MEPS 501:53-66 (2014)  -  DOI: https://doi.org/10.3354/meps10681

ABSTRACT: Food web structure along depth gradients in the South China Sea and off northeastern Taiwan were investigated by analyzing stable isotopic compositions of demersal fish, benthic megafauna invertebrates and fish stomach contents. Various prey items were found in fish stomach contents, which covered a wide range of δ¹³C values; however, most fish had empty stomachs at catch. Cnidarians and ophiuroids had relatively low δ¹³C and δ¹⁵N values, while asteroids had higher values for both isotopic compositions. Molluscs and decapods showed the widest ranges for δ¹³C (-25 to -15‰) and δ¹⁵N (2 to 16‰), indicating feeding on different food sources and trophic niche expansion in deep-sea environments. A positive relationship between depth and δ¹⁵N was found in cnidarians, molluscs and fish. In addition, depth was negatively related to δ¹³C of cnidarians, decapods and fish. This disparity can be explained by different degrees of biological reworking of particulate organic matter (POM) along depth gradients, and a variety of food sources, e.g. phytodetritus, terrigenous matter, chemosynthetic matter and wood falls, used by different taxonomic groups. At the continental shelf and upper slopes, phytodetritus from surface phytoplankton production was the main food source for consumers, while reworked POM including both marine and terrestrial organic matter might contribute to food sources for consumers in the deep-sea basin. Other food sources, e.g. organic material synthesized in hydrothermal vents, could have provided nutrients to the surveyed site in the southern Okinawa Trough since the base consumer (shrimp) and top predators (fish) all showed δ¹³C values (between -24 and -26‰) lower than those found in sinking POM and surficial sediments (between -23 and -21‰).

KEY WORDS: Deep sea · Megafauna · Food web · Stable isotope