Stable isotope analysis of simple food web in coral reef ecosystem of Daya Bay

In order to clarify the food web structure of coral reef ecosystem in Daya Bay, carbon and nitrogen stable isotope technology was applied to analyze carbon and nitrogen stable isotope ratios of major biological samples in the coral reefs, calculate the nutritional levels of major consumers and draw a continuous nutritional spectrum to construct the nutritional structure of coral reef in Daya Bay. In this paper, the trophic levels of the main consumers were calculated and the continuous nutritional spectrum was drawn to construct the nutritional structure in the coral reef area. The results showed that the δ¹³C values ranged from −23.22‰ to −10.76‰ and δ¹⁵N values ranged from 4.32‰ to 15.82‰ of reef ecosystem food web in Daya Bay. The average δ¹³C and δ¹⁵N values were −16.47‰±2.89‰ and 11.46‰±2.37‰, respectively. The carbon and nitrogen isotope ratios of potential food sources and consumers were significantly different. According to the calculation of δ¹⁵N value, the trophic levels of the main consumers ranged from 1.70 to 3.64 as a whole in the reef area of Daya Bay. The trophic levels of gastropods, bivalves and fishes ranged from 1.84 to 2.68, 1.70 to 2.49, and 2.45 to 3.64, respectively. There were no significant seasonal differences both in the carbon and nitrogen isotope ratios of macrobenthos. Six quantitative community indices of nutrient structure of macrobenthos were calculated using the SIBER model. The level of food source diversity (CR), trophic level (NR) and bio-ecological space utilization (SEAc) were the highest in spring; species aggregation density parameter (MNND) and species aggregation uniformity parameter (SDNND) were the highest in winter. The results showed that different sources of organic matter contributed differently to the coral reef food web in Daya Bay. Phytoplankton and algae were important primary producers and important carbon sources driving the food web. Some consumers fed on common bait organisms, and there was a phenomenon of niche overlap. The contribution of organic matter from different sources to the coral reef food web varied, and each consumer group had different nutritional positions. The average trophic level of cephalopods was the highest, followed by fish and bivalves, mainly related to their diet. The consumers in Daya Bay coral reef ecosystem showed a nutrient level distribution with less nutrient species at both ends and more in the middle. There was no significant seasonal difference in carbon and nitrogen stable isotopes of coral reef benthic communities in the Daya Bay, which could be related to the characteristics of coral reef ecosystems and the size of benthic animals. In general, benthos had relatively stable levels of nutrient diversity and food webs, among which, there were large differences in nutrient levels and fierce competition within the population in spring, and the highest nutritional redundancy in the community in winter.