Performances of Seagrass meadows in monsoon dominated dynamic coast of Bangladesh

Seagrass meadows are one of the most widespread and productive ecosystems in the coastal and estuarine waters worldwide. They trap and stabilize bottom sediment as ecosystem engineer, reducing the water currents through their physical structure that facilitate benthic faunal diversity and offer habitat for resident and transient fauna. Despite these ecological functions, this ecosystem has been suffering in the Bangladesh coast due to coastal morphological changes that are influenced by the monsoonal climate. This study aims to investigate the seasonal changes in seagrass density and biomass along with sediment erosional and depositional rates at seagrass vegetated dynamic estuarine sites, located in the south-eastern coast of Bangladesh. Additionally, this study made an extensive survey to record the benthic faunal assemblages and their spatial variation at four different seagrass sampling sites (i.e. St₁ – St₄). Moreover, water parameters and sediment characteristics were measured and compared among sites to understand their role in community assemblages. The results indicated that shoot density and biomass of the seagrass, Halophila beccarii showed monthly variation, which ranged 109–3143 shoots m^-2 and 0.38–13.21g DW m^-2 respectively. The shoot density rapidly increased after the end of monsoon (i.e. June–August) and reached maximum (3143±362 shoots m^-2) in October. It also showed sharp decrease during dry winter months (i.e. November–February) due to sedimentation. Sediment deposition began during the post-monsoon (i.e. October–November), when the sea state conditions were relatively calm and continued until February. By the end of the winter (i.e. February) sediment accumulation reached maximum levels (11.31±1.56 cm) compared to October. Seagrass was absent for the period between February–June in the investigated sites as a result of unstable coastal morphology. Water temperature, salinity and conductivity ranged 28.5–29.6 ºC, 20.3–21.2 ppt, and 34.87–36.75 mS cm^-1 respectively, which were not significantly varied among the stations. On the other hands, turbidity (10.3–20.2 NTU), total particulate matter (17.5–87.4 mg l^-1), pH (7.81–8.23), dissolved oxygen (4.67–5.36 mg l^-1), chlorophyll-a (1.9–7.3µg l^-1), nitrate (5.92–28.47 mg l^-1), and phosphate (0.37–3.98 mg l^-1) contents in the water varied significantly in four sampling stations. Soil pH were found significantly lower (pH, 5.44±0.31) in the St₂ than other stations (pH, 5.79–6.08) as a result of high organic carbon content (3.46±0.22%) therein. Moreover there was no significant difference in soil salinity (21.04–23.28 ppt) and texture composition (i.e. percentages of sand, silt, and clay) among those four sampling stations. However, the benthic sediment core sampling yielded a total of 6 polychaete species, 4 bivalve species, 7 gastropod species, 5 crustaceans and 2 other invertebrate species including sea cucumber and sea anemones. nMDS analysis for seagrass habitats explained 98% (R² = 0.98) of observed variation in the data on benthic sediment macrofauna, which also indicated that abundances (i.e. shoot density) of seagrass and soil organic contents positively influences the benthic macrofaunal communities in the sampling stations. Considering the natural dynamics of the seagrass ecosystem, this study formulated conservation strategies involving coastal communities. 

 

 

Key words: Monsoon, Halophila beccarii, sedimentation, benthic macroauna, seagrass conservation