Effects of aeration on the suspended matter from a tropical and eutrophic estuary

Abstract

A comprehensive understanding of the complex biogeochemical interactions between organic matter and persistent contaminants in the suspended matter is vital for eco-efficient estuary recovery. However, little is known regarding aeration effects in suspended particulate aggregates. Therefore, this study aimed to investigate the effects of aeration on the suspended matter from a Tropical and Eutrophic estuarine environment. Anoxic water with 60 g/L of suspended particulate matter (SPM) was collected from Guanabara Bay, Rio de Janeiro, Brazil, transferred to experimental boxes and aerated for 61 days. SPM aggregates monitoring included abiotic variables measurements and, determination of total organic matter (TOM), biopolymers composition, bacterial activity, trace metals, and polycyclic aromatic hydrocarbons (PAHs) concentrations. The aeration enhanced dissolved oxygen (DO) concentration and the redox potential (Eh). However, from days 0 to 61 the predominant bacterial activities were denitrification and fermentation. Electron transport system activity increased after day 10, and aerobic activity was detected after day 19. In summary, aeration increased aerobic bacterial activity, lipids (LIP) and trace metal concentrations, although diminished protein/carbohydrate ratio and PAH concentration. Trace metals concentration (Ni, Pb, Cu, Cr, Mn, and Fe) were the highest on day 19 when the pH was 5.9. Copper presented toxic values (Cu > 20.0 μg/g). The pH showed a strong negative correlation with Eh (r = −0.94; p < 0.001). Acidic environment (pH ≤ 5.9) in marine ecosystems with high loads of toxic trace metals is unsafe for biota. Therefore, managers must be aware of the environmental and biological risks of introducing the aeration technique into a eutrophic marine environment.