Lac Bay

Bird communities of contrasting semi-natural habitats of Lac Bay, Bonaire, during the fall migration season, 2011

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.

Date
2013
Data type
Research report
Theme
Research and monitoring
Report number
C164/12
Geographic location
Bonaire

Extent and health of mangroves in Lac Bay Bonaire using satellite data

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. 
Date
2013
Data type
Research report
Theme
Research and monitoring
Report number
C190/11
Geographic location
Bonaire

Ecological rehabilitation of Lac Bonaire by wise management of water and sediments

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. 

Date
2013
Data type
Research report
Theme
Research and monitoring
Report number
Alterra Report 2448
Geographic location
Bonaire

Crustose, calcareous algal bloom (Ramicrusta sp.) overgrowing scleractinian corals, gorgonians, a hydrocoral, sponges, and other algae in Lac Bay, Bonaire, Dutch Caribbean

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

Date
2010
Data type
Scientific article
Theme
Research and monitoring
Document
Tags
Journal
Geographic location
Bonaire

Eutrophication status of Lac, Bonaire, Dutch Caribbean Including proposals for measures

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.
Date
2011
Data type
Research report
Theme
Research and monitoring
Report number
C093/11
Geographic location
Bonaire