Abstract
The microphytobenthos (MPB) are known to play important ecological roles in estuarine ecosystems. While tidal flat MPB are widely studied, mangrove MPB are often overlooked due to the common belief that the light-limited environment under the mangrove canopy is not conducive to MPB growth. We assessed the biomass and community characteristics of the MPB in two contrasting mangrove forests (silty substrate and brackish influence versus sandy substrate and oceanic influence), their seasonal photosynthetic performance, as well as their role in mediating porewater carbon flux. Biomass measured as chlorophyll a concentration indicated that MPB abundance was equal, if not higher, in mangroves than on tidal flats. While mangrove sediments fostered an equally rich assemblage of MPB (estimated 23 to 45 genera) as the adjacent tidal flat, the MPB genus composition was different across all habitats, sites, and seasons, indicating the strong environmental influence on MPB assemblages. This study identified porewater salinity, temperature, and pH, as well as irradiance level and surface soil temperature to be accountable for 23.5% of this variation. MPB in all sampling locations were dominated by pennate diatoms, while cyanobacteria were common in the estuarine (as opposed to oceanic) site. Compared to tidal flat MPB, mangrove MPB exhibited photosynthetic performance characteristic of lower light acclimation. A mesocosm experiment using enriched stable isotopes of carbon and nitrogen indicated that a tight coupling exists between mangrove-derived carbon and MPB. Mangrove MPB are abundant and different from tidal flat MPB, and are functional photosynthetic entities with a regulatory role in nutrient cycling in mangrove ecosystems.