Complex spatial and temporal variation of subtropical benthic macrofauna under sewage impact

•We assessed variation in benthic fauna at nested spatial and temporal scales.•Differences between Seasons were significant due to lower faunal abundance in summer.•Interactions between Fortnight and Tidal Flat were significant for most variables.•Benthic community may change significantly at short time scales.

To better understand variation in sewage-impacted benthic macrofauna from subtropical tidal flats over time and space, we applied a five-factor linear model at a hierarchy of spatial (Condition – Contaminated or Non-Contaminated, Tidal Flat and Plot) and temporal scales (Season and Fortnight). The Contaminated site showed high levels of coprostanol and the presence of Paranais cf frici as markers or indicators of organic enrichment. Differences between Seasons were more pronounced for the faunal variation patterns than for the other parameters, with lower species richness and abundance in summer. There were significant interactions between Fortnight and Tidal Flat for most variables, reflecting marked heterogeneity within Tidal Flats. Benthic community has significantly changed over short periods of time. These rapid changes may lead to erroneous interpretations and mask the true sources of variation. Our results clearly demonstrate the need to better understand benthic temporal variability even at small scales.