How eutrophic and pristine estuaries export acidic waters to the coastal ocean: a case study in SE Brazil

Chair: Yuri Artioli

Leticia Cotrim Da Cunha (1)*, Helen Soares (1), Michelle Araujo (1), Cassia Farias (1), Claudia Hamacher (1), Gleyci Moser (1)

1 Faculdade de Oceanografia, Rio de Janeiro State University (UERJ), Rio de Janeiro – RJ, 20550-900, Brazil

Large amounts of untreated domestic sewage are discharged into many coastal ecosystems in Brazil, resulting in excessive algal growth and consequent microbial degradation of this biomass. As a consequence, dissolved oxygen becomes subsaturated, along with a decrease in pH and increase in water pCO2. However, tropical pristine areas may also have high loads of organic matter, as mangrove-fringed estuaries. There, water column metabolism may also lead to pCO2 supersaturated surface waters.

Here we assess the interplay between DO, pH, and pCO2 in four coastal ecosystems in Rio de Janeiro state, SE Brazil. Three of them are heavily impacted by domestic sewage and/or industrial effluents (Guanabara Bay, Canal da Joatinga, R. de Freitas Lagoon), and one is considered relatively preserved (Barra Grande Estuary). They all vary in size and water renewal time.

Diurnal variability of biogeochemical parameters is extreme in all ecosystems. Seasonal surveys have shown that tidal forcing influences stratification, especially the smaller estuaries, and therefore CO2 saturation in its surface waters. Biological activity also largely influences the CO2-system parameters, and it is commonly observed increasing dissolved CO2 in bottom waters, related to increasing respiration metabolism. Surface waters may remain seasonally undersaturated in CO2, as in the pristine estuary, the choked urban lagoon or the large Guanabara Bay.

Generally during flood-tide, the acidic/hypoxic conditions are alleviated, while during ebb-tide these organic-matter-rich, low-oxygen, CO2-saturated waters are exported to the coastal ocean. How these and other “acidic plumes” either from pristine or sewage-polluted ecosystems impact near-shore areas in Brazil remain yet to be assessed.