The structure and productivity of the Thalassia testudinum community in Bon Accord Lagoon, Tobago Rahanna A. Juman Institute of Marine Affairs, Hilltop Lane, Chaguaramas, Trinidad and Tobago; [email protected] Received 15-I-2004. Corrected 10-IX-2004. Accepted 29-III-2005. Abstract: The Thalassia testudinum dominated seagrass community in the Buccoo Reef/ Bon Accord Lagoon Marine Park, measures 0.5 km2 and is part of a contiguous coral reef, seagrass bed and mangrove swamp sys- tem in southwest Tobago. T. testudinum coverage, productivity and percent turnover rates were measured from February 1998 to February 1999 at four sample locations, while total T. testudinum biomass was measured at two locations in the lagoon from 1992-2002. Productivity and turnover rates varied spatially and seasonally. They were higher in the back-reef area than in the mangrove-fringed lagoon, and were lowest at locations near to a sewage outfall. T. testudinum coverage ranged from 6.6% in the lagoon to 68.5% in the back-reef area while productivity ranged from 3.9 to 4.9 g dry wt m-2 d-1. Productivity and percentage turnover rates were higher in the dry season (January–June) than in the wet season (July–December). Productivity ranged from 3.0 in the wet season to 5.0 g dry wt m-2 d-1 in the dry season while percentage turnover rates ranged from 4.2% to 5.6%. Total Thalassia biomass and productivity in Bon Accord Lagoon were compared to six similar sites in the Caribbean that also participate in the Caribbean Coastal Marine Productivity Program (CARICOMP). This seagrass com- munity is being negatively impacted by nutrient-enriched conditions. Key words: Thalassia testudinum, Bon Accord Lagoon, CARICOMP, productivity, biomass. Seagrasses are an important marine especially important in the Caribbean, where resource (Hemminga and Duarte 2000, fisheries are already stressed from over-fishing Nagelkerken et al. 2001) that continues to and mismanagement of the natural resources decline worldwide. While this decline has been (Muehlstein and Beets 1989). Limited research caused by both natural and human-induced dis- has been conducted on seagrass communi- turbances, nutrient over-enrichment of coastal ties around Trinidad and Tobago. Most of the waters has been mostly cited as the main rea- information available has been on their loca- son (Orth and Moore 1983, Short and Wyllie- tion with brief qualitative descriptions in some Echeverria 1996). Nutrient loading is derived instances (Goreau 1967, Massiah 1976, Alkins from the watersheds (Valiela et al. 1992, Short and Kenny 1980, Anonymous 1990, 1992). In and Burdick 1996) as a result of increased 1997, Alleng and Juman (unpublished) reported human population in the coastal zone. Increase the disappearance of five Thalassia dominated in dredge and fill, coastal construction, dam- communities around the islands. age associated with commercial exploitation of The seagrass community within, and adja- coastal resource and recreational boating, have cent to the Bon Accord Lagoon (BAL) is one also led to reduced seagrass cover (Short and of the largest in Trinidad and Tobago, and part Wyllie-Echeverria 1996). of a mangrove-seagrass-coral reef continuum. Loss of seagrass beds impact on fisheries While some baseline data for this system productivity (Walker 1989), as organic produc- were provided by Alleng and Juman (unpub- tion and nursery habitats are decreased. This is lished) and productivity and biomass data are Rev. Biol. Trop. (Int. J. Trop. Biol. ISSN-0034-7744) Vol. 53 (Suppl. 1): 219-227, May 2005 (www.tropiweb.com) 219 available for one sample site within the lagoon Tobago between 11°08’ to 11°12’N and 60°40’ (Anonymous 1997), a detailed assessment is to 60°51’W (Fig. 1). BAL has a surface area lacking. This study assesses seagrass structure of 1.2 km2 and forms the southern boundary of and productivity in the lagoon. The data col- the BR marine park. This ecosystem complex lected are compared to existing monitoring covers an area of 7 km2, and drains a terres- datasets from CARICOMP. trial area of 7.7 km2 in a built-up part of south- west Tobago. The seagrass community within and adjacent to BAL covers 0.5 km2. Turtle MATERIALS AND METHODS Grass, Thalassia testudinum Banks ex König is the dominant species in the lagoon, compris- Site description ing 80% of the community. Smaller areas of Halophila decipiens Ostenfeld and Halodule The BAL and adjacent Buccoo Reef (BR) wrightii Ascherson are also found interspersed are located on the leeward coast of south among the turtle grass . The dominant algal Fig. 1. Location of Bon Accord Lagoon/ Buccoo Reef, Southwest Tobago. 220 Rev. Biol. Trop. (Int. J. Trop. Biol. ISSN-0034-7744) Vol. 53 (Suppl. 