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Revista de Biología Tropical

On-line version ISSN 0034-7744Print version ISSN 0034-7744

Rev. biol. trop vol.58  suppl.3 San José Oct. 2010


The distribution of seagrasses in Dominica, Lesser Antilles

S.C.C. Steiner, K.J. Macfarlane, L.M. Price & D.A. Willette

Institute for Tropical Marine Ecology Inc. (ITME), P.O. Box 944, Roseau, Commonwealth of Dominica;



Seagrass beds are the largest organism-built marine habitat in Dominica, yet have only been surveyed since 2007. Standardized examinations along a depth gradient between 0 and 24m, focusing on magnoliophyte species composition and benthic cover of shoots at 17 seagrass bed sites, were carried out between September 10 and December 7, 2008. The Cymodoceaceae Syringodium filiforme (Kuetzing 1860) and Halodule wrightii (Ascherson 1868), as well as the Hydrocharitaceae Halophila decipiens (Ostenfeld 1902), H. stipulacea (Fosskal & Ascherson 1867) and Thalassia testudinum (Banks ex König 1805) displayed distinct regional and horizontal distribution patterns. Syringodium filiforme is the island’s dominant seagrass along the western and northern coasts, occurring at depths between 2 and 18m and with a mean benthic cover ranging from 0.9-10% along the West coast. Along the North coast it grew between 0.2 and 1m depth with a mean maximum benthic cover of 48.9%. Halodule wrightii grew along the North and West coasts, in depths between 1 and 14m in areas of recent and chronic disturbances. Its delicate morphology and sparse benthic cover (<0.1%) did not constitute seagrass beds. Halophila decipiens grew along the deep, shallow and lateral margins of west coast S. filiforme beds and monospecifically in depths between 3 and 24m. Halophila stipulacea, an invasive species, was widespread along 45km of the West coast and was found in depths between 5 and 24m. Both Halophila species formed extensive beds at depths beyond the survey limit of 24m thus playing a potentially important role in the resettlement of shallow areas after storms. H. decipiens and H. stipulacea are currently the second and third most common seagrasses on the island respectively, despite their absence along the North coast. T. testudinum was confined to North coast’s sheltered reef flats at depths 1m or less with mean a benthic cover ranging from 2 to 76%. It grew monospecifically in the most turbulent and in the calmest locations, yet intermixed with S. filiforme in areas of moderate turbulence. Strong surge along the West coast (October 15-16, 2008), associated with Hurricane Omar, caused uprooting and burial of seagrass beds in varying degrees, in particular along the shallow margins between 2 and 10m depth. This event also demonstrated the dynamic nature of Dominica’s shallow seagrass bed margins and the resistance level of individual beds to storm disturbances. Rev. Biol. Trop. 58 (Suppl. 3): 89-98. Epub 2010 October 01.

Key words: Seagrasses, Syringodium, Halodule, Halophila, Thalassia, Dominica.


Pastos marinos son los ambientes más grandes constituidos por organismos en Dominica. Sin embargo, sólo se han examinado desde 2007. Entre el 10 de septiembre y 9 de diciembre 2008, se examinaron la composicion de especies y la densidad de magnoliofitas en profundidades de 0 a 24m. Los Cymodoceaceae: Syringodium filiforme y Halodule wrightii, tal como los Hydrocharitaceae: Halophila decipiens, H. stipulacea y Thalassia testudinum,mostraron una distribución regional y horizontal muy distinta. Syringodium filiforme fue la especie dominante en las costas del oeste y del norte de la isla. Se encontró en profundidades de 2 a 18m y con un promedio de cobertura béntica de 0.9-10% en la costa del oeste. En las costas del norte creció entre 0.2 y 1m de profundidad con un pormedio de cobertura béntica de 48.9%. Halodule wrightii creció en las costad del norte y oeste, en profundidades de 1 a 14m en áreas de perturbaciones recientes o crónicas. Su morfología delicada y su baja cobertura béntica (<0.1%) no constituyeron pastos. Halophila decipiens creció en los márgenes profundos, llanos y laterales de pastos dominados por S. filiforme, pero también en forma mono-específica entre 3 y 24m. Halophila stipulacea, una especie invasora, se encontró comúnmente a lo largo de 45km de la costa del oeste en profundidades entre 5 y 24m. Ambas especies de Halophila formaron pastos extensos en profudidades mayores al límite de este estudio y puden ser importantes en la recolonización de áreas llanas después de tormentas destructivas. H. decipiens y H. stipulacea ahora son las especies más comunes después de S. filiforme, a pesar de su ausencia en la costa del norte. T. testudinum, sólo creció en la costa del norte, encima de áreas arrecifales con un pormedio de cobertura béntica entre 2 y 76%, en aguas con 1m o menos de profundidad. Crecieron mono-específicamente en áreas de turbulencia máxima y mínima, pero entre S. filiforme en áreas de turbulencia moderada. Oleaje fuerte en la costa del oeste (octubre 16), asociado con el Huracán Omar, causó la erosión y el enterramiento variado de las márgenes llanas de pastos marinos, particularmente entre 2 y 10m de profundidad. Este evento demostró la dinámica en las márgenes llanas de pastos marinos de Dominica y el nivel de resistencia a perturbaciones de pastos individuales.

