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

[Display omitted] •Stable isotope analysis showed that epiphytic algae was poorly used by consumers in Hepu seagrass bed.•Halophila ovalis is the most important basal carbon source of macrozoobenthos and nektonic trophic groups.•H. ovalis instead of its epiphytes fuelled food web in Halophila-based seagrass bed and adjacent waters.•Energetic and ecological roles of seagrass itself need to be re-examined in seagrass-based food web. 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.