Water quality

Abstract

For this thesis a hydrochemical groundwater survey was carried out on the Caribbean island of Curaçao in 2020 as part of the NWO SEALINK project. In order to be well-adapted against anthropogenic and natural pressures, Curaçao needs representative data, yet thorough hydrochemical datasets were only sparsely collected in the wet seasons of 1977 and 1992. The aim of this thesis was to determine the current chemical state of the groundwater and analyze for long-term pollution trends with an extended database spanning four decades, also including data that was collected in another fieldwork campaign in 2021.

Curaçao was divided into a western and eastern section to facilitate data interpretation. Willemstad is located in the east, with a higher population density and most drinking/wastewater treatment plants and pipelines. The west is less populated, more rural and mostly agricultural. Four geological formations occur: Diabaas, Knip Group, Limestones, and Mid Curaçao Formation, of which the Diabaas is distinctly present as both Diabaas East (DE) and Diabaas West (DW). During the 2020 fieldwork campaign, wells that were sampled in 1992 were revisited (n=96), of which 20 could be remeasured, but 76 were inaccessible. To acquire more data, an additional 71 wells were sampled, bringing the total to 91 (neast=52; nwest=39). To obtain representativity, wells were as homogeneously distributed across the island as fieldwork conditions allowed for. 27 water quality parameters were analyzed in the field and lab (EC, DO, pH, turbidity, T, alkalinity, NO3, NO2, NH4, Al, B, Br, Ca, Cl, Fe, F, K, Mg, Na, Ni, P, PO4, S, Si, SO4, V and Zn). Datasets were assessed with a variety of boxplots, diagrams, descriptive and multivariate statistics (e.g. cluster analysis).

Of the groundwater wells measured in 2020, 29% were fresh, 53% slightly brackish (EC = 1.5 – 5 mS/cm), 17% brackish (5 – 15 mS/cm) and 1% saline. The majority was used for private irrigation (51%) or commercial agriculture (11%). The expectation was that the 2020 and a later obtained 2021 (n=72) dataset could be combined, but the results showed that this was not possible; reasons are further discussed within this thesis. To determine the influence of field methodology, wells measured in 2020 and 2021 (n=8) were compared for EC and sampling strategy, and wells situated closely together (n2020=7; 21-74 m) were assessed for heterogeneity (EC, pH, NO3, Ca, Cl), showing the effects of different sampling techniques (3 mg/L’92,’20,’21=81%, 87%, 65%; NO3>50 mg/L’92,’20,’21=38%, 39%, 47%), likely linked to wastewater and fertilizer inputs. Throughout the years, groundwater acidified, but the rural west did so with a higher onset pH (pH1977=7.83, DW) and steeper decline (ΔpH=0.7) than the urbanized east (pH1977=7.49; ΔpH=0.25, DE). An upward alkalinity trend is observed for the east, but not the west. Eastern acidification is likely caused by leaking wastewater. This is substantiated by higher eastern NO3 concentrations (east: 74±63 mg/L, west: 27±44 mg/L; p<0.001), more exposure to wastewater due to the presence of Willemstad, and a negative correlation between alkalinity and pH only found in the east, probably caused by the conversion of wastewater-related organics to HCO3/H+ . For the rural west, acidification is also attributed to wastewater pollution, but to a much lesser extent than the east. Instead, exploratory assessments point towards a plausible influence of atmospheric pollution spreading westward from the petrochemical facility “Isla Refinería” during its operating years. More vegetation, increasing the degradation of subsurface organic matter, is speculated to have an acidifying effect in the west, but no detailed analyses was done. Overall, further research into the causations and influences on long-term groundwater trends is recommended.

As was shown in this thesis, multi-annual datasets are valuable tools to disclose long-term groundwater quality trends regarding contamination, freshening, salinization and acidification. Such knowledge can greatly support the management and protection of groundwater resources and interlinked systems, such as marine aquatic life. This underlines the value of extended research, and also stresses the importance of an accessible network of groundwater wells, allowing for more representative data, further improving the advisory potential of such fieldwork campaigns.

