Trophic structures of benthic food web in the Halophila-invaded seagrass ecosystems of Korea using stable isotope signatures

Abstract

Seagrass meadow is one of the most productive ecosystems in coastal environments and this plant strongly controls benthic environments in coastal system. Recently,distributional range extensions of seagrass species have been reported from various locations worldwide, caused by climate change. The invasion and expansion of the tropical seagrass Halophila nipponica have been observed on a shallow subtidal flat in temperate coasts of Korea since 2007. The trophic importance of phytoplankton, microphytobenthos (MPB), epiphytes, native Zostera marina, invasive Halophila nipponica, SOM, SPOM in benthic food webs was studied using natural stable carbon and nitrogen isotopic analyses. The benthic macroinvertebrates and seven food sources of the Namhae estuary were sampled at three different habitats (Halophila nipponica, Zostera marina beds and a shallow subtidal flat) in July and November in 2012. Although the carbon isotope values of invertebrates considerably varied among consumers in three habitats, the isotope signatures of consumers showed similarities among different sites. Cluster analysis based on their isotopic similarity showed that the isotopic variability of the benthic consumers was related more to functional feeding groups rather than to habitats or taxonomic groups. The isotopic mixing model suggested that micro primary sources (phytoplankton and MPB) play an important role in supporting the benthic food web in summer and fall. However, benthic invertebrates in three habitats fed not exclusively on H. nipponica and Z. marina in the present study, indicating a minor contribution of these plants to the benthic food web of shallow subtidal habitats. Our findings suggest that benthic and pelagic microalgae made a large contribution to benthic consumer diets, while invasive seagrass, H. nipponica may not have a large role in supporting food webs in this estuarine system. Although there are no differences between native and invasive seagrass beds in the benthic food web structure and ecological function, the invasion and expansion of this tropical seagrass may alter the nutrient foundation of resident and migratory consumers and thus significantly impact the coastal ecosystems there.