DIN

Spatial & Temporal monitoring of Bonaire’s near-shore water quality

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

Date
2022
Data type
Research report
Theme
Research and monitoring
Report number
Wageningen Marine Research Student Report
Geographic location
Bonaire
Author

Nutrient Enrichment and Eutrophication on Fringing Coral Reefs of Bonaire and Curaçao, Netherlands Antilles, 2006-2008

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

Date
2011
Data type
Other resources
Theme
Research and monitoring
Geographic location
Bonaire
Curacao