Habitats

Saba Bank is a 2,200 km2 submerged carbonate platform in the northeastern Caribbean Sea off Saba Island, Netherlands Antilles. The presence of reef-like geomorphic features and significant shelf edge coral development on Saba Bank have led to the conclusion that it is an actively growing, though wholly submerged, coral reef atoll. However, little information exists on the composition of benthic communities or associated reef fish assemblages of Saba Bank. We selected a 40 km2 area of the bank for an exploratory study. Habitat and reef fish assemblages were investigated in five shallow-water benthic habitat types that form a gradient from Saba Bank shelf edge to lagoon. Significant coral cover was restricted to fore reef habitat (average cover 11.5%) and outer reef flat habitat (2.4%) and declined to near zero in habitats of the central lagoon zone. Macroalgae dominated benthic cover in all habitats (average cover: 32.5 – 48.1%) but dominant algal genera differed among habitats. A total of 97 fish species were recorded. The composition of Saba Bank fish assemblages differed among habitat types. Highest fish density and diversity occurred in the outer reef flat, fore reef and inner reef flat habitats. Biomass estimates for commercially valued species in the reef zone (fore reef and reef flat habitats) ranged between 52 and 83 g/m2 . The composition of Saba Bank fish assemblages reflects the absence of important nursery habitats, as well as the effects of past fishing. The relatively high abundance of large predatory fish (i.e. groupers and sharks), which is generally considered an indicator of good ecosystem health for tropical reef systems, shows that an intact trophic network is still present on Saba Bank

The Indo-Pacific lionfish species [Pterois volitans (Linnaeus, 1758) and P. miles (Bennett, 1828): Family Scorpaenidae] are the first nonnative marine fishes to establish in the Western North Atlantic and Caribbean Sea. Despite the continued documentation of its range expansion and highly publicized invasion (including public-driven removal efforts) there remains a paucity of basic information on lionfish ecology. This knowledge gap limits effective long-term management. In this study we conducted a multi-scale investigation of habitat occupancy of a newly established population of lionfish in Roatan, Honduras. Based on field surveys and citizen sightings in Roatan Marine Park we found that lionfish occurred more frequently on aggregate coral reef habitats (54% of sightings) compared to patch reef habitats (30%) and sea grass lagoons (16%). In general, these aggregate and patch reef habitats contained adults (mean total length =118.9 mm and 114.7mm, respectively) whereas sea grass habitats contained juveniles (mean total length=89.5 mm). At the micro-habitat scale lionfish occupied areas dominated by hard coral and overhanging structure; the same microhabitats containing native fishes of concern – grouper (Nassau grouper, Epinephelus striatus; yellow fin grouper, Mycteroperca venenosa) and snapper (dog snapper, Lutjanus jocu; mutton snapper, Lutjanus analis). Results from this study contribute information on basic habitat requirements of lionfish and inform current management removal efforts focused on containing spread and mitigating their impacts on native species

Findings

Abundant coral growth within the Saba Bank is restricted mainly to the two large windward reefs. These two reefs carry a very rich reef fauna.

The rest of the Bank only has a very small growth of corals due to several factors:

• Most of the food supply is probably filtered away by the large windward reefs.
• These reefs are located at a least favorable leeward position.
• These reefs are located at a considerably greater depth.

As in most other atoll lagoons, coral growth in the lagoon area is restricted to small patch reefs; the number of species is not significantly lower than on the reef, but the colonies are smaller in size and number.

The Saba Bank undoubtedly has a volcanic base but no information on this has been collected. This study did uncover some black sand, presumably of volcanic origin, on the southwestern part of the Bank. Geologists believe that a composite volcanic island is buried under the more recent formations.

Only the eastern and largest part of the Saba Bank can be called a living atoll with an open lagoon, while the western part is a bank with drowned fringing reefs.  Whether the western part is included or not, the Saba Bank ranks among the largest atolls in the world.

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.

A semi-detailed landscape-based vegetation map (scale 1:50,000) is presented for the southern Caribbean arid island of Bonaire (mean annual precipitation is 463mm). Color aerial photographs (1:8,000) taken in 1995 and 1996 were used to produce the map. A total of 302 vegetation sample plots were analyzed using a stratified random sampling design and twinspan cluster analysis.

A total of 18 vegetation types, and 32 (sub-)landscape types were distinguished. The three principal vegetation types, Casearia tremula-Prosopis juliflora type (Type 17), Croton flavens-Haematoxylon brasiletto type (Type 14) and Prosopis juliflora- Opuntia wentiana type (Type 16), account for 40% of total vegetation cover. The four principal landscape types also cover 40% of the island and are: D3 (Prosopis- Casearia Landscape), TH1(Haematoxylon-Croton Higher Terrace), D2 (Haematoxylon- Casearia Landscape) and TM7 (Acacia-Croton Middle Terrace). The vegetation on the volcanic Washikemba Formation is more uniform than that on the limestone forma- tions. Most of the vegetation types can be categorized as secondary. This is consid- ered mainly to be the result of the impact of introduced grazing mammals (principally goats and donkeys) and woodcutting in the past. Six vegetation types are considered of relatively high natural value. Three of these (Types 1, 9, and 10) are comparable to Stoffers’ less degraded communities. The other three have been selected based on cri- teria such as structural complexity, diversity and number and rarity of rare species.

A comparison with a vegetation map from the 1950’s shows that three types of areas can be distinguished: areas in which the vegetation has remained more or less the same, areas in which the vegetation shows improvement and areas that show broadscale de- terioration of the vegetation. The largest area that shows deterioration is the southern part of Bonaire. The northern part of the Washington-Slagbaai National Park is the largest area with improvement. The findings are discussed in relation to the Nature Management Plan for Bonaire and conservation recommendations are made. 

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 

Although coral reefs are well developed in the Leeward islands of the Netherlands Antilles, they are poorly developed in the Windward group. Coral communities are common in the Dutch Windward islands, but no structural reefs have been observed. Flat, sandy bottoms there seem to prevent reef development, as is also the case on large parts of the southwest coast of Aruba.

The zonation of corals on the reefs, with respect to depth, distance from shore, and conformation to the bottom, resembles that of other Caribbean reefs. Density of living-coral cover ranges in the several zones from nearly zero to almost 100%. Below 20 to 25 m on the forereef slope the corals are areally less abundant than crustose coralline algae. Generic diversity of hermatypic corals is comparable in the Leeward and Windward groups of the Dutch islands, with 24 and 23 genera present, respectively. These numbers are comparable to those of other highly diverse reefs in the Caribbean. The number of species in the Windward group, however, is relatively low. The differences in abundance of coral genera (and species) throughout the Caribbean needs more thorough investigation.

Findings

The main conclusion from this study is that the Saba Bank has little modern material/ sediment deposited in relation to present sea level. This contradicts with previous beliefs such as Spencer’s claim that the bank surface “has been leveled by coral growth and sands derived from them”.

The study found the following:

• Minimal reef development.
  • Limited data from the survey revealed a sparse coral cover but   abundant crustose coralline algae and sponges.
  • The Saba community showed no sign of forming an interlocking reef framework.
• Sediments rich in residual material derived from late Cenozoic limestone bedrock.
• Most samples from this area had relatively large quantities of calcite, which suggests erosion of bedrock on Saba Bank.
• The internal structure of Saba Bank remains unknown; the surveys lacked sufficient sub-bottom penetration to disclose information about internal structure.