Water quality monitoring Bonaire. Results monitoring November 2011 and recommendations for future research


On the island Bonaire, eutrophication is a point of serious concern, affecting the coral reefs in the marine park. Eutrophication can cause altered balance of the reef ecosystem because algae can outcompete corals, leading to a disturbed composition and deterioration of the biodiversity of the reef. The reef of Bonaire faces nutrient input by various sources, of which enriched groundwater outflow from land to the reef is considered to be a substantial one. Groundwater is enriched with nutrients e.g. due to leaking septic tanks.

In order to reduce the input of nutrients on the reef via sewage water, a water treatment plant is being built on Bonaire. The treatment of sewage water will be extended in 2012 with a sewage system covering the so called sensitive zone, the urbanised area from Hato to Punt Vierkant. Based on the dimensions of the treatment plant and estimated connections to the plant, it can be assumed that a total of 17520- 35040 kg of Nitrogen a year is removed from the sensitive zone, and will not leach out to the sea at the western coast of Bonaire. No estimates are known of the contribution of other sources to the total nitrogen load.

At the moment limited information is available about concentrations of nutrients in the marine environment. Therefore, Rijkswaterstaat Waterdienst asked IMARES to conduct a monitoring study. The goal of this coastal monitoring study was to collect baseline water quality data to be able to study the effectiveness of the water treatment plant in coming years.

The study consisted of two phases and resulted in two reports:

1. recommendations for baseline monitoring in 2011,
2. monitoring, data evaluation, and recommendations

In this second report, monitoring data are presented and discussed, and recommendations for future monitoring are provided. Options for dissemination of data and data management are presented.


In November 2011, field monitoring was performed at ten locations at the west coast, at two depths -6m and -20 m. Three of these locations lay with the “sensitive zone” and are suspect of enriched groundwater, being a diffuse source of nutrients. Other locations are regarded as relative reference locations, laying further offshore, north or south from the sensitive zone. The prevailing current is from south to north. The reference locations might be influenced indirectly by the (diffuse) source under study, or can be under pressure by other nutrient sources as e.g. the salt company in the south.

Monitoring data are compared to environmental threshold values for tropical ecosystems. In Figure I, a summary of this evaluation is presented. Data show that during this monitoring study, eutrophic conditions, based on DIN concentration, are observed at four out of ten locations: Habitat, Angel City, Cargill and Red Slave. No clear difference in eutrophic state between the sensitive zone and other locations is observed. Cargill, Red Slave and Angel city are influenced by percolation of enriched groundwater from the salt pans.

Nutrient concentrations in the “sensitive zone” do not clearly differ from reference observations at e.g. Playa Funchi, Karpata and Klein Bonaire, but bacteria counts do. Bacteria numbers at Habitat and Playa Lechi exceed EU, EPA and Caribbean Blue flag standards. Stable nitrogen isotope ratios in macro algae show large variability and low average values near background levels, and are not specifically indicative for nitrogen related to sewage sources. Along developed coastlines with e.g. addition of inorganic fertilizer with low δ15N values will complicate the study for a sewage signal. Analysing δ15N and organic N in groundwater should be considered in next monitoring in order to explain the low ratio found in this study. Statistical similarity analysis between locations shows no similarity and relation to position of the location (within sensitive zone or reference). Location “Habitat” showed a clear dissimilarity compared to the other nine locations, and it is assumed brine effluent from WEB could be a steering factor in this observation.


The study of November 2011 leads to the following conclusions:

  • Benthic surveys were not included in this study, and add largely to a whole ecosystem assessment on eutrophication. In upcoming research this should be included.
  • Based on nutrient levels, in the south and in one location in the sensitive zone a eutrophic status was observed. The other locations did not have nutrient levels harming the development of a healthy coral reef, based on nutrient concentrations alone. Nutrients levels are however in a constant flux, and data should be considered in an ecosystem context.
  • Enriched groundwater with nutrients from sewage is not the only source of nutrients. Other sources as nutrients from the salt pans in the south and from brine near Habitat probably add to the eutrophic status at these locations. Furthermore percolation and surface run off from Salinas and stormwater via roois are probably a source of nutrients as the isotope values at the other locations are low too.
  • Monitoring in the coastal zone alone, will not provide adequate indication of the effectiveness of the treatment plant. Monitoring in the coastal zone is effective to detect areas at risk, and to detect long term changes in overall water quality (= so called “surveillance monitoring”).
  • Monitoring in the coastal zone should be supported by additional so called “investigative monitoring” at the sources to quantify the relative contribution of each of these sources in order to be able to discuss additional measures.

Management Recommendations:

Above mentioned preliminary conclusions need to be considered using additional monitoring. Based on a one time monitoring activity no definite conclusions are possible related to the treatment plant.

“Surveillance” monitoring in the coastal zone will identify areas at risk, determine long-term changes in water quality, and can be used to evaluate environmental risk assessment.
Indicators to include are: nutrients (NH4, NO2, NO3, DIN, PO4, Total P, organic (kjeldahl) nitrogen) bacteria, benthic composition. The added value of N15 is questioned because of the average low response and high variability. A reference locations further offshore has to be added.

A clear advise on minimum frequency cannot yet be given as seasonal and diurnal variance is evident, but the extent not yet identified. Seasonal and diurnal dynamics (and thus variance) in nutrient availability is common at reef systems. Factors steering this seasonal variance are e.g wet and dry season, dynamics in regional upwellings, atmospheric pressure, biannual tidal regime, and irregular discharge in both quality and quantity. Suggestions for getting grip on this variance is provided in the report. A minimum frequency of monitoring in dry (May/June) and wet season (October/November) is suggested by parties involved. This frequency is a starting point, but could however be too low to detect significant trends. Future data have to be evaluated and monitoring has to be adapted according to the new results. Integration of these data with benthic survey data is considered to be a priority.

“Investigative monitoring” should be directed to measurements and evaluation of the quantity and quality of the sources and can be used to establish causal relations. In relation to the effectiveness of the treatment plant, it is advised to direct “investigative” monitoring to:

  • quantity and quality of the influent and effluent of the Water Treatment Plant
  • quantity and quality of other sources of nutrients via e.g. groundwater monitoring
    • Industrial sources (salt company, WEB brine effluent)
    • Salinas and roois

Indicators to include are: BOD, COD, bacteria, nutrients (NH4, NO2, NO3, DIN, kjeldahl N, PO4, total P), and 15N. Scenarios for field work are presented and cost estimates provided in the report.

Synchronization and support of STINAPA research

Options to integrate and support ongoing research by STINAPA are discussed in the report. The processing of obtained data by the benthic surveys is time consuming and therefore not yet available. Second subject is the dissemination of results from project “light and motion” by the university of California. These data could very well fit into an exploration of remote sensing as a cost effective monitoring technique for water quality. Both subjects could contribute largely to the assessment of water quality in the coastal zone of Bonaire and aid management decisions. Data analysis via e.g. student projects should be considered as an option.

Data management and dissemination of results:

Regarding data management and dissemination of results it is advised to further explore and to contribute to the development of the WUR portal on BES data and use the ISO standard by SeaDataNet to describe metadata. The WUR portal provides the opportunity of storing all BES data in a format of choice. Excel tables and figures, including the reports can be uploaded, and could for the time being be suitable enough to disseminate the data. The portal is under development and options for dissemination will be gradually extended and improved. If chosen to describe the monitoring and data with a metadata format prescribed by international standards, in time, the (meta) data could be synchronised with any other system.

The location of the portal is http://scomp0703.wur.nl/bioplanbes/. 


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