Diversity and community structure of zooplankton resting eggs in intertidal mudflats and creeks of an estuary

Numerous studies on zooplankton resting eggs have focused on subtidal seafloors, but ignored intertidal zones. The hypothesis that intertidal zones have comparable community of resting eggs as subtidal seafloors was tested by investigating the mudflats and creeks of the Yangtze River Estuary with the reference of nearby subtidal sediments. A total of 25 species of resting eggs were identified using DNA barcoding of the mitochondrial gene cytochrome c oxidase subunit I. Rotifers and copepods occupied 32% and 24% of taxa, respectively. The remaining species included cladocerans, insects, arachnids, sponges and bryozoans. Both the abundance and species richness of resting eggs did not differ significantly between the intertidal and subtidal zones. Similarly, the intertidal zones had the comparable community structure as the subtidal seafloors. The intertidal mudflats and creeks, however, differed significantly in the abundance, species richness, and community structure of resting eggs. The distance to the low-tide mark, clay content, and chlorophyll a explained the main variations of egg communities. Most abundance and species richness of resting eggs occurred within the upper 15 cm sediments and greatly declined with increasing depth. Zooplankton communities were reconstructed by combining the sedimentation rate and the vertical distribution of resting eggs. Freshwater cladocerans and rotifers declined with an increase of estuarine and coastal copepods over past ten years. These results demonstrate that resting eggs in intertidal mudflats and creeks were comparable to these in subtidal zones, and were natural archives for zooplankton responses to estuarine environments. They may serve as a new link between intertidal zones and open waters, facilitate zooplankton dispersal by waterbirds, and recover past communities.