Recent studies show that late stage pelagic larval fish are not simply drifting with the currents as formerly believed, but are in some cases strong swimmers and more than capable of swimming against the ambient flow. There is evidence that larval fish may select specific habitats in which to settle. Although little is understood about their sensory abilities, both sound and smell have been linked to settlement of coral reef larvae (Leis 1997). On Bonaire, Netherlands Antilles, coral reefs, mangrove forests, and seagrass beds provide refuge and food for young fish. Some fish species are thought to spend the juvenile life stages in mangroves and seagrasses and abundances of certain adult reef fish species have been shown to be greater in coral reefs with surrounding seagrasses and mangroves (Mumby 2004). Larval fish may be able to select environments for settlement based on biological attractions detected by certain senses (Lecchini 2005). This study investigates the potential differences in the larval fish recruiting to mangrove and seagrass habitats with larval fish recruiting to coral reef habitats. Samples of larval fish were taken on the three nights surrounding the November new moon. Light traps and dip nets were used at two different sites, one a mangrove/seagrass habitat, and the other a coral reef habitat. Larger numbers of larval fish and more families were represented in the samples taken in the coral reef habitat than the mangrove/seagrass habitat.
Habitat selection is critical to the development, growth and reproductive success for most marine animals. Organisms select habitat based on food availability, as in gulls of the North Sea, complexity of structure, as in juvenile fish of seagrass beds, or a combination of both factors as in marine snails of the rocky intertidal zone. Mangrove forests are known to be important habitat and nursery grounds for many endangered and/or economically important species of fish. These habitats are currently under threat by coastal development and overexploitation, therefore, scientifically supported restoration efforts are currently being pursued. The mangroves of Bonaire are threatened by development, causing hypersaline conditions and change in water flow. In this study food availability and composition as well as the structural complexity of the mangrove prop root system were assessed as possible attractants to juvenile fish. Snorkeling observations along permanent transects of the fishes in and amongst the prop roots as well as the algae and invertebrates living on the prop roots were conducted to establish a baseline understanding of the mangrove community (consumers and prey). In order to assess the impacts of consumers on prop root epibiota, consumers were excluded from prop roots using plastic mesh and the regrowth of preferred food species was monitored. The caging experiment showed evidence of the impacts of herbivores in the mangroves as green algae growth increased significantly in their absence and growth of red algae decreased. Artificial mangrove units (AMUs), modeled after prop roots, were also constructed for this study to test the attractiveness of structural complexity to resident fish. Comparisons between the behavior of the fish community of mangrove prop roots and that of AMUs indicated that there is no distinguishable preference between predators and consumers for structurally complex habitat, but that both prefer more complex habitat over simple structure. The data collected concerning preferred food and habitat structural complexity could be used in restoration projects.
The importance of mangrove and seagrass lagoonal habitats as nursery areas for many reef-associated fish species is well established in the scientific literature. However, few studies have examined the relative use by nursery species of different sub-habitats within such systems. Here, we investigated fish community structure of a variety of interconnected sub-habitats of the tropical lagoon of Lac Bay in Bonaire, Dutch Caribbean. Visual census was used to test the degree to which these sub-habitats may differ in their use by fishes of different species and life stages. We quantitatively sampled the fish species abundance, composition, and size structures at a total of 162 sites distributed among nine different sub-habitats that are common to mangrove and seagrass ecosystems. Fish community variables differed consistently among sub-habitats and were mainly influenced by the presence of mangrove root structure or seagrass cover. Mangrove fringe sub-habitats were a premier habitat since multiple life stages of a variety of species showed highest densities and biomass there. Several reef fish species had a distribution pattern suggesting a unique stepwise post-settlement life cycle migration in which larger juveniles and/or subadults appear to move from the open bay environment (seagrass beds or bay mangrove fringe) to the interior mangrove fringes along mangrove pools before later departing to the adult habitat of the coral reef. In the case of the well-lit and well-circulated central bay sub-habitat, the limiting factor to fish abundance and diversity appeared to be the paucity of three-dimensional shelter due to the lack of Thalassia seagrass beds. In the warm and hypersaline backwaters, physiological tolerance limits were likely a key limiting factor. Long-term changes driven by mangrove expansion into this non-estuarine lagoon have been steadily reducing the net coverage of clear bay waters, while the surface of shallow, muddy, and hypersaline backwaters, unusable by key nursery reef fish species, has been increasing by an almost equal amount. Our study shows how fish density varies along the full gradient of sub-habitats found across a tropical bay to provide insight into the potential consequences for nursery habitat function when the availability and quality of these sub-habitats change in response to the long-term dynamic processes of mangrove land reclamation and climate change.
