An assessment of coral reefs in Tobago

Transcription

1 Caribbean Journal of Science, Vol. 46, No. 1, 83-87, 2010 Copyright 2010 College of Arts and Sciences University of Puerto Rico, Mayagüez An assessment of coral reefs in Tobago Jennie Mallela 1, 2, Richard Parkinson3, and Owen Day 3 1 Department of Life Sciences, The University of the West Indies, St Augustine, Trinidad, West Indies 2 ARC Centre of Excellence for Coral Reef Studies & Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia; < j_mallela@yahoo.com> 3 Buccoo Reef Trust, Carnbee, Tobago, West Indies A BSTRACT. The coral reefs of Tobago represent some of the southernmost reefs in the Caribbean and have developed under the influence of runoff (e.g. terrestrial sediment and nutrients) from South American rivers. Local terrestrial runoff resulting from poor land management practices have also impacted reef development. Benthic surveys were conducted at 11 sites around the island in order to assess reef status. Mean (±SD) coral cover across Tobago was 14.9 (±7.6) % and macroalgae cover was highly variable ranging between 65 % at Bulldog Reef (Atlantic Coast), to 1.2 % at Mt Irvine (Caribbean coast). Montastrea faveolata (Ellis) and Diploria strigosa (Dana) dominated scleractinian coral communities and gorgonians accounted for 12.3 (±7.1) % of total benthic cover. Yellow band disease was observed on the major reef builders, M. faveolata, at most sites. The grazing urchin, Diadema antillarum (Philippi), have not recovered since their demise in the 1980 s. However despite limited grazing, the majority of monitoring sites were still dominated by coral communities. K EYWORDS. assessment, coral, Tobago, sediment, Diadema antillarum, Orinoco, yellow band disease Introduction Tobago s reefs represent some of the most unusual reefs in the region, located at the southern extreme of the Caribbean and close to the South American mainland. The proximity to South America results in coastal zones experiencing major annual runoff events including freshwater, sediment and nutrients from the Orinoco and Amazon Rivers (Muller-Karger et al. 1988; Risk et al. 1992). Consequently, reefs are characterised by a limited number of sediment tolerant coral species (Laydoo 1991). Tobago s reefs are considered to be at high risk (Burke and Maidens 2004). Threats identified by reef scientists in the 1960 s (Goreau 1967) and 1970 s (Kenny 1976) still exist today. Threats from human activities include coastal development, tourism, sedimentation, sewage, marine based pollution, overfishing and poor marine resource management practices (Agard and Gobin 2000; Lapointe et al. 2001; Wilkinson 2002; Mallela and Harrod 2008). Tobago s reefs have been heavily impacted by marine diseases and terrestrial pollution. In the 1970 s, white band disease destroyed the dominant Acropora species (Laydoo 1984; Julian 83 Kenny unpublished photos). By the 1980 s, disease was responsible for loss of the keystone herbivore, Diadema antillarum, and Aspergillosis was first noted on seafans (Goreau 1998). By the 1990 s, sponge disease was occurring on slit barrel sponges (Goreau 1998) and during the rainy season of 1999, a large scale reef fish mortality occurred. The cause of the fish kill was never confirmed; however, it was suggested that deteriorating water quality associated with runoff from South American rivers may have been the cause (Siung-Chang and Lum-Kong 2001). In 2005, a coral bleaching event heavily impacted Tobago s reefs with up to 85% of coral colonies bleaching. This subsequently led to high levels of mortality (75 %) for framework building species such as Colpophyllia natans (Houttuyn) and Diploria spp. (Wilkinson and Souter 2008). Recent observations confirm the occurrence of several coral diseases: yellow band, black band, dark spot, white plague, white pox, white band type 1 and Aspergillosis (Hoetjes et al 2002; Harding et al 2008; Mallela and Parkinson 2008). Mallela and Crabbe (2009) documented reductions in coral recruitment in years following bleaching, hurricanes

