Trophic importance of the seagrass Halophila ovalis in the food web of a Hepu seagrass bed and adjacent waters, Beihai, China

Abstract

The current paradigm emphasizes the trophic role of epiphytic algae in seagrass-based food webs. However, a growing body of literature demonstrates that grazers would directly cause considerable damage to seagrass rather than targeting epiphytes, perhaps depending on seagrass traits. Here, we analyzed δ13C and δ15N signatures of macrozoobenthos, nekton and their potential organic carbon sources in Halophila ovalis seagrass bed and adjacent waters on the Hepu coast (Beihai, China) to test the hypothesis that Halophila with high nutritive values and fragile leaf-fracture traits may be a key carbon source. The δ13C values of most consumers either fell between H. ovalis (−14.7 ± 0.7‰) and benthic microalgae (microphytobenthos and Halophila’s epiphytes, −19.9 to −19.3‰), or approached the δ13C of H. ovalis, suggesting that H. ovalis and microalgae is basal carbon sources in Halophila-based food web. Further quantification based on a 4-end-member MixSIAR model showed that H. ovalis is the most important basal carbon source, supporting 4 out of 6 trophic groups of macrozoobenthos, and 4 out of 7 nektonic trophic groups (a total of 22 species, accounting for 84.6% in nekton). The mean contribution was 37.2–75.3% for macrozoobenthos and 51.1–64.4% for nekton, respectively. Most macrozoobenthos directly or indirectly assimilated H. ovalis or its detritus and were then mainly utilized by nekton except for bivalves which largely fed on suspended microphytobenthos and particulate organic matter (POM), and porifera filtered POM. Our results re-examined the trophic function of seagrass in seagrass-based food web and emphasize the importance of protecting Halophila resources.

Keywords

• Trophic link
• Stable isotope
• Macrozoobenthos
• Nekton
• Carbon flow