Herbe à tortue

Thalassia testudinum forms extensive dense seagrass beds throughout its range. This is the most important seagrass habitat-forming species in the Caribbean. There have been some declines in seagrass beds in developing areas, particularly in areas of nutrient enrichment and sedimentation. Generally, however, this species still very abundant throughout its range.

Out of 103 published studies on this species, 29 showed an increase in abundance (mix of biomass, aerial extent, or density), 17 decreased and 57 showed no change (Global Seagrass Trajectories Database, Carruthers pers. comm. 2007). The overall population trend is stable.

Caribbean regional biomass ranges from 3.2 g dw/m² (recorded at the latitude 17.8^o in the Virgin Islands) to 820 g dw/m² (recorded along the 28.3^o latitude on the Florida coast). These findings are summarized in Gacia (1999) and include data from publications ranging from 1959 until 1996.

The T. testudinum population in the Florida Bay covered 90% of mud bank and basins until a massive die off in 1987. Four thousand hectares were completely lost and 24,000 ha were affected due to primary die off caused by higher water temperatures, hypersalinity and increased biomass, leading to high respiration demands. This was followed by secondary die-off from increased turbidity due to increased phytoplankton beginning the fall of 1991. Between 1984 and 1994, mean short shoot density dropped by 22% throughout the Florida Bay and standing crop biomass dropped by 28%. While light attenuation may have been a factor, the patchy distribution of die-off suggests that primary die-off was the principle reason for the decrease in abundance (Hall et al. 1999). Between the spring of 1995 and spring of 1999, despite individual basins in the Florida Bay exhibiting changes in abundance by between 12-100%, the total abundance throughout the Bay exhibited little change due to declines in abundance in the western part of the Bay being offset by increases in abundance in the central and eastern basins (Durako and Hall 2000). Increases in seagrass coverage along with large occurrences of flowering plants were observed during the spring of 2000; however, primary die-off among high density stands was also observed north of Barneys Key with similar symptoms noted to the 1987 die-off (Nuttle et al.  2003).