Slijkerman, D.M.E.

Lac Bay, Bonaire is the most important mangrove and seagrass area of Bonaire and has been undergoing steady ecological decline in the last decades. Based on an initial assessment of conservation management issue and potential solutions, as, conducted by IMARES in June 2010, the Dutch Ministry of Agriculture, Nature and Food Quality (LNV) asked IMARES to return to Bonaire to work with Stinapa Bonaire to choose narrower priorities and jointly make a short-list of topics as a working document for cooperation and action. In the beginning of September, site visits and discussions were held in Bonaire with the manager of Lac Bay and various stakeholders to identify and agree on priority issues for action. This working report gives the results of that visit.

Four action spear point projects were identified, based on urgency and feasibility based on local Bonaire and Dutch IMARES expertise. The projects are as follows:

1. Mangrove restoration demonstration pilot study
The basic objective is to reestablish water depth and tidal connection in high marsh salt areas that have resulted from infilling with sediment, and restore them as effective mangrove and low marsh fish nursery habitats. By collecting baseline data before the restoration activities take place, it will be possible to monitor and compare and assess changes in fauna and flora at the restoration sites and hence evaluate the effectiveness of the measures implemented.

2. Baseline ecological study of the zonation of aquatic communities
The goal is to complete a scientific description of Lac’s aquatic community zonation as it exists at landscape level today. This will provide the framework against which large-scale community change and effectiveness of mitigation measures can be monitored and evaluated.

3. Recreational and land use survey for Lac Bay and its catchment area
The goal is to identify user problems and potential solutions by mapping and assessing user density and pressures in Lac

4. Study of avifaunal habitat use of Lac Bay
The goal is to identify bird habitat use problems and potential solutions by assessing habitat use of Lac by birds during the migratory season.

(A fifth project for implementation by Stinapa and Dienst LVV was identified)
5. Lac mangrove channel clearing project
Re-establish water flow from the Bakuna dam to Lac using a pipe system.

The Lac mangrove channel clearing project of Stinapa was reviewed and judged to be valuable and important. The baseline study of zonation of aquatic communities (project 2) is urgently needed in this respect to allow short and long-term evaluation of this project which need to become a structural part of Lac Bay management. Routine mangrove channel maintenance was identified as ideal work for involvement of Bonaire youths and volunteers, to rekindle public involvement in caring for Lac and its rich natural and cultural-historical heritage. 

Management Recommendations:

A project plan is presented by which all four projects can be delivered by December 2012. These projects can count on government and broad community support. In this all, Stinapa indicated to be willing to provide basic free lodging to interns and scientists at their science accommodations at the entrance of Washington-Slagbaai National Park. The ability and willingness of IMARES to recruit and guide students and interns for these projects was an important selection criterion to help restrain total project costs. The action spear points will, nevertheless, require funding as well as permits from the Island Government of Bonaire. With LNV various funding options were reviewed and discussed, and the need for permits was discussed with DROB (Dienst Ruimtelijke Ontwikkeling en Beheer) Bonaire. DROB envisioned few problems with the required permits. The visit was concluded by the joint resolve to work out ways to maintain momentum and proceed towards the implementation phase.

Abstract:

The aim of this study was to draft a generic ecological assessment framework for coastal systems in Caribbean Netherlands (CN) that offers guidance in the process of license-applications of planned activities that could impact coastal systems, as well as a general guidance towards environmental and ecological monitoring related to proposed projects and existing activities. The study was limited to the review of (inter)national ecological assessment frameworks and monitoring initiatives; peer-reviewed academic literature was not consulted. Though this draft framework provides guidance, it limits itself to standard practice and general regulations. Further fine-tuning of the framework is required to be applicable to the specific situation in CN. Furthermore, it is the responsibility of the individual initiators to customize an adequate and comprehensive ecological impact assessment and monitoring-plan adjusted to the spatial and temporal scale relevant of the type of activities and possible impact resulting from the project.

