Insights into Bioindicator Redundancy for Monitoring Water Quality based on Functional Units of Periphytic Protozoan Communities in Marine Ecosystems

Abstract Although periphytic protozoan communities have long been used for the bioassessment of water quality, their utility is hampered by functional redundancy leading to high “signal to noise” ratios. In this study, a 1-year baseline survey of periphytic protozoan communities was carried out in coastal waters of Yellow sea, northern China, in order to determine redundancy levels in conditions of differing water quality. Samples were collected at four sampling sites along a pollution gradient. Environmental variables such as salinity, chemical oxygen demand (COD), and concentrations of dissolved oxygen (DO), soluble reactive phosphates (SRP), ammonium nitrogen (NH4-N) and nitrate nitrogen (NO3-N) were measured to compare with biotic factors. A total of 53 functional units (FUs) were identified from 144 observed protozoan species based on four biological traits, i.e., feeding type, body size, movement type and source of food supply. For reducing the “signal to noise” ratios of species-abundance/biomass data, the peeling procedure was used to identify the bioindicator redundancy levels based on these FUs. Three consecutive subsets of response units (RU1–RU3) with correlation coefficients >0.75 of the full FU dataset were identified, comprising 12, 21 and 9 FUs, respectively. Algivores and bacterivores were dominant in RU1 and RU2 among the polluted sites, whereas raptors were dominant in RU3 at the unpolluted site. In terms of relative abundance, RU1 was the primary contributor to the protozoan communities during the 1-year cycle and its relative abundance increased with increasing pollution, whereas RU2 and RU3, with complementary temporal distributions, generally decreased with increasing pollution. Ordinations based on boot-strapped average analyses revealed a significant variation in functional pattern of all three RUs among the four sampling sites. Biological-environmental match analysis demonstrated that the variability was driven by the increasing concentrations of nutrients (e.g., NH4-N, NO3-N and PO4-P) and decreasing concentrations of DO (P<0.05). Based on these findings, it is suggested that there were high levels of functional redundancy among periphytic protozoan communities used as bioindicators of marine water quality.