Coral cover

This report characterizes the state of Bonaire’s reefs as of March 2005. We pay particular attention to structural and functional attributes of reefs that have changed in so many other Caribbean reefs. We characterize coral reefs by their resident organisms and the forces regulating their distribution and abundance. Thus, corals, algae and fish define the “structure” of coral reefs but climate changes, diseases, hurricanes, overfishing, sedimentation and excess nutrients may affect how they “function”. Recent unfavorable changes in the structural and functional attributes of reefs have caused “the coral reef crisis” (Bellwood et al. 2004). In Caribbean coral reefs the most alarming changes have been the declines in the abundance of corals, sea urchins and reef fishes and the accompanying increases in large harmful seaweeds (called “macroalgae”). The decline in coral and increase in macroalgae, called a “phase shift”, represents a significant change in the structure of coral reef ecosystems that could lower its resilience.

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In March of 2005, a team of graduate students from the University of Maine revisited six study reefs on Bonaire to determine the status of those reefs and to detect if any change has occurred since March of 2003 when the last such survey was conducted. The study sites established in 2003 from north to south are: Karpata, Barcadera, Reef Scientifico, Forest on Klein Bonaire, Plaza and Windsock. Bonaire’s shallow (10 m) reefs remain in good condition. Coral cover averaged 47% in 2005 compared to 46% in 2003 (no change). Turf algae have increased and coralline algae have declined slightly over the past two years. Harmful seaweed “macroalgae” abundance remains low (2% in 2005 and 5% in 2003; see Steneck in this report) at the 10 m depth we studied. At depths below 20 m, macroalgae are now and have been (for at least the past 30 years) much more abundant (e.g. Van den Hoek et al. 1975) The absence of macroalgae in Bonaire most likely relates to the abundance of seaweedeating species or “herbivores”. Caribbean-wide, harmful macroalgal seaweed abundance corresponds inversely with the abundance of grazing fish such as parrotfish and tangs (Fig. 1). No comparable plot exists for seaweed abundance and any other measured factor on reefs.

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Changes over the past two decades
Comparisons between the status of reefs over a few years tell us little about long-term changes. For example, today there is a distinct demarcation between where Bonaire’s fringing reefs begin at 5 to 10 m depth and the shore. This region today is largely coralfree and dominated by rubble and sediment laden turf algae. However, this may not have always been the case. Prior to whiteband disease that killed nearly 90% of the elkhorn and staghorn corals in the Caribbean (i.e. Acropora palmata and A. cervicornis) (Aronson et al. 1998, Aronson and Precht 2001), most of the near shore zone was coral-dominated.
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Coral cover in the near shore zone surrounding Bonaire has declined dramatically and is now dominated by dead coral rubble where once elkhorn and staghorn corals had formed near monocultures prior to white band disease. Five of our six study sites have changed dramatically over the past 20 years except for Karpata. The decline of the Acropora species may have allowed competitively inferior species such as lettuce, pencil, finger and fire corals (Agaricia spp, Madracis spp, Porites porities and Millepora complanata) to expand since all have increased in abundance since the Van Duyl study (1985). Corals are not the only group to have changed dramatically since the 1980s. Diadema antillarum, the dominant grazing sea urchins was abundant in the near shore zone until it succumbed to the mass mortality of the mid 1980s. Today, more than 20 years later it remains below detectable levels at most of the sites we studied (Smith and Malek this report, Steneck this report). These changes, along with the significant declines in large predator finfish (see Bonaire Report 2003) indicate that several key players for the resilience of coral reefs (e.g. Fig. 3) have declined in abundance.

Bonaire’s reefs remain among the best in the Caribbean. However, our monitoring has revealed some potentially troubling trends that may require management action. In 2005, we reported to the Bonaire Marine National Park on the status of Bonaire’s coral reefs, and we suggested a strategy for monitoring trends among four key reef attributes we believe track the health and resilience of Bonaire’s reefs (Steneck and McClanahan 2005). Here we report the results of monitoring studies conducted 2003, 2005 and now 2007 at each site. Where appropriate, we drew from Bonaire’s first AGRRA assessment conducted in February 1999 (Kramer and Bischof 2003) to extend temporal trends over a period of eight years. 
Troubling trends
We see three troubling trends of increased macroalgae, declining herbivory from parrotfish, and increases in damselfish populations. Of these, the first two are most serious (see Chapters 1, 2 and 3). Secondary trends of concern, increases in damselfish populations (Chapter 4) and declines in coralline algae (Chapter 1), could lead to reduced recruitment of reef corals (Chapter 7), but to date this is not evident (Chapter 7). Importantly, coral cover remains relatively high (Chapter 1). The monitored group of carnivorous fishes, the lutjanid snappers, are holding constant but we remain concerned about the past (Steneck and McClanahan 2003) and continued loss of other larger bodied reef carnivores such as groupers and barracuda. The positive ecological role of parrotfish is well documented (e.g. Mumby et al. 2006) so their decline is troubling. It is unclear exactly why their population densities are declining. While parrotfish are not currently a widely sought group of reef fish (Chapter 8), fishing pressure on them is growing. It is possible they are vulnerable to even modest fishing pressure, particularly from fish traps. Accordingly, we recommend that the capture and killing of parrotfish be stopped because of their key ecological role on Bonaire’s coral reefs. Further, other groups of grazing herbivores such as the longspined sea urchin (Diadema antillarum) are increasing but too slowly to effectively replace the functional role of parrotfish (Chapter 1). We suggest continued monitoring of key drivers of reef health (coral cover, algal abundance, herbivory and coral recruitment). Some standard protocols such as the Atlantic and Gulf Rapid Reef Assessment (AGRRA) are entirely commensurable with the data presented in our reports in 2003, 2005 and 2007 (this report). A streamlined monitoring protocol is likely to be most useful to managers to alert them as a potential problem is growing and, perhaps more importantly, to show improvement when it occurs.
 