Fundamental to the draft framework is a network approach in which the impact chain between the activity and ecosystem components is specified by a suit of pressures. The broad strategy of the ecological assessment framework consists of three major phases:

1. Establishing the context in which the project will take place.
2. Scoping of the project activities, their pressures and the environmental descriptors relevant to the projected area.
3. Assessment and evaluation of the pressures on the environmental descriptors.

For each phase practical guidance is provided in the form of questions. While answering these questions an overview is established of all relevant activities, pressures, and environmental descriptors. Each phase is further elaborated upon in the report. An adaptive and interactive management approach is required for the processes of the three phases. Informative environmental descriptors groups were identified based on international monitoring initiatives (Benthic diversity, Coral health, Species requiring special attention, Fish diversity, Chemical water quality, Physical structure) and for each descriptor indicators are proposed. Further study is required into which indicators are most appropriate for CN.

Threshold levels are not commonly available for each of the environmental descriptors. Significance testing in the absence of threshold levels is discussed in de report. A practical guidance is proposed to evaluate and categorize the significance of an impact by listing questions related to the nature, magnitude and intensity of the (expected) impacts. Reference is made to relevant (inter) national treaties or ordinances in which qualitative goals are reported.

This report provides practical guidance and considerations on how to establish appropriate reference situations in a changing environment. The reference situations must be chosen using best available information about the physical and biological characteristics of the environment to ensure that they represent suitable reference conditions. Important factors to consider are summed up in this report. A well set-up monitoring design should include multiple reference sites (spread across space and time) to allow the authorities and the initiator to tease apart natural variability and general trends in decline (e.g. due to climate change) from changes caused by the initiated project.

The proposed framework has not yet been tested with pilot situations or cross-referenced with the legal framework in CN, nor has it been evaluated with stakeholders. It is highly recommended to evaluate this framework by applying it to pilot or actual cases, and to adapt were necessary. 

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

On the 10th of October 2010 the governmental entity known as the Netherlands Antilles is scheduled to cease to exist. Each island will aquire a new status within the kingdom. Following the declaration of an Exclusive Fishery Zone (EFZ) in 1993, an Exclusive Economic Zone (EEZ) has been declared in the Dutch Caribbean on the tenth of June 2010.

The EEZ area concerned, is a large expanse of sea which harbours exceptional biodiversity, and represents an important natural renewable resource potential. The Netherlands Antilles, Aruba and The Netherlands have, therefore, opted to draft a management plan for the EEZ. This initiative began in the year 2005 when the first conference regarding the management of the biodiversity in the EEZ was held. The consensus was that despite a fragmented Dutch Caribbean, the EEZ should always be integrally managed. In 2009 the participants of the second conference confirmed the need for common management and developed common goals, principles and a framework for the management of the Dutch Caribbean waters. Resulting from this conference a management plan was drafted, circulated to all stakeholders and discussed on the 1st of June 2010. Based on the input and feedback received, as well as subsequent correspondence, this final management plan was jointly developed.

The Dutch Ministery of Agriculture, Nature, and Food Quality (LNV) gave financial support to facilitate the process. This management plan initially stems from the framework policy plan “Natuurbeleid van de Nederlandse Antillen” (2000) that recognized the urgent need for a management plan for the Saba Bank, situated inside the EFZ waters, to ensure sustainable fisheries and protection of its rich biodiversity. It also closely follows the intention of the Kingdom with regards to the goals set forth in the Dutch biodiversity policy programme “Beleidsprogramma Biodiversiteit 2008-2011”. In this respect, as a party to the Convention on Biodiversity, The Netherlands also has a strong international commitment to help stem the global decline in biodiversity.

This management plan outlines the purposes and manner in which the Caribbean Exclusive Economic Zone and Saba Bank in particular may be used in a sustainable manner, based on a shared vision and common set of goals. It outlines the management objectives, as well as key policies, and strategies with which to achieve sustainable management. It also addresses the administrative structure, resource use, financial support, key information needs, and action points most urgently required to set sustainable management in place.

