Spatial and seasonal metal variation, bioaccumulation and biomonitoring potential of halophytes from littoral zones of the Karachi Coast

Highlights: • Three littoral zones of the Karachi coast were studied of which site III (Korangi Creek) near industrial area was most polluted. • Metal bio-concentration, translocation and bio-indication were both site and species specific. • Aeluropus lagopoides appeared ideal bio-indicator species for cleaning tested metals (Mn, Zn, Pb and Cr). • All species had potential for cleaning Pb and Zn affected soils while A. marina for Pb only. Coastal wetlands primarily serve as natural sinks of trace metals and their importance for phytoremediation is well known at global level. There are some reports on trace metal availability in soil sediments of the Karachi coast but studies on accumulation and translocation to harvestable (Shoot) parts among halophytes of the littoral zones have not been conducted. Hence, phytoremediation potential of six naturally existing halophytes (Aeluropus lagopoides, Arthrocnemum macrostachyum, Atriplex stocksii, Avicennia marina, Cressa cretica and Suaeda fruticosa) was assessed for cleaning metal (Mn, Zn, Pb and Cr) polluted soils of the Karachi coast. Seasonal (winters, pre and post-monsoon summers) and spatial (three littoral zones: viz., site – I: Sandspit, site – II: Do-Dariya/Clifton and site – III: Korangi creek) variations in soil and plant metals of the Karachi coast were studied. Soil Zn, Pb and Cr were generally higher in winters, Mn and organic matter in summers (7–11%) while pH values ranged between 7.15 and 7.5 in all seasons at site – III. All tested species had potential for cleaning Pb through their harvestable part (shoots) with A. stocksii as prominent candidate (16 mg kg⁻¹) at site – I. Cressa cretica emerged as exclusive candidate for Zn phytoremediation (96 mg kg⁻¹) at site – I, while S. fruticosa, A. macrostachyum and A. lagopoides showed bioaccumulation in pre-monsoon summers at site II. Aeluropus lagopoides with higher Mn in post monsoon summers (62 at site – III and 53 mg kg⁻¹ at site – II) and Cr (7.1 mg kg⁻¹ at site – II and 14 mg kg⁻¹ at site – III) appeared exclusive bioindicator with potential of for cleaning all metals (Mn, Zn, Pb and Cr) at different sites. Metal bioaccumulation at study sites appeared species specific and varied seasonally among tested halophytes.