Palabras clave: algas marinas, Syringodium, Halodule, Halophila, Thalassia, Dominica.

Seagrass beds throughout the world provide many ecological and economic benefits to the coastal regions they line, including their role as a nursery for juvenile fishes and invertebrates (Randell 1965, Nagelkerken et al. 2000, Beck et al., 2001, Peterson & Heck 2001) and shelter for adult organisms (Baelde 1990, Boström & Mattila 1999, Danovaro & Gambi 2002); as well as their functions to stabilize shorelines through wave attenuation (Fonseca & Fisher 1986, Fonseca & Cahalan 1992, Grizzle et al. 1996, Chen et. al. 2007), decrease suspended material in the water column (Agawin & Duarte 2002, Marbà et al. 2006), recycle nutrients, increase oxygen loads and sequester CO2 (Lee & Dunton 1999, Gacia et al. 2005, Suzuki et al. 2003). Seagrass beds also rank among the most productive ecosystems on the planet in terms of biomass and species diversity (Constanza et al. 1998, Duarte & Chiscano 1999, Duarte 2002). In Dominica, a young volcanic island, the characteristically narrow sublittoral zone provides a comparatively limited area for seagrasses and other light-dependent benthic communities to flourish. The first comprehensive island-wide survey of the distribution and size of benthic marine habitats that encompassed the assessment of seagrasses, was carried out in 2007 by Steiner & Willette (2010). Results from these initial studies included that seagrass beds were (i) found along extensive portions of the West coast, on the reef flats of fringing reefs along North coast, and in the form of a single patch (>100m2) along the East coast, yet absent along the South coast, (ii) that they are the island´s largest organism-built habitat covering a minimum of 265ha, more than the equally marginal coral reef systems covering only 72ha and (iii) that five seagrasses have been identified so far, the Cymodoceaceae Syringodium filiforme and Halodule wrightii, and the Hydrocharitaceae Halophila decipiens, H. stipulacea and Thalassia testudinum. An earlier report by Onuf et al. (2003) including seagrass distribution maps encompassing Dominica, was only marginally in accordance to the findings of Steiner & Willette (2010), as it illustrates the distribution of seagrasses, without naming species, at locations along the North, West and East coast that do not have seagrass beds. The Government of Dominica (2005) illustrates the distribution of seagrasses, also without naming species and omitting the largest seagrass beds along the West coast, while claiming the presence of seagrass beds at northern and eastern locations marked by a high-energy rocky sublittoral zone, which is devoid of seagrasses. Similarly inaccurate is the distribution map published by UNEP-WCMC (2007), which shows seagrasses growing around the entire island. The information provided by Onuf et al. (2003), the Government of Dominica (2005) and UNEPWCMC (2007) does not suffice as baseline of Dominica´s seagrass resources and highlights the fundamental benefits of field surveys.

