Water quality

Bonaire is considered to harbor some of the best remaining coral reefs of the Caribbean, but faces multiple pressures including eutrophication. We measured multiple water quality indicators twice annually, from November 2011 to May 2013, at 11 locations at the west coast of Bonaire. This study resulted in 834 data points. DIN concentrations ranged from below quantification to 2.69 μmol/l, phosphate from below quantification to 0.16 μmol/l, and chlorophyll-a from 0.02 to 0.42 μg/l. Several indicators showed signs of eutrophication, with spatial and temporal effects. At southern and urban locations threshold levels of nitrogen were exceeded. This can be a result of brine leaching into sea from salt works and outflow of sewage water. Chlorophyll-a showed an increase in time, and phosphorus seemed to show a similar trend. These eutrophication indicators are likely to exceed threshold levels in near future if the observed trend continues. This is a cause for concern and action.

doi:10.1016/j.marpolbul.2014.06.054

Approach:

Eutrophication is a common threat to the integrity of coral reefs as it can cause altered balance and integrity of the reef ecosystem. On the island Bonaire the former waste water treatment is limited which is a point of concern to the quality of the marine park. The reef of Bonaire faces nutrient input by various sources, of which enriched groundwater outflow from land is considered to be a substantial one. It is assumed that groundwater is enriched with nutrients e.g. due to leaking septic tanks.

In order to reduce the input of nutrients on the reef via enriched groundwater, a water treatment plant is being built on Bonaire. The treatment of sewage water is extended in 2012 with a sewage system covering the so called sensitive zone, the urbanised area from Hato to Punt Vierkant, including Kralendijk, the islands largest town. Based on the dimensions of the treatment plant and estimated connections to the plant, it is estimated that a total of 17.5 to 35 tonnes of nitrogen a year will be removed from the sensitive zone, and will not leach out to the sea. No estimates are known of the contribution of other sources to the total nitrogen load.

Limited information was available about concentrations of nutrients in the marine local environment and its eutrophic state. Therefore, Rijkswaterstaat asked IMARES to conduct a study on water quality aspects. The goal of this coastal monitoring study was to collect baseline water quality data to be able to study the impact of the water treatment plant in coming years. The following research questions are discussed based on the results:

• Are environmental safe threshold levels of water quality exceeded?
• Is temporal (over the years), or seasonal variation (November-May) of water quality observed?
• Does water quality vary among locations or regions in Bonaire?
• Based on experience and results, what are recommendations for future monitoring of water quality?

The study area was the west coast of Bonaire, and included 12 field locations. Water was sampled during early morning field trips at each location twice a year (May and November) starting November 2011 till May 2013. Indicators for water quality related to the nutrient status on the reef were selected and analyzed.

Based on their relevance to general water quality aspects and steering primary production, their relevance to the outflow of enriched (polluted) groundwater (and thus possible impact of the treatment plant in future) the following indicators were included:

• Inorganic nutrients
  • NO2, NO3, NH4, PO4
  • DIN (calculated based on NO2+ NO3+ NH4)
• Organic nutrients
  • Total nitrogen, ureum and total phosphorus
• General water parameters
• Chlorophyll-a
• Fecal bacteria

Concentrations were assessed against environmental threshold values from peer reviewed literature or (inter)national standards. If not available, outlying concentrations were highlighted taking the 80th percentile as a representative level.

Results and discussion

Water quality indicators measured at the west coast of Bonaire show signals of eutrophic conditions. Spatial and temporal variation in water quality is however observed. At some locations and certain moments environmental safe levels of nutrients are exceeded (see overview of data in Figure 1- Figure 4). Especially at locations in the south and in the sensitive zone concentrations of nitrogen and phosphorus exceed the threshold levels. Southern locations are probably affected by the salt pans, and locations in the sensitive zone by outflow of sewage water.

Furthermore, an increase of phosphorus and chlorophyll-a is observed in the last 2 years, whereas nitrogen (DIN) decreases slightly over the years. However, despite the decrease of nitrogen, its threshold levels are exceeded at Red Slave, Tori’s reef, Angel City, 18th Palm, Cliff. Phosphorus and chlorophyll-a do not yet exceed environmental threshold levels, but if the increase continues, this might be relevant in near future.

