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Highlights • The functional distinctness were generally significantly related to nutrients and pH. • The functional distinctness may detect the departure from an expected functional pattern. • All functional diversity/distinctness measures are not sensitive to samples effort. • The functional distinctness of periphytic ciliates may be used to indicate water quality status. Abstract As trait-based ecological parameters, functional diversity/distinctness measures are robust parameters for biodiversity research by reducing "signal to noisy" ratios due to functional redundancy in a community, and have an advantage for bioassessment due to a statistical framework for testing departure from expectation by combining ecosystem functions and community structure. The effectiveness of four functional diversity/distinctness measurements for water quality assessment was based on a study of additional ciliate datasets for biological evaluation of coastal waters along the Yellow Sea in northern China. A fuzzy-coding trait system was used as a trait pool, which comprised six characters of feeding types, movement features, body size/form/flexibility, and sociability. Results showed that (1) the functional diversity (Δ χ) significantly related to the spatial variation in water transparency; (2) the average functional diversity index (Δ χ +) and variation in functional distinctness (Λ χ +) represented a significant correlation with the changes in pH and nutrients, especially ammonium nitrogen, and soluble active phosphate; and (3) ellipse test for the paired indices (Δ χ + and Λ χ +) revealed a clear departure from an expected functional trait pattern. According to our findings, it is recommended that the functional diversity/distinctness measures may be served as a robust bioindicator of water quality status in marine ecosystems. [ABSTRACT FROM AUTHOR]