2 84 J. MALLELA, ET AL. and tropical storms. However, despite multiple disturbances, corals were still successfully recruiting on Tobago s reefs, albeit in low numbers. There is a clear lack of current islandwide information regarding the coral reefs of Tobago. The aim of this work was to report on the current status of reefs across Tobago. The following indicators were used to assess reef state: sedimentation rates (mg cm -2 d-1 ), live coral cover (%), macro algae cover (%), the occurrence of yellow band disease (YBD), and the abundance of grazing urchins, Diadema antillarum (individuals m -2 ). Methods Reef sites characterised by different levels of terrestrial runoff and anthropogenic disturbance were selected for monitoring around the coast line ( Figure 1 ): Kariwak, Buccoo Reef (outer reef), Mt Irvine, Plymouth, Culloden, Little Englishman s Bay (LEB), Sisters Rocks, Pirates Bay, Black Jack Hole (BJH), and Cove ( Figure 1 ). Sedimentation rates (mg cm -2 d -1 ) were used as an indicator of sediment stress. Sediment traps (n = 3 site -1 ) were positioned at 10 m depth on the reef, 0.5 m above the substratum from April to May Sediment traps were retrieved monthly, or when logistically possible. Benthic reef communities were assessed using 20 m long transects (n = 3 site -1, depth = 10 m). Data was recorded using an underwater digital video recorder. Digital video footage was analysed using the Coral Point Count (CPCe) software (Kohler and Gill 2006). All values, unless otherwise stated, are presented as mean (± SD) benthic cover (%). Since the bleaching event of 2005, the major framework building species in Tobago, Montastrea faveolata, has been affected by YBD (Mallela and Parkinson 2008). Presence or absence of YBD was noted by the divers on coral colonies along transects. Diadema antillarum presence was noted (individuals m -2 ) 0.5 m either side of Fig. 1. A map of Tobago showing the 11 monitoring stations and mean benthic cover (± SE) at each site.

3 TOBAGO CORAL REEFS 85 the transect line. The data presented here are from surveys conducted during the dry season in Results and Discussion Results from benthic surveys and sediment traps were highly variable around the island (Figures 1, 2 and 3 ). All data presented here was collected during the dry season (April and May, 2007) and mean (± SD) sedimentation rates (mg cm -2 d -1 ) for this period ranged from 5.2 (± 0.9) at Pirates Bay, a site influenced by terrestrial runoff, Fig. 2. Box whisker plots showing dry season sedimentation rates (mg cm -2 d-1 ) at the 11 study sites in Tobago (April May 2007). to 0.4 (± 0.1) at Sisters Rocks, an offshore site, not subjected to local land based runoff. Such values suggest that during the dry season the reefs of Tobago are not heavily impacted from sediment inputs (sedimentation rate: < 10 mg cm -2 d -1, Rogers 1990). However, during the wet season terrestrial runoff increases dramatically at reef sites (Agard and Gobin 2000, Mallela and Harrod 2008) with underwater visibility often being reduced to < 1m. Community composition (% benthic cover) varied between the Caribbean and Atlantic coastlines with scleractinian corals (herein referred to as coral) and gorgonians dominating Caribbean reefs (mean coral cover: 17.3 ± 6.1 % and gorgonian cover: 15.2 ± 6.0 %). An exception was Little Englishman s Bay, which although located on the Caribbean coast was dominated by macroalgae. This site has been heavily impacted by terrestrial sediment inputs associated with road building activities in 2006 (e.g. sediment smothering of corals was observed). Macroalgae (e.g. Dictyota spp) dominated reefs on the Atlantic side of the island (40.5 ± 22.3 %) ( Figure 2 ). Coral cover was not significantly different between the Caribbean (17.3 ± 6.1) and Atlantic side (8.5 ± 8.7) of the island (t = 1.9, d.f. = 9.0, P > 0.05), however live coral cover was negatively correlated with sedimentation rates (Pearson correlation: r = , P <0.05). Fig. 3. Benthic cover (%) for each coral species observed at each site (circle sizes represent % cover), total coral cover (%) per site, and presence (Ö) of Yellow band disease (YBD) on M. faveolata.

