Abstract:
Seagrasses are marine angiosperms, thriving in brackish and shallow marine waters.
This study aimed to assess the impacts of rainfall changes due to climate anomalies
and anthropogenic land use changes on the distribution, species richness and diversity
of the seagrass beds in selected locations in the Negombo Estuary. Locations with
abundant seagrass beds were randomly selected and surveyed from August to October
2024. Three transects perpendicular to the shoreline were established across four
distinct ecozones. Water quality and seagrass beds were assessed to determine their
current status compared to past changes in rainfall and land use within the estuarine
ecosystem. The blue carbon stocks of seagrass ecosystems were also quantified by
estimating the total above and below-ground organic carbon fractions of seagrass
beds. The present status of the estuarine seagrass spatial distribution, species richness,
and diversity were compared with the published literature. Over the past three
decades, notable changes in monthly average rainfall were observed during the south
west and second inter-monsoon periods. Land use changes during the past two
decades exhibited an increase in vegetation (4.70%), built-up areas (4.24%), and
roads (1.16%), while water areas decreased by 0.76%. Estuary sections near
Basiyawaththa exhibited the highest diversity, while Kadolkele and Munnakkaraya
recorded no diversity, being composed only of Halodule pinifolia. Quantified
seagrass coverage of three sections Munnakkaraya, Kadolkele and Basiyawaththa
were 77%, 74% and 65%, respectively. Seagrass coverage and density were
significantly correlated to Secchi-depth, indicating an indirect link between turbidity,
rainfall, and water influx. Sea surface temperature exhibited a negative correlation
linking the adverse effect of elevated temperatures, likely caused by thermal stress.
The blue carbon stock of the ecosystem was significantly higher at Munnakkaraya
(24.15 Mg C ha⁻¹) compared to Kadolkele (14.35 Mg C ha⁻¹) and Basiyawaththa
(13.45 Mg C ha⁻¹). This study underscores the sensitivity of the Negombo Estuary’s
seagrass ecosystem to rainfall fluctuations, land-use changes, water quality
variations, and blue carbon content.
