Water quality, its environmental drivers, and its effects on the benthic community of coral reefs on Saba

Coral reefs are among the most valuable ecosystems in the world and provide natural, economic, and social services. The coral reefs in the Caribbean suffer from global, regional, and local stressors and are widely degraded. Saba is a small island in the upper leeward Caribbean that is no exception to the degrading reefs in the Caribbean. Where global and regional stressors are difficult to manage, local stressors can be regulated and managed as they are often a result of human activity onshore. Eutrophication and high turbidity levels are among these local stressors. This report provides the findings on research under Wageningen Marine Research’s (WMR) project “Restoration of Resilience of Nature and Society in the Caribbean Netherlands”, where the tempo-spatial variation of turbidity and chlorophyll a concentrations around Saba were measured and compared with threshold values found in literature. On top of that, the drivers behind the chlorophyll a and turbidity levels, and the health of the benthic community were assessed. Lastly, a risk matrix was used to categorize locations
into 5 risk categories; No risk, low, moderate, high, and very high. The Reef Health Index (RHI) was used to further categorize the health of the benthic community, and compare sites around the island. Overall, the chlorophyll a concentrations around Saba are above threshold values for eutrophication
on most sites and are categorized as high-risk zone and thus represent locations of concern. The values are averaged over all sites, 70% above the lower limit of 0.2 µg/L threshold, and 9% above the upper threshold limit of 0.4 µg/L. However, the high values at the reference sites and the samples at the Saba bank suggest that not only local coastal activities influence the measurements but also regionalwide impacts should be considered. Except for one sample at site 3, the turbidity levels around Saba on all sites never exceeded the protective threshold value of 3 Nephelometric Turbidity Units (NTU), often staying below 1 NTU. Thus, the data presented in this report indicate no concern regarding turbidity as a stressor on the coastal environment. Nevertheless, some individual measurements directly after rain events showed that short-term high increases in turbidity levels occur and might be better captured if the water is not strictly biweekly sampled or if long-term measurements become available. Furthermore, elevated turbidity levels at site 10 indicate an influence of the landfill, which
should be incorporated into improved future regulations. Rainfall is a driver behind both parameters, whereby the lag time between the event and the effect on turbidity is shorter than the effect on chlorophyll a, 2, and 7 days, respectively. Currents also play a role in the spatial distribution of
chlorophyll a, as well as wave height and wave direction for turbidity. Furthermore, currents might influence the regional impact from urbanized neighbouring islands such as St. Martin.Compared with previous studies, this study showed that coral cover on Saba is further declining. Where hard coral cover in 1993 ranged from 11.4-33.6%, it declined to 17% in 1999 to an average of 7.7% in 2019. This study reports a cover of 0.55% (0.23 - 1.13%) of hard coral. With a coral cover that low and turf and macroalgae combined reaching 95% cover, the coral reefs around Saba are in critical health condition according to the Reef Health Index (RHI). No recovery and further degradation are expected with the current eutrophicated states of the water. Furthermore, high eutrophication can increase susceptibility to coral diseases, while local run-off could serve as a vector for pathogens and pollutants. However, some healthier spots around Saba that were not measured in this study could act as a refuge; when conditions are favourable again, recovery can start from there. Lastly, this report recommends future research, management, and monitoring practices.