The risk of higher nutrient levels is that algal growth can outcompete corals, and can change the structure of the ecosystem. Furthermore, increased levels of nutrients affect the coral reefs integrity due to decreased stability of the skeleton.

The increase of bioavailable phosphate alters the nutrient ratio (DIN:SRP ratio) and species composition can evolve from this change in relative nutrient availability. Relating these data with observations in benthic composition and chlorophyll-a trends is advised to support this hypothesis.

Fecal bacteria numbers exceed several standards for human health safety. High fecal bacteria numbers are more frequently found in the south and in the sensitive area, and are likely to be related to rainfall events. Bacteria are found in surface samples as well; indicating surface run off as a possible source.

Actual rainfall, especially just before or during sampling is an important steering factor in the concentrations measured. Rainfall is very scattered during the rainy season, and we believe so is the outflow of nutrients to the reef.

In short it is recommended to continue the monitoring of water quality over several years at the same frequency and locations. Next to the regular program, make sure that interval sampling during heavy rains are included as these moments indicate point source discharges which can be missed when rainy season is shifted. No locations should be discarded from the program. In order to prepare the monitoring program for future measures taken outside the current zone (Hato- Punt Vierkant) additional locations just north and south of the sensitive zone are advised to be included. The set of indicators can remain the same, with some slight adaptations such as the addition of coprostanol (measure of faecal discharge) and discard of ureum.

As nutrient levels are in a constant flux, data should be considered in an ecosystem context. Benthic surveys focusing on macro algae, turf algae and cyanobacteria, were not included in this study, but add largely to a whole ecosystem assessment on eutrophication issues.

Monitoring of water quality in the coastal zone alone will not provide satisfactory indication of the impact of the treatment plant in reducing emissions to the marine environment. To monitor the impact of the treatment plant, several factors should be considered. These are related to the treatment plant itself, groundwater quality, coastal water quality, benthic coverage and benthic quality. Actual reduction of emissions to the marine environment can be retrieved from monitoring and reporting of the efficiency of the treatment plant. Monitoring of groundwater wells provides knowledge on the groundwater quality that outflows to the reef. Water quality monitoring in the coastal zone gives knowledge on conditions contributing to environmental health. It is advised to synchronize the monitoring programs, and to analyze the datasets in a coherent way.

In the end, eutrophication is not the only pressure potentially affecting a reef. Besides the focus on the research related to the treatment plant it is advised to consider additional research on a “whole ecosystem basis” in which the contribution of other pressures as well, such as run off via canals and overflows of salinas with nutrients and sediments (in rainy season), fisheries impact and the impact of climate change/acidification on the reef are included. 

Abstract:

The Saba Bank is a large submerged carbonate platform of approximately 2,200 km2 in the Caribbean Sea which lies partially within the Exclusive Economic Zone of the Netherlands and partially within the territorial waters of Saba and St. Eustatius. It was declared a protected area by the Dutch Government on 15 December 2010 and has been registered as such in the Specially Protected Areas and Wildlife (SPAW) protocol of the Cartagena Convention for the Protection and Development of the Marine Environment of the Wider Caribbean. Applications for a Particularly Sensitive Sea Area (PSSA) at IMO and Ecological or Biological Significant Area (EBSA) at CBD are pending.

As part of the Saba Bank research program 2011-2016, commissioned by the Dutch Ministry of Economic Affairs (EZ), an expedition to the Saba Bank was conducted from 22 to 29 October 2011. The Saba Bank research program aims to obtain information on the biodiversity, key ecological processes and carrying capacity for commercial fisheries to facilitate sustainable management of the area. The primary objectives of the 2011 research expedition were to collect data on benthic and reef fish communities; sponges and nutritional sources of the sponge community; seabirds and marine mammals; water quality, water velocity and other physical parameters. A multidisciplinary team conducted video and visual surveys on benthos, fish and sponges during 10 SCUBA dives at 20-30m depth, while sea birds and marine mammals were surveyed by means of on-board visual surveys and acoustic data loggers. Water velocity and water quality were also measured on-board using an Acoustic Doppler Current Profiler (ADCP) and Conductivity, Temperature and Depth (CTD) device.