To set the stage for implementation and properly initiate this process governments of The Netherlands Antilles, Aruba and The Netherlands have, among others resolved to:

1. take those steps needed to legally designate the Saba bank as a specially protected national marine area,
2. take all steps necessary to legally designate the Dutch Caribbean EEZ as a Marine Mammal Sanctuary,
3. install a EEZ Marine Resourses Committee to guide the process of further management implementation and
4. allocate the required core funding.

These important intentions are formalized and set in action by means of an agreement between parties. 

Abstract:

Lac is a semi-enclosed lagoon located on the south-eastern side of Bonaire, and contains a diversity of shallow water coral reef associated habitats in close proximity such as mangroves, seagrass beds, Halimeda algal beds, the back reef and sand flats. These habitats support a diversity of fish and invertebrates. The bay has numerous international and national legal protections. The Bonaire National Marine Park regulations and various Island Decrees facilitate from the local perspective. However, despite all regulations, the bay faces several changes, and management and protection of the bay is hampered by a lack of scientific information regarding current environmental status.  

Nutrient poor waters are a requirement for healthy coral reefs. When these become enriched with nutrients, it results e.g. in increased algae and affected reef condition. One area of interest for management is the eutrophication status of Lac. Eutrophication is a pressure that might explain some of observed changes in the bay. However, no baseline on the eutrophication status of Lac exists. IMARES and Environics NV conducted a snapshot assessment of the eutrophication status for current understanding and as a basis for future management. Environics conducted the field measurements at Lac, and most of the data analysis. IMARES analysed geographical data and together with Environics cowrited the report.

The purpose of this baseline study was to assess the trophic status of Lac by analyzing 4 potential indicators of eutrophication simultaneously:

• Nutrient levels
• Levels of fecal indicator bacteria (enterococci)
• Epiphyte loads of seagrasses,
• Benthic community composition of the back reef

The monitoring was performed at 32 sites within the bay and 1 control site outside the bay in December 2010.

In this study, three of the four observed indicators point towards an ecosystem that is under stress from eutrophication. The levels of nutrients in the bay exceeded thresholds for open coral reef systems due to lack of better. Overall, concentrations show that enrichment with nitrogen was widespread and levels commonly exceeded threshold values. No clear source or “hotspot” could therefore be identified in this study. Phosphate only exceeded threshold values at a few locations, but no clear source was identified. The diffuse enrichment of nutrients across the bay probably results from multiple factors such as water circulation, residence time, freshwater input, rainfall, groundwater contamination, tidal range, and geology. Besides the (semi-) natural conditions the nutrient status is likely to be affected by human impacts as greywater inputs and lacking of proper sewerage. All these factors should be considered regarding the future state and measures to tackle the eutrophication of the bay.  

Enterecocci bacteria were detected at levels above acceptable levels as determined by ISO for bathing waters. The mean levels of enterococci decreased as the distance from shore increases with the highest levels found at groundwater sites and zero enterococci found on the back reef sites. Based on this dispersion we assume that sources of enterococci in this study are most likely birds and cattle (donkey and goat manure). The identification of the true sources of enterococci in Bonaire is however compelling and further study on this aspect is necessary to protect public health.  

The levels of epiphytes on seagrass blades, showed differences in biomass among studied stations. This could mean that seagrass beds in different regions of the bay are experiencing different levels of water column nutrients but no clear relation between nutrient levels and epiphyte cover was observed in this study.

The benthic composition monitoring revealed high abundance of calcareous algae (Ramicrusta sp.). This abundance is likely to be a bloom (pers. observations over time). The bloom of Ramicrusta sp. might be indicative of nutrient enrichment and uptake occurring in Lac. The alga is currently taking over habitat where hard corals lived and changes the benthic composition of the back reef and potentially affecting the integrity of the reef crest. The degradation of the reef crest will diminish the protective role provided by the structure and increase exposure to wave and storm action from the adjacent sea.  

