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.
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.
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.
A prominent process of photoacclimation accounts for the change in pigment concentration to improve photosynthetic performance. It occurs in algae due to their inherent characteristic of living underwater and receiving limited wavelength and irradiance of light from the sun. This process was investigated on the macroalga Ventricaria ventricosa, one of the largest known unicellular organisms. It contains a central liquid-filled vacuole that pushes the organelles towards the cell membrane. Upon perforation, the inner cell structure disassembles, facilitating pigment concentration analysis. On a first experimental stage, individuals at 8-10 m and 16-18 m deep were collected and analyzed for chlorophyll a (Chl a), chlorophyll b (Chl b) and carotenoids (Car) concentrations via spectrophotometry. The overall pigment concentration was not significantly different (ranging from 0.54-14.74 μg ml-1 Chl a, 0.36-19.29 μg ml-1 Chl b, 0.19- 4.09 μg ml-1 Car). The second stage included covering alga from the same depth belts with a green or transparent (control) filter cage for one week to then analyze pigment concentration. Control cages had pigment concentration values considerably lower from those of the first experimental stage (ranging from 0.02-3.38 μg ml-1 Chl a, 0.02-2.20 μg ml-1 Chl b, 0.01-1.44 μg ml-1 Car). Membrane perforation upon detachment from the reef, decreased water flow, and the cells’ size may have influenced this outcome. It is concluded that the depth gap used was not large enough to promote photoacclimation. It is also suggested that V. ventricosa prioritizes healing over photoacclimation, resulting in loss of pigment concentration while reestablishing turgor pressure.
Land-based nutrient pollution is a major stressor on coral reef communities around the Caribbean region and globally. To assess the status of nutrient enrichment and eutrophication on Bonaire and Curacao’s coral reefs, we conducted a comparative nutrient monitoring program that included seasonal sampling for nutrients (ammonium, nitrate, DIN, SRP, TDN, TDP), phytoplankton biomass (Chl a), stable nitrogen isotopes (ð15N) in reef macroalgae, and biotic cover (point count analysis of video transect data) of shallow and deep reef sites between March 2006 and June 2008. Ammonium dominated the DIN pool on both islands and the highest concentrations (~10 µM) occurred on Bonaire’s reef sites adjacent to the Cargill salt ponds. DIN concentrations averaged > 1 µM on both shallow and deep reefs of both islands, indicating that these reefs are now above the DIN threshold noted to support expansion of algal turf, macroalgae and coral diseases. SRP concentrations averaged ~ 0.1 µM on both islands, a level that also represents the SRP threshold for eutrophication on coral reefs. DON and DOP dominated the TDN and TDP pools; TDN/TDP ratios averaged 52 on Bonaire and 45 on Curacao, indicating strong P-limitation of algal growth. Mean Chl a concentrations were higher on Curacao (0.25 µg/l) than Bonaire (0.19 µg/l) and the highest Chl a concentrations on both islands occurred on reefs adjacent to urbanized, nutrient enriched areas. In contrast, low Chl a values of ~ 0.05-0.1 µg/l occurred at the upstream reef sites and the offshore reference site, underscoring the importance of land-based nutrient enrichment to microbial growth and eutrophication on fringing reefs of both islands. The highest macroalgal ð15N values (> + 3 ‰) occurred at the MegaPier and Piscadera Bay on Curacao, which reflects the highest watershed sewage nitrogen inputs of all reef sites in the study. The lower ð15N (< + 2 ‰) values at the other reef sites reflect lower levels of sewage treatment as well as contributions from other nitrogen sources (nitrogen fixation, atmospheric inputs) that have lower source ð15N values. Reefs on both islands were dominated (~ 75 % cover) by benthic algae but showed distinct differences in algal composition; Bonaire’s reefs had high cover of turf and low cover of macroalgae compared to the opposite pattern on Curacao. Our results suggest that the recent expansion of benthic algae and loss of coral cover on reefs in Bonaire and Curacao are not simply the result of top-down human pressures (e.g., overfishing) alone, but also reflect strong bottom-up effects from land-based nutrient pollution.
The data of the study can be found here