Net Ecosystem Metabolism and Nonconservative Fluxes of Organic Matter in a Tropical Mangrove Estuary, Piauí River (NE of Brazil)

Abstract

Net ecosystem metabolism (NEM) was measured in the Piauí River estuary, NE Brazil. A mass balance of C, N, and P was used to infer its sources and sinks. Dissolved inorganic carbon (DIC) concentrations and fluxes were measured over a year along this mangrove dominated estuary. DIC concentrations were high in all estuarine sections, particularly at the fluvial end member at the beginning of the rainy season. Carbon dioxide concentrations in the entire estuary were supersaturated throughout the year and highest in the upper estuarine compartment and freshwater, particularly at the rainy season, due to washout effects of carbonaceous soils and different organic anthropogenic effluents. The estuary served as a source of DIC to the atmosphere with an estimated flux of 13 mol CO 2 m −2 year −1 . Input from the river was 46 mol CO 2 m −2 year −1 . The metabolism of the system was heterotrophic, but short periods of autotrophy occurred in the lower more marine portions of the estuary. The pelagic system was more or less balanced between auto-and heterotrophy, whereas the benthic and intertidal mangrove region was heterotrophic. Estimated annual NEM yielded a total DIC production in the order of 18 mol CO 2 m −2 year −1 . The anthropogenic inputs of particulate C, N, and P, dissolved inorganic P (DIP), and DIC were significant. The fluvial loading of particulate organic carbon and dissolved inorganic nitrogen (DIN) was largely retained in two flow regulation and hydroelectric reservoirs, promoting a reduction of C:N and C:P particulate ratios in the estuary. The net nonconservative fluxes obtained by a mass balance approach revealed that the estuary acts as a source of DIP, DIN, and DIC, the latter one being almost equivalent to the losses to the atmosphere. Mangrove forests and tidal mudflats were responsible for most of NEM rates and are the main sites of organic decomposition to sustain net heterotrophy. The main sources for this organic matter are the fluvial and anthropogenic inputs. The mangrove areas are the highest estuarine sources of DIP, DIC, and DIN.

Air‐water CO2 fluxes in the microtidal Neuse River Estuary, North Carolina

“…Survey data from the lower estuary during July 2009 were excluded due to an incongruent transect route. (Figures 2 and 3) [Frankignoulle et al, 1998;Cai et al, 1999;de la Paz et al, 2007;Zhai et al, 2007;Guo et al, 2009;Souza et al, 2009]. In the spring and summer, conditions in the wind-driven NRE were most similar to those seasonally observed by Koné et al [2009] in the microtidal Aby Lagoon, Ivory Coast.…”

A strong CO<sub>2</sub> sink enhanced by eutrophication in a tropical coastal embayment (Guanabara Bay, Rio de Janeiro, Brazil)

“…In Brazil, most previous studies have concerned riverdominated estuaries, especially along the northern and northeastern coast, which all behave as CO 2 sources (Souza et al, 2009;Araujo et al, 2014;. In contrast to Guanabara Bay, highest CO 2 fluxes correspond to denser population in the watersheds of these net heterotrophic systems .…”

“…However, the limited number of available studies suggest that tropical estuaries are generally sources of CO 2 to the atmosphere (Souza et al, 2009;Sarma et al, 2012;Araujo et al, 2014), with the exception of one lagoon (Koné et al, 2009). Also, most studies are potentially biased by the lack of information on the diurnal variations in CO 2 , which corresponds to a crucial component of mass balance calculations (Borges and Frankignoulle, 1999;Zhang et al, 2013;Maher et al, 2015).…”

Nutrient Input and CO<sub>2</sub> Flux of a Tropical Coastal Fluvial System with High Population Density in the Northeast Region of Brazil

“…A recent study [12] reported an efflux of 13 mol CO 2 m −2 ·yr −1 for a tropical estuary (the Piauí estuary, Brazil, latitude 10˚S). As indicated by [6], the current calculation is based on a very limited data set from subtropical and tropical regions.…”