The impact of biotic and abiotic factors on the distribution of surface sediment dinoflagellate cyst assemblages on the Nanji Island in the East China Sea

The dinoflagellate cyst assemblages on the Nanji Island in the East China Sea, are documented at the first time to construct a quantitative overview of the cyst bank from 2014 to 2015. Thirty-four morphotypes from six groups are identified and quantified at eight sampling sites around the island, including a high proportion of potentially toxigenic species (14%). Autotrophic dinocysts constitute 74% of the total cyst counts, which is relatively low (two to thirty-three per millilitre sediment) compared with previous studies in adjacent areas. Scrippsiella trochoidea and Protoperidinium avellana are the most abundant autotrophic and heterotrophic species, respectively. A multivariate analysis is performed to assess associations between dinocysts and abiotic or biotic variables. Differentiation among seasons is evident in the detrended correspondence analysis (DCA) ordination plot, while a spatial pattern is not clearly revealed despite heterogeneity of the hydrodynamic conditions between sampling sites. Soluble reactive phosphate, the ratio of nitrogen to phosphorus concentrations and Karenia mikimotoi bloom are the three factors significantly ( P <0.05) related to surface sediment cyst assemblage defined by the canonical correspondence analysis (CCA), highlighting the importance of nutrient regime to a dinocyst distribution in this area. Although attempts to address the origin of HAB events in recent years using seed banks have failed, knowledge can be valuable for further investigation of dinocyst dynamics and potential toxin threats on the Nanji Island.