Management Recommendations:

Despite the current eutrophic state of Lac, studies elsewhere indicate that eutrophic bays may begin to recover within months after implementation of proper measures. To do so, natural sources of nutrients should be distinguished from anthropogenic sources. Based on the results of this study and historical accounts of other bays in the Caribbean that have been degraded by eutrophication; the following recommendations for Lac are suggested:

• a. Reduction of nutrient and fecal bacteria inputs by removing donkeys and goats from the watershed, and ensuring adequate toilet facilities and sewerage at Cai and Sorobon, including greywater disposal.
• b. Continuation of nutrient monitoring nutrient in order to locate clear sources and fate of the eutrophic state of Lac. We recommend adding urea to the suite of nutrients monitored in this study.
• c. Implementation of a regular monitoring program to identify sources and fates of fecal bacteria in order to support public health. Effectiveness of above measures can then be assessed as well.
• d. In general, to understand the outcomes of the water quality management plan it would be of great value to have an understanding of groundwater flows, circulation patterns and residence time of water in Lac.

At St. Eustatius a project has started for the improvement and expansion of the Seaport St. Eustatius. The planned activities related to the project need to be evaluated in order to comply with the legal requirements for a licence from the competent authority Rijkswaterstaat Noordzee in the Netherlands.

A quick scan of the potential environmental impact for the planned activities regarding the St. Eustatius harbour extension was commissioned by Rijkswaterstaat Noordzee. This quick scan was performed within limited time and based upon limited background information. No additional research on site was performed. Consequently, this report provides an environmental impact assessment based only on a review of literature and expert judgement.

Within the project it is foreseen that an estimated 10.000 m3 of sediment will be dredged from the turning basin and dock in the new harbour and from the old harbour. The dredged material will be disposed of in the sheltered inner harbour and south of the breakwater. This deepening of the St. Eustatius harbour and associated activities can potentially negatively impact the environment (directly) through:

1. Destruction of habitat on the dredged site and on the site where the dredge material is deposited
2. The amount of sediment that will be dispersed into St. Eustatius coastal waters, and the cascading impact thereof on marine habitats
3. Noise in marine habitats caused by the placement of piles and moorings

The resulting deepened harbour, the disposal sites and changes in future use may cause long term (indirect) negative impact on the environment due to:

1. Dispersal of the dredge spill deposits, and thereby threatening marine habitats
2. Increased turbidity due to harbour sediment erosion, increased sediment trapping and more shipping movements
3. Changing current and wave patterns, thereby threatening key monuments of human history close to the shoreline
4. The increased risk of spills (fuel, oil, bilge water), introduction of nutrients and marine litter, and introduction of invasive species (bio pollution)

These potential impacts have been investigated in this report and have resulted in the following findings:

The tidal and residual currents around St. Eustatius are weak and estimated to be up to 20 cm/s. Near the harbour area, the residual flow is probably dominantly north. The wave height is low throughout the larger part of the year, except during hurricanes and tropical storms. From December to April cold fronts in Florida regularly generate swells from the north to northeast (“brown seas”). These events occur once or twice a month, last for a day to a week, and may generate swell waves 3 to 5 m high.

The marine substrate in the harbour area consists of a hard substratum overlying a more loosely packed conglomerate including sand and pebbles. This hard substratum consists of large rock fragments and cemented conglomerates. Removing this hard layer makes the underlying softer material available for erosion, especially since the deepened area will be exposed to the winter swells and has a water depth where the swells may break (and hence lead to high near-bed shear stresses). The risk for increased levels of suspended sediment due to erosion is probably small, but depends on the fine silt content of the sediment to be exposed. The available information does not indicate the presence of such fine material in the sediment.

During the dredging works, the sediment spill is expected to be limited. It is assumed dredging will be done during calm conditions. Some sediment will enter the water column during dredging, but due to the low ambient currents most will immediately settle from suspension. If present, silt and flocculated mud will be transported 1-2 km northward. Unflocculated mud can be expected to mix with ambient currents within days, leading to only limited increase in turbidity.