During the expedition 8 sponge species were collected and 37 scleractinian coral species and 85 fish species were identified.Fish biomass varied per site between 1.3 kg to 4.4 kg.
Part of the measurements on water velocity, water quality and benthic cover are still in the process of being analysed. Data collected will also be used as baseline for future monitoring and analyses of biodiversity and key ecological processes within the framework of the 2011-2016 research program. 

Findings

The main issues that Lac Bay faces were identified as follows:

1. Filling-in of Lac and reduced water circulation. Over-grazing by extensive livestock husbandry as well as non-sustainable land-use practices (e.g. barren fields) has resulted in an accelerated infilling of the bay with sediment, which hampers water circulation and causes mangrove die-off. This has lead to a gradual reduction of the effective nursery and habitat surface of the bay over the last decades.
2. Increase in uncontrolled recreational pressure. The Lac ecosystem has been modified or altered by construction of roads, the building of hotels, subterraneous nutrient enrichment by untreated sewage and more. Trampling is causing an important decrease in sea grass bed coverage in the bay. Endangered species such as turtles and nesting birds are vulnerable to human disturbance (Lac is intensively used for various kinds of recreation).
3. Litter contamination. Marine litter washed in from the open ocean and abandoned fishing lines in the deeper parts of Lac are big issues.
4. Algal blooms. While the outer reef is in very good health, many of the inner reef’s corals, gorgonians and sponges are being overgrown by the crustose calcareous alga Ramicrusta sp. This may cause a serious decline in living corals inside the bay.

Management Recommendations:

The highest priority is to start habitat restoration.

Direct enforcement of existing and new legislation is crucial as well as a permanent presence of one or more officials.

Filling-in of Lac and reduced water circulation

• Tackle the livestock overgrazing problem in the whole watershed.
• Regularly open up the former channels to the rear areas of the mangroves and re-establish circulation and water quality.
• Remove filled-in sediments and reforest with red mangroves in the rear stagnant areas of Lac so as to re-establish mangrove and fish nursery habitat.

Address increase in uncontrolled recreational pressure

• Set upper limits for the various users.
• Strictly limit public access to seagrass areas using a combination of zoning, demarcation and enforcement.
• Upgrade the visitor facilities designed to limit or steer user impact towards low sensitivity areas.
• Monitor the human use of the bay.
• Assess Lac’s current bird use and their vulnerability to disturbance.

Litter contamination

• Conduct regular cleanups with volunteers and monitor litter densities.
• Limit and regulate fishing inside of Lac.
• Conduct PAH  (polyaromatic hydrocarbon) studies of the water in Lac.

Algal blooms

• Periodic annual monitoring of enteric bacterial presence at high risk locations.
• Install a monitoring program to assess the nutrient situation in Lac at several locations.
• Continue monitoring of coral overgrowth by Ramicrusta sp. 

Findings:

The main conclusion from this study is that the combined levels of anthropogenic impact on the bay currently exceed sustainable levels. Lac Bay is experiencing a long-term decline in productive habitat area all the while non-sustainable grazing of vegetation, eutrophication, seagrass trampling and high levels of litter contamination have been documented.

Lac catchment area

• The Lac catchment area was mapped using satellite imagery combined with field verification and gave a preliminary estimated size of about 22.6 km2 of surrounding lands. This area consists of a mix of semi- natural deciduous and dry-evergreen vegetation types and at least 213 small part-time farms.
• There are at least 52 dams that obstruct or retard water flow and many wells from which groundwater can be or is being extracted.
• Extensive livestock husbandry (goat and sheep) occurs at densities higher than 1 animal per hectare. Such densities well exceed densities that permit ecological recovery (0.1 animal per hectare).

