Mangroves

Executive Summary

This Site Characterization report provides a comprehensive coverage of past and present scientific research that had been conducted and recorded in Lac Bay during the 20th Century. Elements of environmental change are evaluated in this writing by comparing information from the prior studies, and drawing implications from them against recent data collected from a year long ecological monitoring series that were conducted throughout 1999 by Environics, N.V. Consulting.

Additional implications were based on ground-truthing a series of aerial photographs that were completed over the years. The consultant activities were proposed to the Bonaire Marine Park and VOMIL to respond to the management needs listed in the Bonaire Marine Park Terms of Reference LAC001/98, by investigating the following research questions:
• What is the current health and status of the bay area and surrounding environs, and how has it changed from the past to present, as determined through the interpretation of aerial
photographs, scientific research and local historical knowledge?
• What is the status of the globally endangered species, the queen conch (Strombus gigas), which inhabit Lac Bay?
• How is the bay currently being used by marine life, wildlife, and by humans?
• Are there indications of threats on the bay’s natural carrying capacity?

These would begin to provide relevant data in which to support decisions pursuant to the associated public concern issues of the following that are addressed in Section 9 of this report:
• Opening channels to “refresh” the dying mangrove wetlands;
• Extracting sand materials from the Sorobon site; and
• Fisheries exploitation of the queen conch.

The following key environmental problems were found in the research:
• Conch Population Decline Field monitoring activities determined that the conch population size class distribution of a 51,000-m2 area found approximately 111 conch individuals, with an average age of 2.5 years old. No
adult conchs were found in Lac Bay. In brief, the statistical results indicate that for both conch and invertebrate species in general, the spatial distribution patterns for conch locations in the bay, are found to
be in clumps, implying potential constraints on the population. For conch, fishing pressure may be likely, as the mapping results show the remnant conch population is located in the deeper bay channel or boat
routes, where the species have least potential accessibility by people fishing for conch. Evidence of juvenile conch species being fished out before reaching adult reproduction size can also be found in the
discarded conch piles at Cai, re-enforcing the field data that no adult conch exist in Lac. Taking juveniles may contribute to the extinction of conch in Lac Bay.

• Mangrove Die-off and Hydrology
Hydrological monitoring in the mangrove sub-basin areas found that annual salinities ranged from 44 0/00 to 180 0/00 throughout the area (normal bay salinity averages 44 0/00 ). Two main feeder channels
that circulate bay water from the Lac lagoon, into the mangrove sub-basin areas occur in the Kreek di Pedro and Kreek di Coco areas. A third, less regular tide water delivery channel occurs across the Isla di
Chico during high water events. Annual tidal flooding and draining phenomena occur during the March and September solstice periods that influences the environmental conditions in the mangroves wetlands.
During the March solstice, the tidal waters drain rapidly, depleting the oxygen balance in the natural system, and fish kills frequently occur during this annual event. This dynamic process is part of a natural
annual cycle that is typical for the mangrove environment at Lac.

Fresh water flow has been altered over history by building levees, such as at Mona Lisa levee, as well as ill-designed road construction and failing culverts. Periodic fresh water flow into the mangroves is a part of the natural system that provides balance for mangrove viability; it has been grossly altered at the mangrove sub-basin areas, and no longer functions.

• Sedimentation Processes in Lac
Sedimentation processes occur due to deposition of sands that are carried in from currents crossing over the coral “dam” that naturally separates the Lac from the coastal oceans. Sedimentation creates the
Awa Blanku area. The circulatory patterns in the bay move in a clockwise pattern, that also diverge into the feeder channels that provide water into the mangrove sub-basin areas of the north, depositing sand in
the process. In addition, sedimentation occurs in an ephemeral time scale in the area behind the Isla di Pedro. Historical interpretation of maps since the 1961 aerials, show sand deposition in 1961, no sand deposition in 1991, but sand deposition again in 1998 aerials. This again appears to be a natural cycle of the ecosystem rhythm, which occurs in Lac.

