Ecological Quality Status (EcoQs) of Chabahar sub-tropical bay based on multimetric macrobenthos-indexes approach: Response of bio-indexes to sediment structural/pollutant variables

Abstract

The present study aims to test the applicability of the macrobenthos indexes (AMBI, M-AMBI, BENTIX, BOPA; Shannon (H′);average taxonomic distinctness (Δ+), variation in taxonomic distinctness (Λ+) and total taxonomic distinctness (sΔ+) to assess the EcoQs of Chabahar Bay. Sampling was performed in the pre-monsoon period of 2017 from 19 stations. Five sediment texture/pollutant variables include grain size, total organic matter (TOM), PAH, heavy metal Pollution Load Index (PLI) and depth were also measured. Distribution pattern of macrobenthos as well as inter/intra relationship of the biotic and abiotic variables was investigated through multivariate statistical approaches. The Pearson and Rosenberg model was used to determine critical limit of TOC to reduce species richness, and as a criterion to select under-stressed stations. PCA analysis of all of indexes was used to evaluate the EcoQs of the polluted, suspected polluted or healthy stations. DistLM analysis showed that the sediment grain size followed by depth and TOM were main variables that significantly determined the distribution pattern of macrobenthos. Sensitive/tolerant indexes showed 84% full agreement as “acceptable” on most stations (including low-high TOC), while H′ values assessed all stations as “unacceptable”. The funnel plot of Λ+ could identify disturbed (high TOC) muddy stations, but was not a strong indicator of TOC disturbance for non-muddy stations. Among all indexes used in this study only the Λ+ showed significant correlation with PAH, TOM and sediment grain size. We believe that the Λ+ index works well in muddy substrates if the stress type is TOM overloading or PAH pollution. However this index alone does not make good judgment on the EcoQs of the studied area, so we recommend multimetric use of different indexes. The findings of the present study suggest that the physicochemical composition of sediment plays a key role in the distribution of macrobenthos and subsequently EcoQs assessment of Chabahar Bay, therefore, any anthropogenic activity affecting sediment could cause dramatic changes in macrobenthos assemblages.