

Virginia Sánchez Barranco, Nienke C. J. van de Loosdrecht, Furu Mienis, Jasper M. de Goeij, Rick Hennekam, Gert-Jan Reichart, Jan-Berend Stuut, and Lennart J. de Nooijer

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Short summary

We studied how pollution from bays in Curaçao reaches nearby coral reefs and changes with seasons. By collecting suspended particulate matter at different locations during dry and wet seasons, we found that sheltered reefs receive more fine particles and pollutants, especially during rainy months, while exposed reefs are less affected. This shows that both location and seasonal rainfall determine reef vulnerability, highlighting the hidden impacts of human activity on these fragile ecosystems.


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