water circulation

Hydraulic circulation of the tides in Lac Bay, Bonaire.

In Lac Bay, Bonaire, a part of the mangrove forest is dying partly because of reduced water circulation through the forest. Tidal waves are the main drivers of the water circulation in the forest. In this study, the tidal behaviour is analysed based on water depth measurements from earlier studies in 2012 in Lac Bay. Time series are used to determine the tidal range (difference between minimum and maximum water depth), the dominant tidal constituent (harmonic analysis) and the water circulation through the forest (difference between the timing of highest water depth). The results show that locations close to open water in the bay have on average a large tidal range (27.8-28.1 cm) than locations in the forest (3.4-8.1 cm), with limited to no seasonal fluctuations. The dominant tide in Lac Bay is the lunisolar diurnal constituent (K1). Water depth changes from stations close to open water could be explained reasonably but in other locations they were distorted by the forest and could not be fully explained. It was found that the water moves slowly through the forest in a circular manner, moving around the islands and through the forest, thereby creating a delay in high water depths up to five-and-a-half hours.


Key words: Mangroves; Lac Bay; Water depths; Tidal range; Dominant tide; Tidal constituents

Data type
Scientific article
Research and monitoring
Geographic location

Improving water circulation into backlands of Lac Bay through channel management

Lac Bay is the lagoon of Bonaire; in that lagoon a mangrove forest of red and black mangroves grows. The mangrove forest of Lac provides birds and fishes of the surrounding
area with several important ecosystem services, as breeding ground, nursery and feeding area. These eco-­‐services are threatened by a reduced water circulation which causes low
water heights in the backlands and thus a reduction of the area which is available to fishes. Water in the backland was measured hypersaline, the obtained result of low water circulation and high evaporation in the backlands.

This hypersaline values may reduce the area of viable mangroves in the backlands. The lagoon and the backlands are connected  through two feeder channels and those natural channels will overgrow when no maintenance is applied causing even less water flowing into the backlands. Calculating appropriate channel dimensions and formulating a maintenance plan a certain discharge into the backlands can be ensured, which may reduce the degradation of ecosystem services.

Data type
Research report
Research and monitoring
Geographic location

Lac Bay: Then and Now… A Historical Interpretation of Environmental Change During the 1900s A Site Characterization of Lac Bay for Resource Managers and Naturalists prepared for Bonaire Marine Park

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.

Data type
Research report
Education and outreach
Research and monitoring
Geographic location