An Assessment of the Health and Resilience of Bonaire’s Coral Reefs
Abstract:
From July 19-26, 2010, a dedicated team of researchers completed transect surveys on 25 reefs located on the leeward side of Bonaire and the adjacent Klein Bonaire to characterize the current status, threats, and resilience of Bonaire’s reefs. The assessments focused on corals, fish, algae and motile invertebrates using belt transects, point intercept methods and photographic documentation, incorporating attributes of the Atlantic and Gulf Rapid Reef Assessment (AGRRA) protocol and the IUCN bleaching resilience protocol. The main purpose of this work was to 1) assess changes in reef structure and health since the last region-wide AGRRA assessments (1998-2000) and other surveys (2001, 2005) by Bruckner; 2) identify sites in excellent health, exhibiting a high biodiversity and cover of reef building corals and an intact fish communities; and 3) characterize the health and resilience of these reefs. The intent of this project was to provide critical information that can assist the Bonaire government and Bonaire Marine Park in the conservation and management of their precious resources.
Between 5-15 m depth, cover of living coral was high on all reefs (approximately 50%), with exception of a few sites impacted by white plague outbreaks and shallow areas scoured by strong waves during previous storms. Cover by fleshy macroalgae was generally low, as compared to reefs in other Caribbean localities, although some deeper sites did have high cover of Lobophora and Dictyota spp. (brown macroalgae), and cyanobacterial mats were prominent in several locations (especially on Klein Bonaire); these algae occasionally carpeted the margins of coral colonies and were competing with living corals. Montastraea annularis (complex) were the dominant corals, in terms of living cover, occupying approximately 20-25% of the benthos, and making up over 50% of the total live coral cover. Agaricia, Madracis and Porites spp. were the other dominant corals, in terms of living cover. M. annularis complex was also most abundant taxa (numbers of colonies) at all sites overall, and also the dominant taxa between 5-10 m depth, while Agaricia was slightly more abundant at 15 m depth. While the proportion (number of colonies) of brooding species (especially Agaricia, Porites) was very high, their contribution to living coral cover was less than M. annularis (complex) because most colonies were small in size.
Based on size structure, abundance, levels of recruitment, and coral condition, coral communities could be divided into two primary groups, the M. annularis complex (M. annularis, M. faveolata and M. franksi) and all other species. Corals lumped into “other species” were small to medium- sized (mean=24 cm), and population structure exhibited a monotonic decline in size; most colonies were < 20 cm in diameter and very few colonies were over 60 cm. Although a small proportion of colonies showed active signs of disease and competition from other biotic stressors, these corals had low levels of partial mortality (8%), few completely dead colonies were observed (0.4%), and they were the predominant species colonizing dead skeletal surfaces of other corals as well as reef substrates.
The original size of M. annularis (complex) colonies was significantly larger (58 cm diameter) than all other species, although many had been reduced in size due to partial mortality and skeletal surfaces of colonies often contained numerous smaller tissue remnants. These corals were being affected to the greatest degree by coral diseases (white plague, yellow band disease, black band disease, dark spots disease) and other biotic stressors, including competition and overgrowth by sponges, encrusting gorgonians, hydrozoan corals and a tunicate, predation by snails and parrotfish, and damselfish algal lawns. Colonies of M. annularis (complex) were missing on average 30% of their tissue, although the largest corals (mean size =61 cm; about 50% of all colonies) exhibited significantly higher amounts of partial mortality (mean loss=50%) than smaller (mean=41 cm) corals (mean tissue loss=11%). The extent of partial mortality, large numbers of completely dead colonies (4.5% of 1602 examined corals), ongoing stressors that continue to plague this taxa, and absence of colonies less than 10 cm in diameter (indicative of a lack of recruitment success) is of serious concern for these reefs, as these are the dominant frame-builders and characteristically the longest lived corals in the western Atlantic. The better overall health and high levels of recruitment observed in other taxa, in combination with recent declines in M. annularis complex, may indicate these reefs are undergoing a shift in species assemblages, with communities being replaced by smaller, shorter lived corals.
Fish communities on Bonaire were relatively high in diversity, with a dominance by herbivores (especially parrotfishes and damselfishes). Many species of important predatory fishes were present, including the dominant western Atlantic species of snapper, grouper, jacks and grunts, although these predatory fishes may be declining as the size structure was dominated by small and medium-sized fish. In particular, grouper over 30 cm total length were very rare. Large- sized groupers are the most important members of the family, as these species change sex (large individuals are females) and the larger fish produce an exponentially higher number of offspring.
In general, Bonaire’s reefs show signs of high resilience and a good ability to recover from acute disturbances. Reefs had high coral cover, low levels of disease, high levels of recruitment, and low amounts of fleshy macroalgae. There are minor problems that need to be addressed through management actions and conservation strategies. This could include 1) a program to eradicate lionfish before their numbers get out of control (these species were seen, but they appear to be rare as compared to other Caribbean Islands); 2) community-based efforts to remove an encrusting tunicate, coral-eating snails, and three-spot damselfish; 3) a nursery/restoration program to propagate A. cervicornis and A. palmata and reintroduce these corals into their former habitat; 4) steps to increase the abundance of herbivorous sea urchin (Diadema antillarum) populations; 5) elimination of fishing on herbivores (parrotfish caught along the shoreline using handlines) and top predators (groupers); and 6) better sewage treatment and other strategies to reduce run-off and nutrient input from hotels located along the coastline.