No abstract

Background: Tidal cycle is an important factor which regularly changes coastal fish assemblages in shallow waters. However, the variations in fish assemblage and trophic structure across tidal cycles in tropical seagrass beds are rarely discussed. We used underwater visual censuses to quantify fish abundance and diversity from the shallow intertidal to the subtidal zone during both flood and ebb tides in seagrass beds surrounding the Dongsha Island, South China Sea. We also recorded fish feeding habits by analyzing stomach contents.

Very little is known about the characteristics of herbivory and selection by various grazers in tropical, multi-species seagrass beds. We used an in situ shoot tethering method to quantify grazing on 3 dominant seagrass species (Thalassia hemprichii, Cymodocea rotundata, and C. serrulata) that co-inhabit extensive seagrass beds surrounding Dongsha Island in the South China Sea. We measured the amount of seagrass grazing as well as leaf production in different habitats and seasons. The dominant seagrass grazers in Dongsha Island were parrotfish, followed by meso-invertebrates, suggesting that herbivory by small invertebrates may also be critical in tropical seagrass ecosystems. Our results revealed significant spatial and seasonal differences in leaf biomass losses. The level of grazing was 3.6× higher in the subtidal than in the intertidal zones, and 2.6 × higher in the warm than in the cool season. Leaf biomass losses were positively associated with parrotfish density, water depth, leaf production, and starch content. However, the nitrogen and total sugar contents did not affect the preferences of the grazers. The 2 Cymodocea species were preferred by most grazers. Collectively, all grazers typically removed <10% of the leaf production in Dongsha Island, which indicates that most seagrass biomass was not directly used by herbivores and flows into detrital pathways.

Seagrass beds and coral reefs are both considered critical habitats for reef fishes, and in tropical coastal regions, they often grow together to form “mosaic” habitats. Although reef fishes clearly inhabit such structurally complex environments, there is little known about their habitat usage in seagrass-coral mosaic habitats. The goal of this study was to examine potential factors that drive habitat usage pattern by juvenile reef fishes. We quantified (1) prey availability, (2) potential competitors, and 3) predators across a gradient of mosaic habitats (n = 4 habitat types) for four dominant carnivorous fishes (lethrinids and lutjanids) in the main recruitment season at Dongsha Island, South China Sea. We found that the coral-dominated habitats had not only a higher availability of large crustacean prey but also a higher abundance of competitors and predators of juvenile fishes. Food availability was the most important factor underlying the habitat usage pattern by lethrinids and lutjanids through ontogeny. The predation pressure exhibited a strong impact on small juvenile lethrinids but not on larger juveniles and lutjanids. The four juvenile fishes showed distinct habitat usage patterns through ontogeny. Collectively, mosaic habitats in the back reef system may be linked to key ontogenetic shifts in the early life histories of reef fishes between seagrass beds and coral reefs.