aggra

MSc internship report

The health and abundance of coral reefs are declining worldwide. Coastal development, terrestrial runoff, the temperature rise of the oceans and coral diseases have decreased the health of coral reef systems in the Caribbean. Terrestrial run off has been shown to affect water quality through nutrient influx. The water quality changes and community composition of the reefs of Bonaire have been recorded in the past but only on smaller scales and most of the time in combination with sites on Curaçao. The project “Resilience Restoration of Nature and Society in the Caribbean Netherlands” (the Resilience project) aims to improve the resilience of the coral reef ecosystem on Bonaire, by implementing an extensive monitoring plan for water quality and benthic community composition. This internship project continues surveys of benthic community composition of Bonaire (1), assess the risk of anthropogenic pollution through nutrient influx (2) and relate the water quality assessment of Bonaire to the benthic community data (3).

Benthic community composition was assessed at 8 sites on the leeward coast of Bonaire, between 5 and 60 meters depth. This was done by Remote Operated Vehicle assisted, picture gathering. Dissolved Inorganic Nitrogen (DIN) and Dissolved Inorganic Phospor (DIP) concentrations were also collected at 39 locations around the leeward coast of Bonaire. 11 of the locations were sampled biweekly between November 2021 and May 2022 and nutrient concentrations from these locations were used in a water quality Risk assessment for eutrophic pressure. This was done by calculating a Risk Quotient and comparing it to the percentage of exceedance of a nutrient concentration threshold for eutrophication on coral reefs. Additionally a "Generalized Additive Model" (GAM) was used to explore the relationship between mean coral coverage and mean DIN concentration.

Turf algae showed to be the most dominant functional group between 5 and 20 meters depth, followed by cyanobacteria as the most dominant group between 40 and 60 meters. Coral cover increased until 20 meters depth, up to 25%, and showed a sharp decline afterwards. The benthic index based on AGGRA benthic indicators suggests that the coral reef ecosystem is more dominated by algae and cyanobacteria than corals. DIN concentrations are close to the eutrophic threshold of 1 μm/L and display a relatively high exceedance percentage of that threshold. DIP concentrations stay safely below the threshold value of 0.1 μm/L. Risk assessment of the worst case scenario for DIN shows that almost all locations sampled have low risk or higher for pressure by nutrients. The GAM showed no significant relationship between DIN and coral coverage. Further nutrient concentrations need to be gathered on a temporal scale to get a better view of the water quality over the whole year. The benthic community composition of locations towards the north of the island also need to be mapped to assess the effect of the elevated nutrient concentrations on the coral reef. This study hopes to support the “Resilience Restoration of Nature and Society in the Caribbean Netherlands” project with their extensive monitoring plan.

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

Raw data of fish biomass based on the Atlantic and Gulf Rapid Reef Assessment method (AGGRA) for the locations:

• calabas
• no dive reserve
• front porch
• 18th palms
• bach beach
• barkadera
• forest
• karpata
• oilslick
• reef scientifico

Please contact the DCBD admin, or STINAPA for more information.

Coral reefs at 10-20 m depth off eastern and western Curaçao, Netherlands Antilles had high abundance and high cover (25-50%) of stony corals, although the latter declined between 1998 and 2000, primarily from impacts associated with Hurricane Lenny in November 1999 and coral disease. Most corals had lost 15- 40% of their live tissues and the amount of partial mortality declined with depth. Little recent mortality was observed (0.6% in 2000). Reefs were dominated by the Montastraea annularis species complex (46% of all corals ≥20 cm in diameter), which were 40% larger than other species. Overall, colonies of the M. annularis species complex sustained somewhat greater total (recent + old) partial-colony mortality (24%) than other stony corals (19%), and had a higher prevalence of disease. Yellow-blotch disease affected 14.5% of all colonies of the M. annularis species complex in January 2000; infected corals had twice as much total partial- colony mortality (44%) as uninfected conspecifics. Shallow reef communities at 8- 12 m appear resilient to disturbance, as evidenced by low macroalgal cover, a high abundance of stony coral recruits and juveniles, and declining disease incidence and prevalence overall. However, the high incidence of yellow-blotch disease in the M. annularis species complex and the absence of recruits of these species suggests their condition may continue to decline and a shift in species dominance may be underway.