Environmental impact of heavy metals from dredged and resuspended sediments on phytoplankton and bacteria assessed in in situ mesocosms

Past and on-going reclamation, dredging, construction and shipping activities impact Ponggol Estuary, located on the northeastern coast of Singapore. Tin, lead, nickel, cadmium, and copper in particulate and dissolved fractions and sediments ranged from ND (undetectable)–92 ppm, ND–303.2 ppm, ND–2818.4 ppm, ND–74.4 ppm and ND–1117.7 ppm, respectively. Intensive dredging activity during the monitoring period may have led to the resuspension and bioavailability of particulate metals. This was tested by the exposure of phytoplankton and bacteria in mesocosms to previously measured environmental levels of heavy metals and the contaminated sediments with the highest heavy metal concentrations from one of the impacted sites. The results showed significant copper toxicity to phytoplankton and autotrophic bacteria, followed by nickel and lead at all concentrations tested. Enhanced rates of heterotrophic bacterial production and total bacterial abundance were observed in treatments with higher metal concentrations. Among the various treatments, particulate and sediment metal concentrations were significantly different from those of the control. Mesocosms using contaminated sediments with the highest metal concentrations compared with the control showed a bioavailability of metals that resulted in the inhibition of phytoplankton and autotrophic bacteria. High concentrations of copper (5.52–11.35 mg L−1) and nickel (2.42–2.71 mg L−1) observed in the aqueous phase of treatment mesocosms, and attributed to release from the contaminated sediments could account for the toxicity to phytoplankton and autotrophic bacteria.