All of Dominica´s seagrass beds are close to human settlements and are historically and presently used as fishing areas, locally also referred to as "conch beds". The fisheries are artisanal and the fishing techniques applied include seine net fishing, the use of baited fish pots, spear fishing and occasionally the collection of the gastropods Strombus gigas and S. costatus and the echinoid Tripneustes ventricosus. Visible signs of stresses affecting seagrasses in Dominica are the burial and erosion of seagrasses, the contamination by solid wastes, and pulses in the colonization and growth of epiphytic organisms and macroalgae. Seagrasses are not yet specifically protected by the laws of Dominica or other conservation mechanisms as no major seagrass beds occur within the boundaries of the two existing marine protected areas, the Cabrits National Park and the Scott´s Head – Soufrière Marine Reserve. In order to expand our recent understanding of Dominica´s largest marine organism-built habitat type, and to provide the primary data based on field surveys, necessary for the sound development of environmental policies, this study examines the species composition and topography within and between seagrass beds at depths ranging from 0 to 24m.

Materials and methods

Seagrass beds at fifteen sites along the West coast and two sites along the North coast (Fig. 1) were surveyed between September 10 and December 7, 2008 (Table 1). The sites were strategically selected to encompass all major areas of seagrass beds as ascertained by Steiner and Willette in 2007. At all west coast sites SCUBA dives were carried out following a compass heading perpendicular to the shoreline. Snorkeling gear was used to examine north coast sites. The coordinates of the starting point, and the heading, length and maximum depth of each transect were recorded for future reference (Table 1). The survey protocol entailed a "downward" and an "upward" transect. During the downward transect, from shore to a depth of 24m or to where no more seagrasses were seen (whichever came first), qualitative descriptions of the seagrass bed margins (intact or eroded, species composition, height and benthic cover of leaf shoots) and the central parts (species composition, height and benthic cover of shoots, signs of endofauna, epiphytes, sessile and vagile fauna) were recorded. Seagrasses observed beyond the deep  end of each transect were also noted and their benthic cover was estimated. At Espagnol Bay the deep margin of the S. filiforme bed was further than 335m from shore and was not reached during the survey. Each "upward" transect included quantitative assessments of "plots" consisting of three 1m2 quadrats, aligned perpendicular to shore and 1m apart from each other, first at a distance of 5m from the deep margin of the seagrass bed or at the deepest point of the "downward" transect if a seagrass bed margin was not reached, and repeated at 20m intervals towards shore. Intervals were measured with a 20m transect line extended in  the opposite direction of the compass heading used during the "downward" transect. At Toucari and Espagnol Bay a 40m interval was used to allow for the completion of the survey during a single dive. Within each quadrat, seagrass species composition, benthic cover and height of leaf shoots were assessed and the presence of macroalgae, epiphytes, sedentary and vagile epifauna was noted. The benthic cover of leaf shoots was visually estimated using 1m2 quadrats subdivided into four 0.25m2 squares. Each estimate was derived from the consensus of at least two surveyors and was based on eight hours of consistency training in the field and with the aid of projected images, to ensure that surveyors estimated the percent cover congruently. Shoots closest to five pre-marked spots along the quadrat were selected for in situ height measurements to the closest centimeter. Quantitative data of the three quadrats constituting each of the plots were pooled and the means used for the comparison within and between seagrass beds and depth. Species identification in the field was based on Littler & Littler (2000). Voucher specimens were stored at the Institute for Tropical Marine Ecology.

Outer wind bands of Hurricane Omar caused strong swells along Dominica´s West coast during the night of October 15 and through most of the day on October 16, 2008. Field surveys were suspended due to poor visibility caused by the storm and resumed on October 23, 2008. Storm impacts in the form of erosion and burial of seagrasses in varying degrees, were evident in many seagrass beds. The width of seagrass beds and the species presence were subsequently reassessed.


