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

Status: this preprint is open for discussion and under review for Biogeosciences (BG).

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

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'Comment on egusphere-2025-4873', Anonymous Referee #1, 24 Dec 2025 reply
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.

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Citation: https://doi.org/10.5194/egusphere-2025-4873-RC1

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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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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