Organic Matter Composition Related to Methane Ebullitive Flux of an Urban Coastal Lagoon, Southeastern Brazil

Abstract

In 2016, Brazil emitted 18.25 Tg of methane. Some of these emissions occur through continental aquatic ecosystems, locations of fate and accumulation of organic and inorganic matter. Thus, the aim of this study is to evaluate the origin of organic matter in Rodrigo de Freitas Lagoon, an eutrophic, coastal and chocked lagoon in Rio de Janeiro, Brazil, through the determination of n-alkanes and sterols compounds in the upper two centimeters of sediment and compare these data with methane ebullitive flux. The concentrations of n-alkanes varied between 2.43 and 25.82 µg g-1. C29 was predominant compound in most sites, but also with important petrogenic source evidenced by the occurrence of Unresolved Complex Misture. The total concentration of sterols ranged from 2.76 to 56.01 µg g-1. β-sitostanol was the most abundant compound and coprostanol was the most relevant at the sites under influence of domestic effluents. CH 4 ebullitive flux averaged 199 mg m-2 d-1 in the dry period and 9.3 mg m-2 d-1 during the wet season. We conclude that the organic matter reaching Rodrigo de Freitas Lagoon through the rivers discharge added or enriched with irregular sewage input and petroleum derivates are the main source of eutrophication and generate higher methane emissions to the atmosphere.

Bacterial production prevails over photo- and chemosynthesis in a eutrophic tropical lagoon

“…Biological oxidation of ammonia to nitrite and its subsequent oxidation to nitrate via nitrification or reduction of nitrite and ammonia to N 2 via anammox are known chemolithotrophic processes, but they do not seem to be very relevant in tropical coastal lagoons (Enrich-Prast et al, 2016). Evidence of methane in the water column at high concentrations near the bottom was reported in RFL (Viana et al, 2019), a fact that suggests the availability of substrate occurrence of methane oxidation. Methane oxidation represents an important benthic-pelagic carbon and energy link in lakes, with methanotrophic bacteria being the main actors (Bastviken et al, 2003), and has been reported to be important in tropical aquatic food webs (Sanseverino et al, 2012).…”

“…and sewage (coprostanol), which are considered biomarkers for the presence of animal feces and sewage contamination (Viana et al, 2019). Specifically for station 4, the combination of pluvial and fluvial waters with effluents input resulted in high rates of HBP, extremely high concentrations of total and fecal coliforms and Enterococcus (sometimes 50 times higher than other stations, van Weerelt et al, 2012), associated with high ammonia emissions (Barroso-Santos et al 2012), as well as relatively higher nutrients and lower salinity, pH and oxygen (e.g.…”