Ecological structure and microbial composition of a Thalassia hemprichii -dominated reef meadow in Southern Sri Lanka

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.