Deterioration of mangrove forests is occurring globally. The loss of mangrove habitats causes a decline in fishery resources, livelihood and biodiversity loss. Deterioration is also been seen in the mangrove forest of Lac Bay on Bonaire. Especially at the landside of the mangroves hyper saline conditions are found and Rhizophora mangles as well as Avicennia germinans trees are dying. The deterioration of the mangroves at the landside is partly compensated by expansion of the mangroves at the sea side. The main causes for the seaward shift seem to be, 1) less fresh water inflow during rainy season, 2) increased sediment transport towards Lac, 3) reduced interaction (tidal flow) between sea side and land side. This project has focussed on the dynamics of the fresh water fluxes toward the mangroves and the associated sediment transport. During two major rain events water discharge and sediment transport was measured for two catchments. Based on these measurements the total runoff amount and amount of sediments transported during the rainy season was determined. It can be concluded that runoff have brought only relatively minor quantities of fresh water to Lac. However, extreme events or extreme annual rainfall sums (1980, 1981, 1985, 1988, 2004, 2005) might have caused extreme runoff amounts and in addition extreme sediment load transports toward the mangrove system. Along with the deterioration of the channels and the associated water circulation, this might have caused root smothering and mangrove die off. This mangrove die off is probably strengthened by a relatively dry period with relatively less freshwater transport by surface runoff toward the mangrove system which enhances the hyper saline waters in the back of the system. Although deterioration of the landscape occurs since the 17th century, due to cutting down forests and overgrazing by goats and sheep’s, heavy rainfall events will not only lead to an increase in water supply but also to more extreme sediment transport rates toward the mangrove system. In addition, also removal of dams would cause an increase in surface runoff towards the mangrove system and an increase in sediment transport. Monitoring rainfall, surface runoff, and sediment transport over a large number of years together with monitoring the growth and the health of the mangrove forest would give more insight in how the mangrove systems reacts on hydrological changes over a number of years.
Mangrove map of Bonaire (GIS), based on aerial photograph interpretation from December 1995 and June 1996, true color, scale approximately 1:8,000. The mangroves of Bonaire are located at the Lac Bay area.
Sponges are major epibionts of mangrove roots in the Caribbean. Mangrove sponge communities in the Caribbean mainly consist of species that are typical to this habitat and community compositions often differ from those found on coral reefs nearby. Heterogeneity in species distributions between locations and within locations between roots is often reported. This study quantifies the diversity and abundance of mangrove associated sponges in the inner bays of Curaçao and Aruba and correlates variability of regional sponge diversity with environmental variables measured along the surveyed sites. Tannin concentrations vary between mangrove roots, and were correlated to sponge cover as a possible cause for habitat heterogeneity on a smaller scale. A total of 22 species was observed. Heterogeneity in species richness and abundance was apparent, and several sponge species were restricted in their depth of occurrence. Statistical data reduction suggests that sponge diversity may be partly explained by the distance towards adjacent reefs and to the degree of eutrophication, in which the latter is comprised of rate of planktonic respiration, total carbon and turbidity. Tannin concentrations did not determine within locality species heterogeneity as a priori postulated, but were positively related to sponge cover for reasons not yet elucidated.
This month’s issue focuses on the development of the Dutch Caribbean Biodiversity Database, which addresses one of the biggest gaps for nature conservation on our islands – lack of access to relevant and reliable biodiversity information. The database, which was recently relaunched at www.dcbd.nl is the central repository for all biodiversity-related research and monitoring data, maps and literature for the six islands.
