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. 

Abstract:

In 1999 a survey was carried out (Lott, 2000) to estimate the conch population and determine the status of the seagrass beds in Lac Bay, Bonaire. Since 1999 the recreational use of Lac has increased, new structures have been built around Lac and illegal conch fishing still goes on (conch fishing is forbidden by Marine Ordinance of 1991). In order to assess the effects of the moratorium and the impact of increased activity in and around Lac the 1999 survey has been repeated, applying the same methods. Fieldwork was carried out from the end of May 2007 through the beginning of October 2007.

Findings:

The main conclusion from this study is that the moratorium on conch fishing seems to be effective as Lac Bay’s conch population has increased; however, the cover of turtlegrass has diminished, which is most likely linked to an increase in anthropogenic disturbance of Lac.

• The conch population has increased: in the 51,000 m2 survey area, 223 live conch were found, against 111 in 1999.
• Adult conch were found (4% of the population) while in 1999 no adult conch were recorded.
• The cover of turtlegrass has diminished in most of the 17 grids (2,142 quadrats of 1 m2 ) studied; these grids were spread over Lac Bay. Grid Cai showed no reduction in cover.
• Trampling from windsurfers, snorkelers and other users of Lac Bay plays a major role in the reduction of turtlegrass.
• Cover by macro algae has changed but not necessarily diminished; in some grids the cover increased, in others it decreased.
• The number of most invertebrates has decreased.

Management Recommendations:

• Further study on Lac Bay’s species composition.
• Further study on Lac Bay’s water quality.

Findings:

Species richness

Species richness is highest in Lac’s mangrove thickets and salt flats. They have a two-fold higher total species richness and a fourfold higher migrant species richness compared to woodland habitats.

The coastal barrens habitats have the lowest number of species.

Composition of bird communities

63 species were recorded. 38% were resident species; 10% had resident populations but also potentially occurred as migrants, 49% were migrants and 3% were migrants that are known to possibly or irregularly breed on Bonaire.

Significant differences were found in the bird communities of the five habitats studied.

• Migrant species are significantly more abundant among the waterbirds (herons, flamingo, waders).
• The highest concentration of migrant birds is in the mangrove thickets and salt flat habitats
• Migratory shorebirds and waders dominate the bird communities of Lac’s salt flat habitat.
• The bird communities of Lac’s woodland habitats are dominated by breeding resident species.
• Migratory passerines are not a major component of Lac’s bird fauna.

Significance of Lac

Lac Bay may be of local significance as a breeding and foraging site to the globally Near-Threatened Reddish Egret, Egretta rufescens.

Lac Bay’s mangroves seem to be of local significance as a nesting and roosting habitat for the Scaly-naped Pigeon, Patagioenas squamosa.

The Yellow-shouldered Amazon no longer uses Lac’s mangrove thickets as a roosting area. Probable causes are decrease in persecution and increase in the quality of the woodlands in many areas of Bonaire.

The Caribbean Coot is not present in Lac’s salt flat habitat. It prefers a number of freshwater locations around the island.

Lac’s number of West Indian flamingo, Phoenicopterus ruber, has grown significantly. This suggests a wider ecological shift taking place within the bay (growth of stagnant backwaters and salt flats behind the mangroves) which threatens Lac’s premier values as a mangrove and seagrass nursery area.

Management Recommendations:

Further observations to document potential egret breeding activity at Lac.

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. 

This report gives an overview of the most prominent problems currently encountered at the Lac bay area. To change the currently negative situation at the Awa di Lodo area with dying mangrove and unfavorable conditions for fish, it is recommended to improve water circulation at the Awa di Lodo area. This will lead to a better water quality and especially to a lower salt concentration. Based on this analyses, recommendations are made for five specific management actions which individually or in combination could be part of a management plan for the ecological rehabilitation of Lac Bonaire.

Management actions are conducted both from the land and sea side, they are short or long term and they can be summarized as follows:

• To clear the upstream (north) section at the labado from sediments in order to restore its function as sediment trap.
• To clear the east and west side feeder channels from sediment and mangrove in order to improve water circulation in the Awa di Lodo area.
• To construct a new central feeder channel following an existing creek pattern to improve water circulation in the Awa di Lodo area. Whether or not this new feeder channel is going to be constructed, will depend on monitoring results of changes in salt concentration in the Awa di Lodo after the existing feeder channels have been cleaned.
• To start a discussion with livestock owners in the greater catchment area on how over-grazing by roaming livestock can be stopped while at the same time these owners can make a living from their business.
• To remove Rhizophora established propagules at the sea side in order to slow down mangrove occupation of the bay area.

Each operation can be separately budgeted and they can be phased in time. 

Findings:

• Ramicrusta overgrows 14 of Lac’s species of scleractinian corals including Acropora, Agaricia, and Diploria, as well as three species of gorgonians.
• Ramicrusta grows upward from the base of coral and causes the death of grown tissues.
• Complete overgrowth by the crustose calcareous algae may happen after several months.

Management Recommendations:

N/A

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.