Recreational use

• The Lac lagoon is intensively used for recreation. From 9 in the morning to 4:30 pm practically every day anywhere from 100 - 400 people are present on or along the shorelines of the bay at any given moment. Highest numbers occur during cruise ship days.
• The majority of recreational use of Lac is concentrated on and around the Sorobon Peninsula.
• The major recreational activities at Lac are sunbathing, windsurfing and swimming or wading. Little current use is directed towards nature activities
• Usage patterns and awareness differ importantly between the four different user-categories of cruise tourists, stay-over tourists, foreign residents and inhabitants born on Bonaire.

Anthropogenic impacts

• The inner borders of the seagrass exclosures display much bare space due to trampling.
• As there is no sewage treatment and as the available toilets and cesspits are generally defunct, beach visitation definitely result in nutrient enrichment in the waters of the bay
• Beach litter contamination is a matter of concern along mangrove shores at entrance of the bay and the lagoon-bottom immediately off the public beach of Sorobon.
• High levels of uses pose issues of disturbance for birds and sea turtles.
• Additional problems are the rapid invasion of the exotic seagrass, Halophila stipulacea and a bloom of an encrusting (possibly invasive) calcareous alga (Ramicrusta sp.) that is smothering live corals at the seaward side of the bay.

Management Recommendations:

• Develop sunbathing and water sport possibilities elsewhere on Bonaire to distribute user densities away from Lac.
• Upgrade user facilities and infrastructure at Lac. These include toilets and septic system, garbage disposal, organized parking, shade, signage and markers for the various management zones.
• Implement a Visitor Centre to provide visitor service (products and added value-information) and enforcement.
• Reduce grazer densities in the watershed and/or around the bay.
• Discourage/prohibit the use of throw-away food and beverage packaging at Lac and participate actively in the regional Marine Litter Action Plan developed by UNEP.
• Design a boom system to herd and trap contaminants entering Lac before they penetrate the mangrove fringes.
• Organize regular beach clean-ups in Lac.

Research to address knowledge gaps

• Further map and quantify anthropogenic effects in the watershed area (pollution, water diversion and extraction, forestation, grazing, farming, erosion) and their effects on Lac (in terms of sedimentation, reduced freshwater influx, nutrient loading).
• Document traffic levels on Kaminda di Sorobon and its effects in terms of disturbance, road-kills and littering.
• Study the concentration and effects of litter-derived contaminants on the environment and biota of the bay.
• Study the distribution and habitat selection of sea turtles in the bay as related to diet, food availability, water temperature, disturbance and other factors.
• Study the use of more and/or larger exclosures to improve seagrass coverage in the Sorobon area.

Abstract

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.

Monitoring:

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.

Conclusions:

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/. 

Abstract

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 shall outcompete corals, eventually leading to a disturbed composition 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 source. Groundwater is enriched with nutrients due to the e.g. 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 will be removed from the sensitive zone, and will not leach out to the sea at the western coast of Bonaire.

At the moment limited information is available about the total amount 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 facility in coming years. No estimates are known of the contribution of other sources to the total nitrogen load.

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

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

The aim of this first report was to define recommendations for the baseline monitoring the, expected positive, impact of the new sewage treatment system on the marine environment of Bonaire, with special emphasis on baseline monitoring. For this an evaluation was made of:

• Parameters/indicators to analyse, including argumentation, critical conditions
• Methods for sampling and critical conditions, including costs
• Potential sampling locations

The Nature Foundation carried out water quality tests on the 22nd, 23rd, and 24th of July 2012 at seven sites surrounding St. Maarten. These tests, which are conducted bi- annually, are carried out in order to determine the levels of pollutants and other factors affecting wetlands and beaches on St. Maarten. Tests were carried out in order to determine Nitrates (which shows that the water is polluted), Phosphates (which shows the presence of Sewage), Nitrogen, Dissolved Oxygen, and the acidity of the water. Tests were carried out on seven sites; Cole Bay Lagoon, Simpson Bay Lagoon, Mullet Pond, Kim Sha Beach, Great Bay Beach, Belair Pond, Fresh Pond, and the Great Salt Pond. The sites of Great Bay Beach and Kim Sha Beach were particularly chosen to test the swimming quality of the beaches.