Syringodium filiforme was the principal component of all seagrass beds examined. It occurred at all sites (Table 2) and grew at depths between 0.2 and 18m (Fig. 2). Its mean benthic cover ranged between 0.9 and 10% among the west coast sites and gradually decreased from the shallow seagrass bed margins in depths ≥2m towards the deep margins. The mean blade height of S. filiforme ranged from 14.3 to 33cm (n=205) and the highest mean values (24.7cm, n=180) consistently occurred within the deeper half of the seagrass beds at 14-15m depth. A single blade of 56cm was measured at Anse Mulâtre at a depth of 11m, outside of the quadrats. The deep margins of S. filiforme beds were all abrupt and intact, linear rather than undulating and without exposed rhizome layers. In contrast, its shallow margins were undulating, marked by a distinct drop in the terrain (ranging from 20 to 35cm) exposing rhizome layers. The widest and bestdeveloped S. filiforme bed of the West coast, in terms of its benthic cover of shoots and blade heights, was at Espagnol Bay. Along the North coast S. filiforme grew between 0.2 and 1m depth, on reef flats, with a mean benthic cover of 34.3% (n=18), a mean blade height of 17.3cm (n=60), and always intermixed with T. testudinum. Across beds, mean benthic cover and blade height increased from the off-shore margins (31% n=6, 15.5cm, n=30) towards the central portions (48.9%, n=6, 21.8cm, n=30) of the seagrass beds, from where they decreased towards the near-shore margins (22.5%, n=6, 14.7cm, n=30). Its off-shore distribution limits at were gradual transitions to areas of coral rubble where T. testudinum grew monospecifically. Its near-shore distribution limit at Calibishie were also smooth and marked by a gradual transition into monospecific areas of T. testudinum, however in sandy substrates. At Anse Soldat the near shore distribution limit of S. filiforme co-occurred with that of T. testudinum less than a meter from shore.

Halodule wrightii grew in depths between 1 and 14m (Fig. 2), in areas of recent (generally in shallow waters) and chronic disturbances (e.g. Layou river mouth), at western and northern sites, but did not occur in any of the 141 quantitatively examined quadrats. Its delicate morphology and sparse benthic cover (<0.1%) did not constitute seagrass beds that shape their substrate. H. decipiens grew along the deep, shallow and lateral margins of the west coast S. filiforme beds and monospecifically in depths between 3 and 24m, however it did not occur in any of the quantitatively assessed quadrats. It was most commonly found along the deep margins of S. filiforme beds forming mono-specific beds with a benthic cover of shoots of up to 30% or in variegated or mixed assemblages with H. stipulacea. The latter was also widespread (Table 2) along 45km of the West coast between Canefield and Toucari and occurred in depths from 5 to 24m, predominantly along the deep margins of S. filiforme beds and with benthic covers of up to 80%. Only at Canefield did H. stipulacea occur within the quantitatively surveyed quadrats, where its mean benthic cover of shoots was 21.4% (n=12) with a mean blade height of 6cm (n=60). Both Halophila species formed extensive beds at depths beyond the survey limit of this study at 24m (Fig. 2) where H. decipiens still covered a greater portion of the seafloor than H. stipulacea.

Thalasssia testudinum was confined to North coast where it occurred on sheltered reef flats at depths of 1m or less (Fig. 2), with a mean benthic cover ranging from 2 to 76% (n=18). It grew monospecifically in the most turbulent locations (coral rubble sediments behind the reef crest) and the calmest locations (along the shore), yet intermixed with S. filiforme in areas of moderate turbulence (central portions of the seagrass bed). At Calibishie the mean benthic cover and blade height within the seagrass beds increased steadily from the offshore margin (most turbulent) to the near-shore margin (calmest) where they peaked at 76.6% (n=3) and 22.8cm (n=15) respectively. The same pattern was observed in the mean blade heights across the narrower seagrass bed at Anse Soldat, which increased steadily from the off-shore margin (11.1cm, n=15) towards shore (22.8cm, n=15). However, the mean benthic cover had a sinusoid distribution with a mean benthic cover of 7% (n=3) at the off shore and near shore margins and peaking at 19% (n=3) in the central portions of the seagrass bed.

The growth of epiphytic invertebrates (primarily hydroids) and algae (primarily phaeophytes and rhodophytes ) was most pronounced on the deep half of western seagrass beds, while northern segrass beds were characterized by dense epibenthic macroalgal communities in the space not covered by seagrasses. Epipsammic macro-invertebrates were uncommon throughout all sites except Bioche, which was characterized by the common occurrence of the gastropod Strombus costatus and the asteroid Oreaster reticulatus. Strong surge along the West coast (October 15 and 16, 2008) associated with Hurricane Omar caused uprooting and burial of seagrass beds in varying degrees, in particular along the shallow margins between 2 and 10m depth, leading to a decrease in the width of seagrass beds and the number of species at several locations (Table 2). The complete or close-tocomplete removal and burial of entire seagrass beds occurred at Salisbury Bay, Mero and Pt. Michel. H. wrightii was the only specie that reappeared in disturbed areas and within the survey time frame and as early as two weeks after the storm. North coast beds were not affected by this event.



