Vertical shift in ciliate body-size spectrum and its environmental drivers in western Arctic pelagic ecosystems

As an inherent functional trait, body-size spectrum is widely used as an informative indicator to summarize community structures in taxon-free space. The vertical shift in the body-size spectrum of pelagic ciliates and its environmental drivers were explored at eight depth layers from the water surface to a depth of 100 m in western Arctic pelagic ecosystems. A total of 85 samples were collected at 23 sampling stations during the summer sea-ice reduction period from August 5 to August 24, 2016. Based on equivalent spherical diameter (ESD), six body-size ranks were identified, of which ranks S2 (15-25 μm), S3 (26-38 μm), S4 (39-60 μm), and S6 (79-91 μm) were the top four levels in frequency of occurrence and ranks S2 and S3 were the dominant levels in abundance. The body-size spectrum of the ciliates showed a clear vertical shift, with a significant succession among the dominant body-size units from the water surface to deeper layers in the water column. Multivariate analysis demonstrated a significant vertical variation in the body-size spectrum of the ciliates among the eight depths, which was significantly correlated with nutrients (phosphate and nitrite + nitrate) and chlorophyll a (Chl a), alone or in combination with dissolved oxygen. Four body-size diversity/distinctness indices were significantly correlated with the levels of phosphate, nitrite + nitrate, ammonium, and Chl a. Our results demonstrated that the body-size spectrum of pelagic ciliates can be shifted by environmental drivers (mainly nutrients and Chl a); thus, we suggest that it may be used to indicate water quality status on a vertical scale in the water column in deep seas.