chlorophyll a

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

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