Abstract:
Nayaru Lagoon, located in the dry-zone lowland of northern Sri Lanka, is a significant blue-carbon
ecosystem adjacent to the sea. Despite ongoing mangrove restoration efforts since 2015, comprehensive
baseline data remain limited. This study assessed mangrove diversity and community structure across
the lagoon. Twelve sampling sites were selected using stratified random sampling to represent the entire
lagoon gradient during the southwest monsoon period (May-June 2025). A 1m wide belt transect was
deployed across twelve representative sampling stations. At each site, the transects were established
perpendicular to the shoreline, initiating at the low-tide water margin and extending landward until the
last true mangrove or mangrove associate individual was encountered. Five true mangrove species were
recorded: Avicennia marina, Lumnitzera racemosa, Rhizophora mucronata, Excoecaria agallocha, and
Aegiceras corniculatum, along with the mangrove associate Acanthus ilicifolius. The highest species
richness was recorded at sites 4, 5, 6 (left bank) and 10 (right bank) (S = 4), while sites 1 and 7 (left
bank) exhibited monospecific stands dominated by A. marina. Shannon diversity index (H') ranged
from 0 (sites 1 and 7) to 1.075 (site 10), while Simpson's diversity index (D) varied from 0 to 0.583 for
the same sites, respectively. The highest species evenness was recorded at site 3 (left bank) (J' = 0.811).
Based on the Sorenson index, sites 1 and 7 exhibit a high degree of similarity since both sites have only
A. marina, whereas sites 4, 5 and 10 also constitute another cluster of similar sites where all A. marina,
L. racemosa, R. mucronata and E. agallocha are present. Results indicate a heterogeneous distribution
of mangrove species across the lagoon. Some areas show moderate diversity, while a single species
dominates others. These findings highlight the importance of site-specific conservation and
management strategies to maintain ecosystem stability.