This study aims to provide historical information on key environmental parameters and describe the current trends of coral growth in Bonaire. In particular, we aim to reconstruct changes in terrestrial runoff and bleaching stress events and establish their effects on the calcification rates for Siderastrea spp corals around Bonaire over multiple decades. For this coral cores of Siderastrea sppcorals will be used to reconstruct the changes in calcification and environmental parameters along a gradient of terrestrial influence and anthropogenic activity. During March 2023 15 coral cores were collected from Siderastrea spp colonies at four locations around the west coast of Bonaire.

Abstract
Anthropogenic stressors such as improper wastewater treatment, dump sites and construction sites can increase nutrient input into marine coastal waters, thereby declining water quality conditions for coral reefs and the surrounding marine environment. Increased nutrient flow in the form of dissolved inorganic nitrogen (DIN) and phosphorous (DIP) is taken up by phytoplankton leading to enhanced algal and cyanobacterial growth which can be measured by an increase of Chlorophyll – a (Chla). As part of the project, “Restoration of Nature and Resilience in the Dutch Caribbean”, water quality monitoring has been implemented on Dutch Caribbean islands to determine local stressors effecting costal water quality. Thus far, the role of land-based nutrient pollution has not been assessed on the Dutch Caribbean Island, Saba. This report analyses the water quality surrounding Saba’s coastal waters with a focus on Chla and nutrients and their relation to anthropogenic stressors on the island. We analysed the distribution of Chla and nutrients spatially and temporally. It is shown that Chla frequently exceeds the 0.2-0.3μM threshold around the harbour and in the two bays: Well’s Bay and Spring Bay/Cove Bay. Temporally, the highest elevated values can be attributed to the hurricane season form August-October. The first round of nutrient sampling from the week of 25-05-2022 shows that DIN exceeds its threshold for Cove Bay and Tent Reef, with ammonium contributing the largest to the DIN content. In future, attention needs to be paid to the DIN and DIP balance since high elevated ratios have been found at Tent Reef, possibly indicating DIP - limitation. With only one week of nutrient data available, no correlation between Chla and nutrients could be made. Overall, sites such as Tent Reef, Well’s Bay, and Spring Bay/Cove Bay demonstrate the most severely declining water quality based on frequently elevated Chla and nutrient levels. Further research on water quality data surrounding Saba is necessary to confirm these findings and to address stressors contributing to decreased water quality.

For full report or more information,  please contact erik.meesters@wur.nl or gulsah.dogruer@wur.nl

Summary
The health of coral reefs is threatened by anthropogenic land-based input, which is a global problem. High nutrient conditions make corals less resilient to environmental stresses like climate change and intense weather. Poor water quality is likely for the island of St. Eustatius due to the lack of sewage treatment and its erodible coastline. However, there are no data on this island’s long-term water quality monitoring. Chlorophyll-a concentrations, used to indicate water quality, were monitored at 13 locations around St. Eustatius twice a month from May to November 2022 (n=13). Additionally, images of the ocean floor at 10m were made using a remotely operated vehicle (ROV) to monitor benthic species communities and their habitat. The main conclusion of this research shows that the reefs are primarily in an algal-dominant state. This may be explained by the frequent, chronic exceedances of the 0.2 g/l chlorophyll-a threshold. Chlorophyll -a thresholds were surpassed more frequently and with higher amounts on the sites with a larger anthropogenic influence. The lower threshold for chlorophyll-a was surpassed at 5 out of the 11 sites by more than 30% of the measurements. This would point to a more pervasive low-level eutrophic condition at all sites. On many of the sand-based substrate areas, seagrass has covered it.

For full report or more information,  please contact erik.meesters@wur.nl or gulsah.dogruer@wur.nl

Abstract
The concurrent rise in the prevalence of conspicuous benthic cyanobacterial mats and the incidence of coral diseases independently markmajor axes of degradation of coral reefs globally. Recent advances have uncovered the potential for the existence of interactions between the expanding cover of cyanobacterial mats and coral disease, especially black band disease (BBD), and this intersection represents both an urgent conservation concern and a critical challenge for future research. Here, we propose links between the transmission of BBD and benthic cyanobacterial mats. We provide molecular and ecophysiological evidence suggesting that cyanobacterial mats may create and maintain physically favorable benthic refugia for BBD pathogens while directly harboring BBD precursor assemblages, and discuss how mats may serve as direct (mediated via contact) and indirect (mediated via predator–prey–pathogen relationships) vectors for BBD pathogens. Finally, we identify and outline future priority research directions that are aligned with actionable management practices and priorities to support evidence-based coral conservation practices.