1): 219-227, May 2005 (www.tropiweb.com) species were from Acanthophora, Dictyota, Sampling methods Halimeda, Padina. Bryopsis, Codium and Caulerpa genera. Aerial photographs (taken by Survair BAL is on average 2 m deep and is International Limited in 1994, at elevation a well-flushed, marine coastal lagoon with 1800 m.a.s.l., at scale 1:12 500; FS160, f/4.0) a reasonably high rate of tidal exchange were used to map the extent of the seagrass between the lagoon and the adjacent coral ecosystem as well as geographical and man- reef (Juman 2004). The tide is mixed, mainly made features. This was then corroborated by semidiurnal with a significant diurnal inequal- ground truthing. The resultant maps were used ity. Because of its geographical setting, BAL to measure the extent of the ecosystem, and is also subjected to wind-generated waves to select sampling sites. Four seagrass study that cause re-suspension of bottom sediment. locations, A-D (Fig. 2) were randomly selected. Tobago experiences a tropical climate with Sample locations A, B and D were within the pronounced dry (January to April) and wet mangrove-fringed lagoon while location C was seasons (June to November) and a mean annu- in the back-reef area. Locations B and D were al ambient temperature of 25.7° C (Berridge west of the Bon Accord sewage outfall. 1981). Southwest Tobago receives a mean total rainfall of 1415 mm yr-1, 84% of which Structure of seagrass/ macroalgae beds occurs during the wet season, based on rain- fall statistics for Crown Point from 1969-1978 The percentage cover by seagrasses (Anonymous 1982). and algae was determined using 50 x 50 cm Fig. 2. Seagrass community in the Bon Accord Lagoon/ Buccoo Reef complex, and selected sampling locations. Rev. Biol. Trop. (Int. J. Trop. Biol. ISSN-0034-7744) Vol. 53 (Suppl. 1): 219-227, May 2005 (www.tropiweb.com) 221 (0.25 m2) quadrats placed at 10 m intervals 3= dry wt of old standing crop (Anonymous along permanent 100 m transect lines. One tran- 1994). The inverse of turnover rate is turnover sect line was randomly established at each of the time, which is the number of days required four study locations, perpendicular to the shore to replace the standing crop. Productivity and (Fig. 2). Two replicate quadrats were measured turnover rate were measured in March, June, at each sampling interval. Each quadrat was September and December 1998. Analysis of divided into 10 x 10 cm grids and percentage variance (ANOVA) was used to examine spa- cover by seagrasses and algae was determined tial and temporal differences. in each grid. Twenty quadrats were surveyed per transect line and estimation of coverage was Total Thalassia testudinum biomass calculated according to English et al. (1994). This estimation was done once within the sam- Total T. testudinum biomass has been mea- pling year between May and June 1998. sured at locations A and B in BAL as part of the CARICOMP program. A polyvinylchloride Thalassia testudinum productivity (PVC) corer (diameter 20 cm and length 60-80 cm) with a serrated edge was used to sample The productivity of the seagrasses in the Thalassia biomass (Anonymous 1994). The lagoon was determined using a biomass accu- corer was forced into the sediment to at least mulation method adapted from Zieman (1974) 45-50 cm to obtain over 90% of Thalassia rhi- and Anonymous (1994). At the four seagrass zomes and roots. Four random cores were taken location, six 10 x 20 cm (0.2 m2) PVC quad- per sample location. Core samples were placed rats were randomly deployed. All the seagrass in individual buckets. Coarse sorting was done shoots in each of the quadrats were marked on a sieve with a mesh of about 2 mm. Samples at the sediment surface by punching holes in were washed with seawater. Thalassia was the middle of the leaf blade using hypodermic then sorted into green leaves, non-green leaves needles. The seagrass was allowed to grow for and short shoots, live rhizomes, live roots, and seven days, after which they were harvested dead belowground material. After sorting, epi- and taken back to the laboratory for analysis. phytes on the leaves were removed by soaking In the laboratory, the leaves of the shoots in 5% phosphoric acid. The sorted sample is were sorted into three groups: (1) new leaves then washed to remove salt and put onto pre- (leaves that had emerged since the time of weighed foil and dried to constant weight at marking), (2) old growth (this was the length of 60ºC. Total biomass is reported as g dry wt m-2. the leaf from the point of marking down to the Thalassia biomass was measured at sample base where the leaf was harvested) and (3) old location A from 1992-2002 and at sample loca- standing crop (this is the section above where tion B from 1997-2002.