• Management Recommendations
After analyzing the GIS mapping results for seagrass habitat abundance, etc., a zonation plan of the bay is recommended to demarcate a sanctuary preservation area to replenish for viable conch and other wildlife habitats, as well as to designate multi-purpose recreational use areas. The map products include a GIS zonation map of the natural communities of Lac Bay in Section 3, with management recommendations for the greater bay area found in Section 10.

An example of a recommended zonation plan is found in Section 10. Thereafter, a fisheries management program could be implemented, enforced through issuance of permits and size class catch
limits is recommended, that would sustain conch population and commercial fisheries, as well as for local family consumption. Outside mangrove, shoreline restoration experts, as well as a hydrology engineer were invited to examine the “environmental” problems at Lac, and provide management recommendations in Section 9.

The conclusions of environmental symptoms, stressors and biological responses to stress that were identified through scientific research are contained in the final Section 10. In addition, community
stakeholders who are concerned for Lac met a consensus on how best to manage Lac; the comprehensive evaluation is provided in Section 10.

The following is a summary list of final management recommendations, as identified by scientific experts and community stakeholders for Lac Bay:
• Permit local, artesinal fishing practices in Lac only with catch quantity and size limits strictly enforced
• Issue permits to control fishing, netting or taking conch
• Initiate a 5-year moratorium against taking conch, and monitor every 3 years for size class distribution for the species in Lac Bay
• Enforce strict size limits of conch to protect taking of juveniles
• Enforce existing legislation to ban taking turtles, protect eggs and nests
• Legally exclude non-Bonaire registered boats to use Lac Bay waters
• Strictly prohibit sand extracting in Lac
• Support water quality standards at Cai
• DROB upgrade Cai road culverts to improve water circulation capabilities
• Disallow cutting of mangroves
• Monitor water quality standards at established sites around bay area
• Sorobon Beach Resort erosion can be safeguarded by removal of lower sections of the groin fence that are below the mean high water line that would greatly help the beaches to the north, without compromising privacy.
• Control car traffic and parking along beach strand of Cai; park outside of Cai area on hard limestone surface
• Establish an environmental education kiosk system throughout the Lac area to explain environmental processes, problems and solutions to promote community self-management and self-regulation.

There has been a recent increase in public awareness of environmental issues as the effects of climate change have become ever more noticeable in our daily lives. As we enter a new decade, it becomes useful to review what conservation efforts have worked so far, and take inventory of what efforts will be required for the future. Starting with the constitutional referendum creating the Caribbean Netherlands (Bonaire, St. Eustatius and Saba (BES), the response to conservation challenges of all six Dutch Caribbean islands have varied. Since 2010, the BES islands have seen an overall increase in funding support and conservation actions, and therefore presumably also saw greater improvements when compared to Aruba, Curaçao and Sint Maarten, though clearly not enough (Sanders et al, 2019).

The goal of this Transboundary Species special edition of BioNews is to provide an update on the latest published research results and highlight the need for transboundary protection. These species know no boundaries, and thus move between the Dutch Caribbean islands and beyond. Their protection will require broadscale conservation efforts which cover the entire Caribbean, including the six Dutch Caribbean islands. Collaboration between all six islands is of the utmost importance. This is one of the Dutch Caribbean Nature Alliance’s (DCNA) main goals: working together and sharing skills, knowledge and resources to maintain a solid network and support nature conservation in the entire Dutch Caribbean.

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.

This student research was retrieved from Physis: Journal of Marine Science II (Fall 2007)19: 32-36 from CIEE Bonaire.

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.

This student research was retrieved from Physis: Journal of Marine Science V (Spring 2009)19: 20-26 from CIEE Bonaire.

Abstract

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.

Abstract:

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:

1. Satellite images are a well suitable for monitoring areal extent, species composition, and health of mangrove areas in Lac Bay, Bonaire.
2. RapidEye satellite images are usable for broad classification, whereas Worldview2 gives better resolution to also include species differences and health assessments.