MSc internship report

The project Restoration of resilience of nature and society in the Caribbean Netherlands aims to improve the resilience of the coral reefs in the BES (Bonaire, Sint Eustatius, Saba) islands, by mitigating the local stressors. One of the methodologies used is water quality monitoring. For this research, the focus was on chlorophyll a and turbidity temporal and spatial variability in Saba, and on how this could be related to anthropogenic and natural land-based local stressors. Chlorophyll a is a key water quality parameter which can give an insight on the eutrophic state of an area, while turbidity can be used as an indicator for coastal erosion and run-offs. These parameters were measured in 13 sampling sites, bi-weekly from May to August 2022, using two CTDs, a sensitive sensor technology. The land-based stressors were mapped as a result of qualitative analysis. Chlorophyll a values were highly variable, and often exceeded the safety threshold (0.2 μg/L) previously chosen, suggesting that the area was in a chronic state of eutrophication. Turbidity instead remained more stable and nearly always below the 3 NTU threshold. On one hand, spatial variability was not always directly linked to land-based stressors, and might be better interpreted including the action of sea currents. Temporal fluctuations, on the other hand, can be only partly explained by the amount of rainfall increasing run-offs from land to sea. Moreover, further research is needed to quantify local stressors, and long-term monitoring is necessary to fully understand the amplitude and nature of their influence on water quality.

For full report or more information,  please contact erik.meesters@wur.nl or gulsah.dogruer@wur.nl

MSc internship report

The health of coral reefs is threatened by anthropogenic land-based input, which is a global problem. High nutrient conditions make corals less resilient to environmental stresses like climate change and intense weather. Poor water quality is likely for the island of St. Eustatius due to the lack of sewage treatment and its erodible coastline. However, there are no data on this island’s long-term water quality monitoring. Chlorophyll-a concentrations, used to indicate water quality, were monitored at 13 locations around St. Eustatius twice a month from May to November 2022 (n=13). Additionally, images of the ocean floor at 10m were made using a remotely operated vehicle (ROV) to monitor benthic species communities and their habitat. The main conclusion of this research shows that the reefs are primarily in an algal-dominant state. This may be explained by the frequent, chronic exceedances of the 0.2 g/l chlorophyll-a threshold. Chlorophyll -a thresholds were surpassed more frequently and with higher amounts on the sites with a larger anthropogenic influence. The lower threshold for chlorophyll-a was surpassed at 5 out of the 11 sites by more than 30% of the measurements. This would point to a more pervasive low-level eutrophic condition at all sites. On many of the sand-based substrate areas, seagrass has covered it.

For full report or more information,  please contact erik.meesters@wur.nl or gulsah.dogruer@wur.nl

MSc report

Land based nutrient input from anthropogenic sources is a worldwide issue and a threat to coral reef health. Corals found in high nutrient environments are less resistant to global stressors such as rising temperature and extreme weather events. The absence of sewage treatment on St. Eustatius and its erosion prone coastline suggests poor water quality. Chlorophyll-a concentrations were monitored at 11 locations around St. Eustatius twice a month from May to August 2022 (n=7). Preliminary results on spatial and temporal variation of chlorophyll-a show an overall good water quality. Average concentrations did not exceed the environmental threshold of 0.3 μg/L at any of the sampled locations. Concentrations varied between locations and over time. Highest average chla values were found close to STUCO's brine outlet, the oil terminal and Golden Rock Dive & Nature Resort. Towards the end of the sampling period, chlorophyll-a concentrations were more variable between locations, which coincided with the onset of more frequent rain showers. No heavy rainfall occurred during the sampling period, so it was not possible to study the effects of higher precipitation on chlorophyll-a concentrations. Current results do not show reason for concern. However they should be re-evaluated when data on stable isotopes and nutrient concentrations are available. Long-term monitoring that includes the rainy season will show if the water quality around St. Eustatius is suitable for coral reefs or if action is needed to improve it.