4 86 J. MALLELA, ET AL. Mean coral cover (%) across all monitoring sites was 14.9 (± 7.6) % with coral cover peaking at 25.8 % at Sisters Rocks and falling to 2.5 % on Bulldog Reef. Coral cover around the island was dominated by massive framework builders in particular Montastrea faveolata (Ellis) (4.8 %), Diploria strigosa (Dana) (2.4 %), and Siderastrea siderea (Ellis) (1.5 %) ( Figure 3 ). The hydrocoral Millepora alcicornis (Linnaeus) (2.3 %) was also commonly observed. Coral species diversity proved to be significantly higher on the Caribbean side of the island (t-test: t = 2.3, d.f. = 9.0, p < 0.05), with diversity peaking at Plymouth (Shannon-wiener: H = 2.04) and lowest at Bulldog Reef (H = 0.5). Cover of macroalgae ( Figure 1 ) also proved highly variable around the island with levels being significantly higher on the Atlantic side of the island (t = -3.5, p < 0.01). Our results have revealed that the majority of reef areas surveyed in Tobago, in particular sites on the Caribbean side of the island, are dominated by corals and gorgonians with scleractinian coral cover being dominated by Montastrea faveolata. However, the health of this species is currently being challenged by the commonly occurring YBD which was observed at all sites with M. faveolata with the exception of Mt Irvine ( Figure 3 ). Montastrea faveolata was not observed on transects at Bulldog Reef and Cove. These two sites were algal dominated (65 and 34 % respectively). The spread of coral diseases such as YBD on the key reef building species on the island could prove highly detrimental to reefal framework accretion, especially as coral tissue has not been noted to recover from YBD (Cervino et al 2001). Tobago s reefs are overfished (Burke et al 2008) and grazing activity by urchins were limited at all reef sites. The grazing urchin D. antillarum were occasionally observed in small pockets in the shallows on the reefs ( 5 m) however, urchins were not noted on any 10 m transects indicating that populations have not recovered since the 1980 s die off. In conclusion, while many sites are still dominated by coral communities, Tobago s reefs are showing signs of deterioration. Key threats include: limited enforcement of reef management policies, deteriorating water quality, algal phase shifts, reductions in coral recruitment, coral disease (e.g. YBD) and over fishing. Acknowledgements. This work was funded by the following organisations: The University of the West Indies in Trinidad and Tobago, the GEF-funded Integrating Watershed and Coastal Areas Management Programme (IWCAM), The Buccoo Reef Trust, The Government of Trinidad and Tobago Research Development Fund. We thank the many dive buddies who helped with this work. We also thank anonymous reviewers for greatly improving this manuscript. Literature Cited Agard, J. B. R., Gobin JF (2000) The Lesser Antilles, Trinidad and Tobago. In: Sheppard C (ed) Seas at the Millennium: An Environmental Evaluation C Sheppard (ed). Elsevier Science Ltd., pp Birkeland, C. (1977) The importance of rate of biomass accumulation in early successional stages of benthic communities to the survival of coral recruits. Proceedings of the 3rd International Coral Reef Symposium, Miami: Burke, L. M., Maidens, J. (2004) Reefs at Risk in the Caribbean. World Resources Institute. P 80 Burke. L. M., Greenhalgh, S., Prager, D., Cooper, E. (2008) Coastal Capital - Economic Valuation of Coral Reefs in Tobago. World Resources Institute. P 76 Cervino, J., Goreau, T. J., Nagelkerken, I., Smith, G. W., Hayes R (2001) Yellow band and dark spot syndromes in Caribbean corals: distribution, rate of spread, cytology, and effects on abundance and division rate of zooxanthellae. Hydrobiologia 460: Fabricius, K. (2005) Effects of terrestrial runoff on the ecology of corals and coral reefs: a review and synthesis. Mar Pollut Bull 50: Goreau T. F. (1967) Buccoo Reef and Bon Accord Lagoon, Tobago; observations and recommendations concerning the preservation of the reef and its lagoon in relation to urbanisation of the neighbouring coastal land. The Economic Planning Unit, the Prime Ministers Office, Government of Trinidad and Tobago: 1-8 Goreau, T. J., Cervino J, Goreau M, Hayes R, Hayes M, Richardson L, Smith G, Demeyer K, Nagelkerken I, Garzon-Ferrera J (1998) Rapid spread of disease in Caribbean coral reefs. Revista de Biología Tropical 46: Harding, S., van Bochove, J., O Farrell, S., Day, O., Gibson, K., Yates, K., Raines, P. (2008) Continued degradation Of Tobago s coral reefs linked to the prevalence of coral disease following the 2005 mass

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