Seagrass beds are highly diverse and productive ecosystems, and are well-known as one of the most economically valuable ecosystems on earth. They provide food and habitat for numerous fish and invertebrate species. Based on present knowledge, the stability of the species communities living in seagrass beds is largely depending on the availability of food, therefore studying the food sources and trophic structures of the living components within seagrass beds is very important for understanding the dynamic process and the sustainability of seagrass ecosystems. So far most publications are focusing on seagrass food webs in temperate regions, such as Mediterranean Sea and the western Australia, and little information is available of those seagrass beds located in tropical-subtropical areas including southern China.
This paper, using the stable carbon isotope analysis method, attempts to identify the main food sources for fish and macro-invertebrates living in a mixed-species tropical seagrass bed at Xincun Bay, southern China, and mainly focuses on their δ¹³C values. The δ¹³C values of the studied organic sources range from -16.9‰ to -6.8‰, with seagrass leaves and leaf litter being the most ¹³C-enriched sources (-7.8±0.2)‰, POM (particulate organic mater) being the most ¹³C-depleted source (-16.9±0.2)‰, and seagrass epiphytes (-12.0±0.9)‰) and SOM (sediment organic matter)(-13.2±0.2)‰ having δ¹³C values in the middle. Three studied seagrass species, Thalassia hemprichii, Enhalus acoroides and Halodule uninervis, have δ¹³C values of (-8.9±0.1)‰, (-6.8±0.4)‰ and (-8.4±0.1)‰, respectively. The studied 12 invertebrate species have δ¹³C values ranging from -12.5‰ to -6.4‰, and the studied 13 fish species have δ¹³C values between -15.4‰ and -8.5‰. The above experimental results suggest that the δ¹³C values of different components from the seagrass bed are significantly different and therefore they can be used as tracers to identify the food sources for different creatures living in seagrass beds.
Using δ¹³C values as references, we then estimate the contribution of four components as food sources for some animal species (including fish and macro-invertebrates) living in Xincun Bay seagrass. IsoSource modeling suggests that the contribution of POM as food sources for three planktonic fish Hypoatherina valenciennei, Sardinella zunasi and Stolephorus spp. are 48% 79%, 42% 77% and 59%-84%, respectively. Epiphytes and SOM are the main food sources for four studied bivalves which have similar δ¹³C values. Seagrass-originated materials are the dominant food sources for the studied 12 animal species, includesing sea urchin (Hemicentrotus pulcherrimus), polychaetes (Dasybranchus caducus and Marphysa sanguinea), crustaceans (Penaeus monodon, Metapenaeus sp., and Thalamita danae) and some fish species. The IsoSource modeling results also indicate that seagrass-related materials contribute 14% to 49% of the food sources for studied two crabs (Thalamita stephensoni and Portunus pelagicus) whose δ¹³C values are between -11.1‰ and -9.9‰, and other four fish species (Terapon theraps, Gerres lucidus, Parascorpaena picta, and Ambassis kopsii) whose δ¹³C values are between -11.4‰ and -10.2‰. Overall, this study shows that seagrass is the most important food source for fauna species inhabiting in seagrass beds.