For full report or more information,  please contact erik.meesters@wur.nl or gulsah.dogruer@wur.nl

Abstract
The coral reefs of Bonaire, providing resources and environmental services, are often ranked among the richest, most resilient and least degraded in the Caribbean, but they are not escaping the global degrading trend in coral reefs. Identifying and combatting local stressors, increases the resilience to global stressors. Research has shown that even the deeper, relatively unexplored reefs, mesophotic coral ecosystems (MCE), ranging from 30 to 150m in depth, are being impacted by anthropogenic disturbances. As the MCEs start where Scuba diving stops, and submersibles are often too costly, this study deployed an ROV to explore and monitor the shallow (5-20m) and the upper-mesophotic (40-60m) reefs at eight sites along the leeward coast of Bonaire. These sites were subdivided into different zones, showing a gradient in human impact and water quality. The imagery obtained by the ROV is of adequate quality, allowing for identification to genus level if not species level, and showed comparable results in estimated percentage coral cover with other recent studies. The benthic community composition changed along the vertical (depth) and horizontal (human impact and water quality) gradient. Benthic cyanobacterial mats were found around 40-60m depth, covering large parts of the ocean floor. Hard and soft corals, sponges, macroalgae and crustose coralline algae occurred at 40m depth at six of the eight monitored sites, indicating the presence of MCEs, and only at one site (Karpata), hard corals were present at 60m depth. Coral cover showed a clear increasing trend with decreasing human impact, addressing the need for a better understanding of heterogeneity among sites and local conservation measures. Developments in underwater robotics and machine learning enable more research on these hidden coral reefs and identification of the effect of local stressors on MCEs.

For full report or more information,  please contact erik.meesters@wur.nl or gulsah.dogruer@wur.nl

Internship report

Although Bonaire’s waters harbours one of the richest reefs of the Caribbean, it has not evaded the unprecedented global decline of these unique and precious systems. Recent research suggests a significant impact of local stressors on coral reef functioning. Future intensification of Bonaire's coastal activities may increase run-off, sedimentation, and eutrophication, which, potentially could induce detrimental changes to the system. However, identifying adverse effects of for example nutrient run-off on coral reefs in field conditions remains challenging. Nevertheless, a new local monitoring infrastructure may help to evaluate the risks posed by nutrient pollution by detecting the frequency and origin of harmful concentrations. We aim to create an integrated seawater quality management plan on Bonaire. For this, we measured levels and spatiotemporal variation of dissolved inorganic nutrients (NH4+, NO2-, NO3-, PO43-) and physiochemical water quality parameters (chlorophyll-a and turbidity).

Preliminary data (NOV 2021-Feb 2023) are presented of this ongoing 4-year monitoring project. Spatial water quality data from thirty-seven study sites collected from November 22nd to December 1st (2021) at 5 and 10m depth on the reef slope indicated that DIN concentration at site B12 (marina) and at the sites located in the area North of Kralendijk exceeded the 1μM threshold value set for the phase shift from coral to macroalgae-dominated coral reefs. Furthermore, geographical differences between in nutrient concentrations and relative abundance of nutrient species were found. Ammonium dominated the DIN pool in the areas Kralendijk and North of Kralendijk, whereas in the northern part of Bonaire DIN pool predominantly comprised of nitrate.

The temporal monitoring showed mean chlorophyll-a concentrations across Bonaire’s west coast approached the upper range of the safe threshold value (0.3 μg/L), indicating that Bonaire’s reefs are experiencing a chronic state of eutrophication. The data presented here of short-time span and should be considered as preliminary results. The outcome of this multi-year project, however, will provide more thorough insight spatiotemporal variation in nutrient and physiochemical water quality parameters. This data will help build scientific knowledge into both sources and resilience to external nutrient loading of coral reef ecosystems. Understanding this heterogeneity in local water quality conditions, will aid effective management, help restore reef resilience, and increase our chances of mitigating the global decline in coastal reef systems.

For full report or more information,  please contact erik.meesters@wur.nl or gulsah.dogruer@wur.nl