Species-specific distribution of intertidal seagrasses along environmental gradients in a tropical estuary (Brunei Bay, Borneo)

Abstract Seagrass diversity and distribution in Southeast Asia have been mainly studied in backreef zones but have rarely been studied on intertidal mudflats in estuaries. This research article aims to study seagrass distribution (species diversity, shoot density, and biomass) and morphology (leaf length and leaf number per shoot) on intertidal mudflats in Brunei Bay, Borneo, to understand seagrass responses to water and sediment conditions along the environmental gradient in the estuary. In total, five seagrass species ( Cymodocea rotundata , Halodule pinifolia , Halophila ovalis , Halophila minor , and Thalassia hemprichii ) were found at five different intertidal sites, and the diversity and biomass were highest at the most upstream site. H. pinifolia was the only species found at all five sites, indicating that this species is the most tolerant of and adaptable to varying water and sediment conditions. H. pinifolia had the highest biomass and longest leaf length at the site with the highest silt and clay percentage (SC%) and organic matter content of sediment, where no other seagrass species were found, suggesting that H. pinifolia could efficiently acquire nutrients without interspecies competition. In contrast, H. ovalis and T. hemprichii were found at the upstream sites and preferred a relatively low SC% in sediment. H. minor was found only at the most downstream site, which had the lowest SC% and is likely to experience the strongest hydrodynamics and receive the least nutrients of all the sites. Based on this study, the seagrass response to environmental gradients in estuaries is proposed to be species-specific, which would have led to the unique seagrass distribution in Brunei Bay.