It was determined that the sites Cole Bay Lagoon, Kim-Sha Beach, Mullet Pond, and Great Bay had medium levels of both phosphates and nitrates in samples tested. Elevated levels of nitrates and phosphates show that there is a presence of various types of pollutants and sewage which can cause toxic algal blooms and mortality events (large scale dying of fish, turtle and crabs) in wetlands and coastal areas. The highest level was recorded in the Great Salt Pond and indicates the presence numerous pollutants and sewage in the tested water. This may cause fish die offs and algal blooms. Taking this into consideration levels will continuously be monitored by the Nature Foundation.

It was further established that the sites Cole Bay Lagoon, Kim-Sha Beach, Mullet Pond, and Great Bay had low levels of Nitrogen in samples tested. Elevated levels of Nitrogen, caused by pollutants, can cause massive fish die-offs in wetlands and coastal areas. The highest level was recorded in the Great Salt Pond at .6 ppm, which is a relatively high number and indicates the presence of elevated nitrogen levels which can pose a threat to aquatic organisms and which may cause fish die-offs between now and the end of the year. The Nature Foundation will continuously monitor Nitrogen Levels at this site in order to give an approximation when fish die offs may be expected.

Almost all levels of oxygen recorded were at sufficient levels to maintain healthy life. The lowest level was recorded in the Great Salt Pond. This site should be closely monitored for a further drop in oxygen levels which may result in fish kills and breeding of airborne insects (i.e. Midges). It is interesting to note that previously recorded levels in April 2011 of Oxygen in the Fresh Pond indicated a low level present. Current levels have been recorded at healthy levels, which is a significant improvement. This is probably related to the installation of aeration pumps in the Fresh Pond by the Public Works (ROB) Department of the Ministry VROMI.

Despite the fact that many sites showed Low to Medium readings, the Nature Foundation will follow up on a monthly basis during the summer months to carefully monitor for changes in the respective levels. Similarly the levels recorded in the Great Salt Pond show that there exists the possibility of fish die-offs and increased midge activity between now and end of 2012. 

Doel en achtergrond werkbezoek

Sinds 10 oktober 2010 maken Bonaire, Sint Eustatius en Saba deel uit van Nederland. Rijkswaterstaat heeft wettelijke verantwoordelijkheden voor deze eilanden. Hieronder valt het opzetten van een operationele maritieme incidentenorganisatie. Gezien de enorme olie-overlagbedrijven op Sint Eustatius en Bonaire, de economische afhankelijkheid van de eilanden van duik toerisme en gezien de biodiversiteit in (de zeeën rond) Caribisch Nederland is dit geen overbodige luxe. RWS Noordzee zet de incidentenorganisatie op. De landelijke coördinatiecommissie milieuincidenten (LCM) ondersteunt hierbij. Doel van de reis naar Caraïbisch Nederland (CN) was om te komen tot een advies over de olie-bestrijdingsmogelijkheden, waaronder mechanisch opruimen en het gebruik van detergenten.

Mechanisch opruimen

Bij oliebestrijding is de keuze voor de wijze van bestrijding belangrijk. Het standaard uitgangspunt van de oliebestrijding is het verwijderen van de gemorste olie uit het milieu (mechanisch opruimen). Gedurende de reis is geïnventariseerd welke middelen beschikbaar zijn en wat noodzakelijk is om de opruimcapaciteit op het afgesproken niveau te brengen (tier 1, 16 m3).

Gebruik detergenten

Er zijn situaties waarbij de inzet van detergenten kwetsbare kustzones kan beschermen. Om het gebruik van detergenten te ondersteunen is een beslisboom detergenten beschikbaar. Voor het Caraïbisch beheergebied moet deze beslisboom worden aangepast. Voor het toepassen van detergenten is het van belang inzicht te hebben in de verspreiding van olie op zee. In Europees Nederland wordt gebruik gemaakt van het operationele model oilmap. Voor CN is het van belang dat dit model ook ingezet kan worden.

Adviezen

Het advies is gericht op het beperken van de belangrijkste risico’s en op maatregelen die op korte termijn gerealiseerd kunnen worden.