Abstract:
Seagrasses are submerged marine flowering plants that form one of the most productive and
ecologically significant coastal ecosystems. This study presents a comprehensive ecological
assessment of a seagrass-dominated in the intertidal zone at Paraviwella, southern Sri Lanka,
with a focus on habitat composition, associated fauna, and microbial associations. The seagrass
meadow, covering approximately 100 m2 at a mean tidal depth of 0.5m, was dominated
by Thalassia hemprichii, which exhibited an average cover of 69.7%. Interspersed within the
seagrass patches were sparse coral colonies, primarily branching Pocillopora damicornis (1%
cover), and macroalgae (0.8% cover). The meadow’s boundaries were defined by biogenic
sand tubes constructed by honeycomb worms (Phragmatopoma sp.), with an estimated abundance
of 3.6%, contributing to build sandy structures mixed with particulate organic materials.
Other substrate consisted of the sand and granitic rock (28%), supporting seagrass anchorage
and providing microhabitats for benthic invertebrates such as Tripneustes sp. and Holothuria
atra. Sea water samples collected from the site noted that temperature ( 28.5±0.5◦C), salinity
(33±1.2 ppt ) and dissolved oxygen (8.5±0.5 mg/L) were within the favourable for growth of
sea grasses. Microbial investigations, conducted using culture-based methodologies standardized
by the Australian Institute of Marine Science (AIMS), revealed high loads of both epiphytic
and endophytic bacteria. Among the identified taxa, Vibrio cholerae demonstrated the highest
abundance, with colony-forming unit (CFU) counts reaching 3.6±2.3 × 106 CFU/mL (epiphytic)
and 2.8±0.8 × 106 CFU/mL (endophytic), significantly exceeding levels of Escherichia
coli and Shigella spp. (p < 0.05, Kruskal–Wallis test). These findings underscore the ecological
status of the Paraviwella intertidal seagrass system, as reflected by microbial abundance during
the study. The data generated provide baseline information for incorporating microbial metrics
into long-term monitoring with the goal of proposing predictive models for conserving marine
ecosystems.
