Carbon sources and trophic levels in the food web of the largest mangrove reserve of China

Abstract

Zhanjiang Mangrove National Nature Reserve (ZMNNR) is the largest mangrove natural reserve in China. We conducted the first comprehensive isotope study on the ZMNNR. Primary producers, including mangroves, grasses, microphytobenthos (MPB), etc. formed the lowest trophic level. Gastropods and bivalves, crustaceans and carnivorous fish were on the 2nd, 3rd and 4th trophic levels, respectively. In Zhanjiang tidal estuary, mangroves contributed about 1/3 of the carbon sources for consumers, substantially smaller than those in swamps and lagoons, but higher than those in river delta mangroves. The major carbon sources to the food web shifted from particulate organic matter (POM), consisting of detritus of mangrove leaves and phytoplankton, perennial grasses and mangrove leaves in the high tide zone to invasive Spartina alterniflora, POM and mangrove leaves in the low tide mudflat. Insignificant contributions of MPB were due to restricted light penetration by the dense mangroves in the high tide zone, and prolonged submersion and sediment resuspension in the low tide mudflat. Aquaculture effluents caused substantially higher δ15N values of all primary producers and consumers in the ZMNNR. In addition, high nutrient content and N/P ratios in aquatic effluents might allow cyanobacteria in MPB to be outcompeted by other fast growing microorganisms. Aquaculture effluents might also contribute to shortened trophic levels and promotion of the dominance of resistant species in the ZMNNR. We call for a sustainable development strategy to conserve the valuable ecosystem in the ZMNNR.