Variations in nano- and pico-eukaryotic phytoplankton assemblages in the Qinhuangdao green-tide area.

Qinhuangdao coastal waters have been frequently hitting by nano- and pico-eukaryotic phytoplankton (NPEP) blooms and green tides (macroalgal blooms) in the recent decade. However, understanding about the impacts of environmental factors and the green tides on the NPEP assemblages in this area is limited. In this study, the composition of NPEP assemblages and their variations were analyzed via amplicon sequence variants (ASVs) assay based on amplicon high-throughput sequencing data with the 18S V4 region as a targeted gene in the Qinhuangdao green-tide area during the green tide. Consequently, average NPEP effective sequences and ASVs of 178 000 and 200 were obtained from each sample, respectively. Although there were 25 classes, 110 genera, and 97 species of NPEP were identified and annotated, the proportions of annotated ASVs at genus and species levels were only 44.7% and 17.8%, respectively. The NPEP communities had a seasonal succession from diatom-dominated to dinoflagellate-dominated. During the three investigations, Skeletonema, Karlodinium, and Gonyaulax were the most dominant genera in May, August, and September, respectively. Species diversity and the abundance of NPEP communities could be increased by a high content of dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) but inhibited by low dissolved inorganic phosphorus content. The outbreak of green tides could alter the composition and content of nutrients and accelerate the succession of the NPEP communities from diatom-dominated to dinoflagellate-dominated under the background of a seasonal increase in seawater temperature. These results preliminarily revealed the impacts of the recurrent occurrences of green tides on the NPEP assemblages in the Qinhuangdao green-tide area exhibiting high DON content and dissolved inorganic nitrogen/phosphorus ratio. [ABSTRACT FROM AUTHOR]