Engel, M.S.

The coastal deposits of Bonaire, Leeward Antilles, are among the most studied archives for extreme-wave events (EWEs) in the Caribbean. Here we present more than 400 electron spin resonance (ESR) and radiocarbon data on coarse-clast deposits from Bonaire’s eastern and western coasts. The chronological data are compared to the occurrence and age of fine-grained extremewave deposits detected in lagoons and floodplains. Both approaches are aimed at the identification of EWEs, the differentiation between extraordinary storms and tsunamis, improving reconstructions of the coastal evolution, and establishing a geochronological framework for the events. Although the combination of different methods and archives contributes to a better understanding of the interplay of coastal and archive-related processes, insufficient separation, superimposition or burying of coarse-clast deposits and restricted dating accuracy limit the use of both fine grained and coarse-clast geoarchives to unravel decadal- to centennial-scale events. At several locations, distinct landforms are attributed to different coastal flooding events interpreted to be of tsunamigenic origin. Coastal landforms on the western coast have significantly been influenced by (sub)-recent hurricanes, indicating that formation of the coarse-clast deposits on the eastern coast is likely to be related to past events of higher energy.

Raw data of Conch monitoring for the Conch restoration project on Lac, Bonaire, including sizes, flared lip, lip thickness, etc.

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Abstract:

Halophila stipulacea (Hydrocharitaceae) is reported for the first time from Aruba, Curaçao, Grenadines (Grenada), St. Eustatius, St. John (US Virgin Islands), St. Martin (France), and St. Vincent and the Grenadines, bringing the total number of known occurrences from eastern Caribbean islands to 19. Native to the Red Sea and western Indian Ocean, H. stipulacea spread to the Mediterranean Sea in the late 1800s and became established in the eastern Caribbean in 2002. The species has dispersed north and south of its first sighting in Grenada and now spans a latitudinal distance of 6° (>700 km), most likely facilitated by a combination of commercial and recreational boat traffic. The continuing range expansion of H. stipulacea indicates the species has successfully acclimated to surviving in the Caribbean environment, warranting further investigation into its ecological interactions with the indigenous seagrasses.

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:

• 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

An encrusting red alga (Ramicrusta sp., Peyssonneliaceae) present in Lac Bay, Bonaire, overgrows and kills corals and other sessile organisms. Living coral tissue comprises 7.2 % of the benthic composition of the shallow reef, while Ramicrusta sp. covers 18.7 % of the substratum. Of 1374 coral colonies surveyed, 45.8 % were partially overgrown by Ramicrusta sp., with P. porites, P. astreoides and M. complanata being the most susceptible to overgrowth. Mean Ramicrusta sp. maximum overgrowth rates ± SD were 0.08 ± 0.05 mm d-1, 0.07 ± 0.03 mm d-1 and 0.06 ± 0.02 mm d-1 for M. complanata, P. porites and P. astreoides, respectively. None of the 71 coral recruits surveyed were growing on Ramicrusta sp. Ramicrusta sp. is an immediate threat to corals, reduces the area of suitable substratum for coral settlement and may have the ability to influence coral species composition. 

Findings:

The main conclusion of this study is that the shallow, warm and saline back-water habitat which is continuing to increase in importance within Lac Bay is unable to support meaningful mangroves, seagrass or algal meadows, nor the key nursery species. As the natural process of land reclamation by mangroves carries on, the bay’s important nursery habitats will come under additional salinity stress and likely continue to decrease in coverage and quality at an accelerated rate.

Distribution of sea grass and algal beds in Lac Bay

• The valuable seagrass and mangrove habitats of Lac are currently trapped in an enclosed bay.
• High light-intensity and well-circulated shallow habitats that fringed the mangroves of the central bay have the richest assemblages with the highest biotic coverage.
• Isolated mangrove pools have the lowest total cover, species richness and biodiversity of all habitats.
• Biotic diversity and cover decrease towards the deeper parts of the bay.
• There is an alarmingly rapid invasion of the bay by the invasive seagrass H. stipulacea.

Fish species utilization of contrasting habitats in Lac Bay

• Fish community variables differ consistently among habitats and are influenced by the percent cover of seagrass vegetation or presence of mangrove-root structure.
• Mangrove fringe habitats are a premier habitat since multiple life stages of a variety of species showed highest densities there. Mangrove fringing open waters had highest overall fish densities and species diversity.
• The various vegetated sub-habitats all play a unique role for different size-classes of different fish species. 

Management Recommendations:

• Management action is needed to stem further erosion of nursery habitat quality and ensure that a tipping-point is not reached beyond which recovery may be difficult or impossible.
• Measures should be taken to help restore water depth and circulation to relieve the bay’s ecosystem of thermal and salinity stress caused by the shallow backwaters. This includes excavating accumulated erosional and biogenic sediments as well as dredging to restore former feeder channels by removal of mangrove overgrowth (as already started by Stinapa).
• Further studies to assess the impacts of the invasive seagrass H. stipulacea on the bay’s flora and fauna.

Abstract:

Although Bonaire’s coral reefs remain among the healthiest and most resilient in the Caribbean, this IUCN report based on the IUCN Resilience Assessment of Coral Reefs highlights some of the threats that exist to Bonaire’s coral reefs, and which could have serious implications for resilience to future climate change and other threats. The report identified recommendations for addressing the current threats, as well as high and low resilience sites.

The threats and recommendations identified include:

Coastal development and artificial beaches.
Recommendation: All coastal construction on Bonaire should be strictly regulated and follow the construction guidelines. The guidelines should become law in order to be enforced appropriately.

Leaching from septic tanks.
Recommendation: It is strongly recommended that Bonaire invest in appropriate sewage treatment facilities to improve water quality and increase the resilience of its valuable coral reefs. It is also recommended that a water quality monitoring program be set up and sustained.

Increasing damselfish populations.
Recommendation: It is recommended that the fishing of predatory fish species on Bonaire’s coral reefs be controlled and managed to a sustainable level to prevent population explosions of prey fish capable of modifying the reef habitat.

Trididemnum and Lobophora.
Recommendation: It is recommended that the populations of Trididemnum and Lobophora are closely monitored and the factors contributing to the unnatural abun- dance of these coral-overgrowing organisms should be studied and then eliminated.

Due to a variety of factors affecting resilience which were assessed using the IUCN methodology, sites were also ranked according to their overall resilience: 

It is noteworthy that sites with lowest resilience ratings (e.g. Chachácha) are those most impacted by coastal development, while sites with highest resilience ratings (e.g. Marine Reserve North, Playa Frans, Karpata, Margate Bay, Vista Blue and South Bay) are those furthest away and least impacted by coastal development.