Amongst others, you will find in this sixth issue:
- Overview of Research and Monitoring Efforts
- Mangrove Management for Conservation of the Queen Conch (by Sabine Engel)
- Invasive Seagrass in Lac Bay, Bonaire (by Sabine Engel)
- ‘Research of the Month‘: Dutch Caribbean Biodiversity Database (by Peter Verweij)
- Maritime Incident Response Workshop on St. Maarten
- Calendar of Upcoming Events, Meetings and Workshops
Remote sensing is an important tool for monitoring the environment. In this report we investigate the use of satellite images from two different satellites for monitoring the extent and health of the mangrove forests in Lac Bay, Bonaire. The different satellite bands were used to produce the Normalized Differential Vegetation Index (NDVI), the Enhanced Vegetation Index (EVI), the Atmospherically Resistant Vegetation Index (ARVI), and the Red Edge index. The above indices, the results from a Principal Components Analysis (PCA), an unsupervised, and supervised classification were used to classify extent and health of the mangroves.
The image from the RapidEye satellite with a spatial resolution of 5 meters in multispectral bands, covering the whole island produced vegetation indexes that generally were able to distinguish broader classes such as mangroves, water, and land. The high resolution image from the WorldView-2 satellite covering only Lac Bay with a spatial resolution of 2 meters in multispectral and 0,5 meters in panchromatic was able to distinguish between different species of mangrove and also appeared to be able to detect differences in health of the different species. Both satellite images can be used to estimate the extent of the mangroves, but only WorldView2 has a resolution that enables the detection of unhealthy (i.e. lower values for certain indices mostly related to chlorophyll content) areas with sufficient confidence. Apart from the technical characteristics, the RapidEye image is cheaper and covers the whole island of Bonaire, providing a full synoptic view.
We conclude that:
- Satellite images are a well suitable for monitoring areal extent, species composition, and health of mangrove areas in Lac Bay, Bonaire.
- RapidEye satellite images are usable for broad classification, whereas Worldview2 gives better resolution to also include species differences and health assessments.
The main conclusion from this study is that at the moment tourists and locals use Lac Bay and its catchment area at levels and in ways that are not sustainable.
- Recreational use is concentrated on and around the Sorobon Peninsula. The major activities are beach and sports-related (sunbathing and windsurfing). The main water based activities are windsurfing and swimming/wading.
- The social carrying capacity for the present kind of visitor and present kind of usage begins to become an issue around 250 beach visitors. Average visitor numbers on cruise days is 359 (highest is 760) and on weekends is 260.
- There is a large difference in visitor numbers between cruise days and non- cruise days, with cruise days having the highest.
- Awareness of Lac Bay’s regulations and zoning plan is low, resulting in certain activities taking place in sensitive zones.
- Fresh water surface-flow to the bay is affected by approximately 54 dams or more, and groundwater flow by many (uncounted) wells
- 213 kunukus (farms on Bonaire) are present in Lac Bay and its catchment area.
Livestock densities within the natural areas surrounding Lac Bay are not sustainable. They exceed the ecological carrying capacity of the area.
- Develop and implement a set of measures that can be used to preserve and enhance the Lac visitor experience in accordance with social carrying capacity.
- Develop sunbathing and water sport possibilities elsewhere on Bonaire to distribute user densities from Lac Bay.
- Develop and implement a set of measures that can be used to preserve and enhance the natural values of Lac Bay.
- Improve the implementation of the zoning plan.
- Organize several facilities at Sorobon more properly.
- Create a visitor centre at Sorobon.
- Reduce livestock densities in the Lac Bay catchment area.
Key recommendations for further study:
- Research on up to what extend tourists facilities are contributing to the eutrophication at Lac.
- Research on the effect of sunscreen on coral bleaching at Lac.
- Further research to obtain a clear overview of land use in Lac’s catchment area.
The main issues that Lac Bay faces were identified as follows:
- 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.
- 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).
- Litter contamination. Marine litter washed in from the open ocean and abandoned fishing lines in the deeper parts of Lac are big issues.
- 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.
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.
- 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.
- 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.