Variations in body-size spectra of periphytic ciliates at different depths: a case study in coastal waters of the Yellow Sea

Abstract

Body-size spectra are inherent characteristics of organisms that can be used to summarise the functional structure of a community and thus be used in both ecological studies and biomonitoring programs. In order to determine the effect of water depth on body-size spectra of marine periphytic ciliate communities, a 1-month baseline survey was conducted at four depths (1, 2, 3.5 and 5m) in coastal waters of the Yellow Sea, northern China. Based on equivalent spherical diameters (ESD), 50 species were categorised into seven body-size ranks: S1, 2–17μm; S2, 22–27μm; S3, 29–36μm; S4, 37–49μm; S5, 53–71μm; S6, 84–92μm; S7, 127–153μm. These seven body-size ranks were composed of four trophic functional groups: algivores (A), bacterivores (B), predators (R) and non-selectives (N). Body-size rank S1 was composed primarily of the B functional group; S2 was composed of the N and R functional groups, S3 and S4 were composed of the A, B and N functional groups, S5 and S6 were composed of the A functional group and S7 was composed of the R functional group. The medium body-size ranks (S4 and S5) were dominant at 1 and 2m, whereas the smallest body-size rank (S1) was dominant at 3.5 and 5m. Canonical analysis of principal coordinates revealed a clear vertical variation in body-size spectra at the four depths. Body-size diversity indices peaked at 2–3.5m and fell sharply at 5m. Body-size diversity indices peaked at 2–3.5m and fell sharply at 5m. Body-size distinctness, as measured by the paired-index (ellipse) test, showed an increasing trend of departure from the expected pattern from surface to deeper layers. These results suggest that the body-size spectra of periphytic ciliates may be significantly shaped by water depth and thus may be used as bioindicators of the ecological integrity and quality of water at different depths in marine ecosystems.