Seagrasses

To date, four indigenous seagrasses have been identified in Dominica, namely Syringodium filiforme (0.5-18m depth), Halodule wrightii (0.5-16m depth), Thalassia testudinum (0.25-10m depth), and Halophila decipiens(5-25m+ depth).The invasive species Halophila stipulacea (originating from the Indian Ocean) has also been identified (5-25m+ depth) along vast expanses of Dominica's west coast seagrass beds.

   
Left: a mixed stand of Syringodium filiforme (thin cylindrical leaves)
and Thalassia testudinum (blade-like leaves). Right: Halophila decipiens

Seagrass beds are Dominca's largest marine habitats built by organisms. Thalassia is more dominant along the northern and eastern coast of Dominica, generally in sheltered bays and back reef areas (e.g. Anse Soldat, Calibishie, Hodges Bay , and Middle Bay in Marigot). Although it mostly occurs in combination with Syringodium it may occasionally also form monospecific assemblages, where the calmest waters lead to stands of Thalassia that are tall (25cm) in local intra-specific comparisons, and which outcompete Syringodium.

By far the largest contiuous stretches of seagrass beds are found along the west coast between Canefield and Pointe Ronde, dominated by Syringodium filiforme. However, the invasive species H. stipulacea, first recognized by D. Willette, has been identified throughout most of the west coast S. filifome beds, in particular along its deep margins (~ 16 m) and deeper (depth limit in Dominica not yet determined). In Prince Rupert's Bay the invasive species has become the dominant seagrass species by disolacing native species.

The native H. decipiens also marks the deep and shallow margins of S. filiforme beds and and forms isolated patches. It is, however, far less storm resistent and aggressive than the invasive H. stipulacea, which now constitutes the second most abundant seagrass in Dominica. Halodule wrightii has a more intermittent distribution along the shallow margins of S. filiforme beds (west coast) and appears to have a shallower distribution limit at about 12m. Along the seagrass beds of Calibishie it has occasionally been found in "open spaces" amidts mixed stands of T. testudinum and S. syringodium.

While the fringing reef systems of the north coast provide well flushed, yet protected (turbulence) back reef areas conducive to shallow-water (0-4 meters depth) seagrass beds, the west coast with its lack of near shore energy-dissipating structures is not the ideal place for shallow-water seagrass beds. Instead, seagrasses along the west coast are generally found in waters from 4-18 meters. In these "deeper" depths, self-shading by seagrasses with blade-like leaves (e.g. Thalassia) would be a disadvantage and so the species with cylindrical leaves, Syringodium, dominate the west coast. Thalassia can occasionally be found in small patches along the west coast in protected shallow areas such as Cachacrou.

Throughout Dominican waters, seagrass beds are not extremely wide, ranging from 90 - 200 m. This is due to the narrow and steep shelf where light becomes a limiting factor within a few hundred meters from shore. Moreover, the shelf morphology offers no dissipation of physical disturbance from waves and storms, thus limiting the expansion of these plant communities. This setting is particularly pronounced along the west coast where dynamic changes in the overall dimensions and density of seagrass beds can regularly be observed after storms. At some locations (e.g. Salisbury) entire seagrass beds "disappear" as a combined result of erosion and burial. In other cases (e.g. Jimmit, Anse Mulatre) seagrass beds are reduced in width by 20-30% along their shallow margins.

Given the fact that the island's underwater topography is not one that provides wide environments suitable for seagrass development, associated organisms like the queen conch and milk conch (Strombus gigas and S. costatus) were never abundant and not a key component in the local fisheries. Such ecological parameters shape the artisanal fisheries of each island, a fact often omitted in direct comparisons of the fisheries from different regions.

SCCS1108/1107