The effects of varying levels of eutrophication on phytoplankton and seagrass (Thalassia testudinum) populations of the southeast coast of Jamaica

Being stationary, seagrass populations respond markedly and cumulatively to continued eutrophication over both time and space. Six seagrass meadows located within and near the eutrophic Kingston Harbour were investigated from October 1995 to June 1996 to determine the effects of increased eutrophication on seagrass population dynamics. The stations were sited at Fort Augusta, Old Coal Wharf (eutrophic, harbor sites), Hellshire coast (mesotrophic) and Port Royal Cays (oligotrophic). Aerial productivity, leaf turnover rate, leaf area, shoot density and total biomass of Thalassia testudinum were assessed during the period. In addition, the water quality at all the sites was monitored, using phytoplankton biomass as the primary index of water quality. These data were supplemented with water temperature, salinity, dissolved oxygen content, nutrients (nitrates + nitrites and phosphates), and light extinction coefficient data at these sites. Although there were no significant differences in labile nutrient data at the six sites, the significant difference in phytoplankton biomass confirmed previous observations of a range of water qualities. Water quality ranged from the eutrophic Kingston Harbour (>2.0 mg m⁻³ chl a), to mesotrophic Hellshire Bays (0.57±0.02 mg m⁻³ chl a), and the oligotrophic Port Royal Cays (0.21±0.01 mg m⁻³ chl a). Thalassia testudinum populations exhibited an increase in leaf productivity rates (3.64±0.39 g dwt m⁻² d⁻¹ at South East Cay to 5.46±1.11 g dwt m⁻² d⁻¹ at Old Coal Wharf) and an increase in leaf area (11.64± 1.2 cm² at South East Cay to 22.85 ± 2.87 cm² at Fort Augusta) with increasing eutrophication. Conversely, there was a reduction in shoot density (917.25 ± 199.92 m⁻²at South East Cay to 459.62±65.35 m⁻² at Old Coal Wharf) and total biomass (972.68±88.39 g dwt m⁻² at Rackhams Cay to 406.24±62.16 g dwt m⁻² at Fort Augusta) with increasing eutrophication stress. Comparisons with previous studies from ten years before indicated that Thalassia testudinum shoot biomass and density decreased at all sites, while leaf productivity rates increased at oligotrophic sites and decreased at eutrophic sites during this period.