Dominica´s west coast (leeward) and east coast (windward) seagrass beds differed in seagrass species composition, depth range, and associated species. West coast beds were dominated by S. filiforme, and were typically found between 2 and 18m, in otherwise sandy regions. The narrow and steep shelf of the West coast, lacking wave energy dissipating structures such as shallow fringing reef systems, limits the space suitable for light dependent seagrasses. Near-shore turbulence creates unstable conditions for seagrasses and therefore Dominica´s west coast communities start growing in no less 2m and extend towards their physiological light limits, which in Dominica was at 18m for S. filiforme. Although T. testudinum is known to have lower minimum light requirements than S. filiforme (Lee et al. 2007), self-shading by T. testudinum with blade-like leaves may be a disadvantage in these depths, possibly explaining its absence along the West coast. In contrast, windward beds were characterized by T. testudinum growing in depths of 0-1m in back reef areas or behind other sheltering features like islands. T. testudinum has the deepest live rhizome mats of the five species (Duarte 1991, den Hartog 1970), and unbranched roots with substantial root-hairs that are adapted for a range of sediment types (Kuo & den Hartog 2006). These attributes provide the necessary stability in the coral rubble substrates behind the reef crests of Anse Soldat and Calibishie. Close to shore, in calm waters where seagrasses lay submerged but flat during low tides, shading by the blade-like leaves of T. testudinum may be among the limiting factors for S. filiforme at these sites.

H. wrightii displayed the sparsest yet most "opportunistic" distribution in that it occurred in areas of disturbance lining or within mixed and mono-specific seagrass beds, and patches of S. filiforme and T. testudinum. Furthermore, it was the first species to reappear at sites disturbed by Hurricane Omar. This is in accordance with their status as pioneer species and described by Mariani & Alcoverro (1999).

Both H. decipiens and H. stipulacea were only found along the West coast. H. decipiens is known to grow at depths of up to 40 m (Williams & Dennison 1990). H. stipulacea generally grows at depths of up to 50m (Beer & Waisel 1981), but as deep as 85 m near the Great Barrier Reef and Cargolos (den Hartog 1970). The deep distribution limit for these species in Dominica has not yet been determined, as this survey did not go beyond 24m depth. However, based on their wide distribution, in particular along the deep distribution limits of the west coast S. filiforme beds, first examined during this study, Halophila spp. must be considered by hurricanes and also have the potential of locally resettling shallower waters after storm disturbances. Although all west coast seagrasses beds were affected by Hurricane Omar, the level of destruction varied greatly between sites. This is an indication as to which seagrass beds are more resistant to such disturbances (Espagnol Bay, Dublanc, Bioche, Colihaut), which in turn is a key insight for establishing conservation measures tailored specifically to the local conditions. 

This study has discerned the differences in the seagrass species distribution of the leeward and windward coasts of Dominica, where all sublittoral ecosystems cover comparatively small areas along the island´s narrow shelf and close to coastal settlements. Dominica´s seagrass beds also undergo dynamic fluctuations in their benthic cover, resulting from storms, as observed here. In such a scenario, marine conservation measures aimed at multiple rather than single habitats (e.g. coral reefs) seem more appropriate given Dominica´s contiguous layout of seagrass beds and coral reefs described in Steiner & Willette (2010). To date, Dominica´s marine habitat conservation measures and two marine protected areas (Scott´s Head-Soufrière Marine Reserve, Cabrits National Park) focus on coral reefs and areas of coral assemblages.Although seagrasses are Dominica´s largest organism-built habitat type, their investigation has only recently begun and non-anecdotal information is now available for the incorporation into broader marine conservation efforts.


Logistic and financial support was provided by the Institute for Tropical Marine Ecology Inc. (ITME RPfa09). The research license for this project (RP-09/248-1) was granted and issued by the Fisheries Development Division, Government of Dominica.


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Correspondencia: S.C.C. Steiner, K.J. Macfarlane, L.M. Price & D.A. Willette. Institute for Tropical Marine Ecology Inc. (ITME), P.O. Box 944, Roseau, Commonwealth of Dominica;

Received 15-VII -2009. Corrected 07-II -2010. Accepted 12-VII -2010.

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