Storage of dredged material occurs in the sheltered inner harbour and south of the breakwater. Little dispersion of this sediment is expected during the dredging and storage activities partly due to the planned placement of bubble screens.

Over longer timescales, the removal of the hard layer will probably lead to higher turbidity in the harbour during storms. The winter storms are associated with southward currents, and therefore some of this sediment may be transported south of the wind breaker.

It is expected that deepening of the harbour will lead to a minor change in alongshore transport in the inner harbour, but will not affect alongshore transport north of the old harbour or south of the breakwater. The wave height near the ruins just south of the old harbour will probably increase due to deepening which may have a small effect on the coast.

Based on the above findings and expert judgement regarding sediment transport and turbidity changes the following conclusions are drawn on the potential impact on the environment:

The sediment that enters the water during dredging works is expected to settle relatively quickly, leading to limited sediment-plumes and turbidity. Therefore, no mayor or irreversible impact from dredging works is to be expected on the surrounding habitats. This is based on the assumption that fine silt is not present at the site. However, if these sediments are present, habitats up to 1-2 km north of the harbour can be affected. The impact is estimated, however, to be limited due to the low expected volumes.

Dredging works will impact living organisms at the dredged site and deposit- sites, covering a total area of approximately 1-2 ha (dredged and dredge-deposit site). Recovery is likely to occur over time if environmental conditions permit. This may take up to several years in case of removal of climax stage ecosystems such as coral reefs and seagrass habitats. 

Direct impact on marine mammals due to pile driving and placement of moorings are considered to be negligible as the migrating season has already passed and noise levels are considered to be relatively low.

During the deposition of the dredged sediment at the two locations, no impact on surrounding habitats is expected due to the minor dispersion and mitigation measures taken (bubble sheets).

Unless added measures are taken it is expected that the dredge deposit on the south side of the breakwater and south of the old harbour will erode and will be dispersed during storm conditions. The rate at which this deposit will erode, and how much that contributes to overall turbidity during the storm-event, cannot be predicted based on available information.

An adverse impact of the deposited sediment over longer timescales on surrounding habitats cannot be excluded. Erosion of the southern deposit during storm events or hurricanes is likely to occur. This means that it cannot be ruled out that an extra total volume of 7000m3 sediment can be transported towards the southern reserve during a single hurricane event, potentially smothering coral and seagrass habitats. This might lead to severe impact on some species of corals and sea grasses. A significant part of the southern reserve is covered with these species. Current species coverage and abundance is not known, and therefore impact cannot be quantified.

Besides the intrinsic ecological value of the habitats of the southern reserve, the southern reserve holds many important dive sites. The environmental quality of the southern reserve habitats is therefore of high importance to the sustainable economic development and prospects of St. Eustatius. Any risk of deterioration of the southern reserve through resuspension of the dredged material and deposition within the southern reserve should be considered with caution and necessary preventive actions should be taken.

Potential indirect impacts on historical monuments could occur as a result of slightly increased wave heights in the harbour but are expected to be minor.

Indirect impact due to more extensive use of the harbour is expected to be a risk, but hard to quantify. Maintenance dredging is not expected, and if needed, the impact due to sediment suspension will no doubt be less than that of the dredging related to this extension, thus limited (assuming calm conditions and no silt content). Risk of bio-pollution is likely. In order to assess actual impact and proper measures, monitoring should be considered. 

Preventive actions should focus on the deposited sediment in the southern corner of the breakwater and lack of information on silt and mud content. Suggestions are to:

• Retrieve information on silt, mud and chalk content in the dredging area
• Make sure sediment deposits cannot erode towards southern reserve. Proper constructions
• should be considered with the contractor and island bureau
• Halt dredging and deposit activities temporarily in case of elevated seawater temperatures
• and during rough seas (to avoid multiple stress)
• Monitor surrounding habitat quality (reefs and seagrass) over time
• Monitor future use and related pressures and mitigate as considered needed