 

Bonaire’s coral reefs remain among the healthiest in the Caribbean. Although the island’s reefs have suffered bleaching disturbances similar to those plaguing reefs throughout the Caribbean, they uniquely show signs of recovery. Here we highlight key findings from our March 2015 biennial coral reef monitoring expedition. We put the findings in the context of both the trends recorded since 2003 when we began our regular monitoring and the most recent research related to the factors controlling the structure and functioning of healthy coral reef ecosystems. 

The Saba Bank is the largest submerged carbonate platform of 2,200 km2 in the Caribbean Sea, which lies partially within the Exclusive Economic Zone of the Netherlands and partially within the territorial waters of Saba and St. Eustatius. The Saba Bank houses an expansive coral reef ecosystem with a rich diversity of species and as such is also an important source of commercial fish for the nearby islands.

The Saba Bank furthermore forms the largest protected area of the Kingdom of the Netherlands, after the Dutch part of the Wadden Sea in Europe. It was declared a protected area by the Dutch Government in 2010 and has been registered as such in the Specially Protected Areas and Wildlife (SPAW) protocol of the Cartagena Convention for the Protection and Development of the Marine Environment of the Wider Caribbean. In 2012 it was internationally declared a Particularly Sensitive Sea Area (PSSA) by the International Maritime Organization (IMO) and an Ecological or Biological Significant Area (EBSA) by the Convention on Biological Diversity (CBD). As there are no large land masses nearby, the Saba Bank can be considered as relatively pristine and remote from human influences. Anthropogenic threats such as fisheries and environmental threats such as climate change, sea surface temperature increase and acidification, however, also threaten the Bank’s coral reefs.

As part of the Saba Bank research program 2011-2016, commissioned by the Dutch Ministry of Economic Affairs (EZ), expeditions to the Saba Bank were conducted in October 2011 and from 19 to 26 October 2013. The Saba Bank research program aims to obtain information on the biodiversity, ecological functioning and carrying capacity for commercial fisheries to facilitate sustainable management of the area. The expedition was funded by the Dutch Ministry of Economic Affairs and the World Wildlife Fund in the Netherlands.

The primary objectives of the 2011 and 2013 research expeditions were to collect data on benthic and reef fish communities, and on sponges and nutritional sources of the sponge community. Studies added to the 2013 expedition were research into the structural complexity of the reef; coral-algal interactions; and connectivity between populations. An international, multidisciplinary team of marine biologists investigated the coral reef structure as well as the spatial variation in species assemblages and population genetic connectivity of corals, algae, fish and sponges during eleven SCUBA dives at 20-30m depth.

During the expedition thirty-three 50m long transects resulted in more than 2000 images of the reef, and over 5000 fish counts of almost 100 fish species. A preliminary comparison with the data from 2011 gives the impression of a reduction in snappers, groupers and grunts, while there were noticeably more sharks. There were fewer algae on the Saba Bank than in 2011, possibly indicating a healthier reef, although there appeared to be a gradient of increasing algal cover towards the island of Saba. It seems unlikely that this is related to anthropogenic activities on the island, but more likely to natural causes.

An overview of collected data and preliminary results is given in this progress report. Further comparative analysis between the data collected in the 2011 and 2013 and further analysis between research components, e.g. between algal biomass, herbivorous fish biomass and nutrient levels, will be performed in 2014. This may give more information on the potential causes of the observed south-north algal gradient.

The expedition elicited large public interest and media coverage in both Dutch and Caribbean media (details provided in Appendix F). The work of the researchers, both above and under water, was also recorded on film as part of the documentary series Marine Life for Discovery Channel.

Abstract:

The reefs surrounding Bonaire have long been renowned for their high abundance and diversity of scleractinian corals and for their long-time status as a protected marine park. As part of a comprehensive study of the current status of Bonaire’s reefs, a quantitative benthic community survey was conducted at 7 locations across depths from 10 to 30 m in early 2008. This showed mean coral cover ranging from 23.7% ± 3.8 (SE) to 38.4% ± 4.4 (SE) on the island’s leeward shore and 1.6% ± 0.4 (SE) to 22.4% ± 3.6 (SE) on the windward shore. Percent cover of macrophytes and turf algae ranged from 41.7% ± 2.0 (SE) to 51.8% ± 3.0 (SE) on the leeward shore and 60.8% ± 4.1 (SE) to 82.7% ± 2.5 (SE) on the windward shore. Comparison of these results to earlier work from 1982 on the leeward shore and 1988 on the windward shore points to a significant, system-wide decrease in corals, increase in macroalgae and increase in the ratio of algae to corals. These observations point to significant degradation of reefs at a site often described as relatively ‘pristine’ in the Caribbean and highlight the sensitivity of coral reefs to anthropogenic and natural stresses even on well-protected islands.