Body-size spectra of biofilm-dwelling ciliates at different layers in water column of coastal ecosystems

Abstract It is increasingly recognized that the body-size spectrum of biofilm-dwelling ciliates is an informative bioindicator of environmental change in marine ecosystems. In order to reveal the effects of water depth on the body-size spectrum of biofilm-dwelling ciliates during the colonization process, a 1-month baseline survey was carried out in coastal waters of the Yellow Sea, northern China. Using glass slides as artificial substrates, a total of 240 microscopy slides were collected from four depths, i.e., 1, 2, 3.5 and 5 m, after colonization periods of 3, 7, 10, 14, 21 and 28 days. A total of 92 ciliate species were collected. Based on its equivalent spherical diameter (ESD), each species was allocated to one of eight body-size ranks (S1–S8): S1 (13– 17 μ m ), S2 (20– 26 μ m ), S3 (30– 34 μ m ), S4 (35– 49 μ m ), S5 (54– 63 μ m ), S6 (66– 93 μ m ), S7 (120– 161 μ m ) and S8 (162– 247 μ m ). The body-size spectrum showed a clear temporal variability in both frequency of occurrence and probability density during the colonization process at the four depths: S1 was dominant in immature samples whereas mature samples were dominated by S3. Multidimensional scaling ordinations (MDS) and bootstrap average analyses revealed a clear shift in body-size spectra from surface to deeper water layers during the colonization process. Ellipse tests demonstrated that the samples showed different patterns of departure from the expected body-size spectrum at each depth during the colonization period. These findings suggest that water depth may significantly shape the body-size spectra of biofilm-dwelling ciliates, and that there may be an optimal water depth when using this feature for monitoring marine ecosystems.