Carbonate dissolution in the turbid and eutrophic Loire estuary

Abstract

We measured particulate and dissolved organic carbon (POC and DOC), chlorophyll, oxygen, partial pressure of CO 2 , pH, total alkalinity (TAlk) and particulate inorganic carbon (PIC) during a late summer cruise in the eutrophic Loire estuary. These parameters reveal an intense mineralisation of organic matter in the estuarine maximum turbidity zone (MTZ) that results in oxygen deficits (down to 20% of the saturation level) and high CO 2 oversaturations (pCO 2 up to 2900 µatm). Several facts revealed the occurrence of carbonate dissolution in the Loire MTZ: large amounts of alkalinity were produced in the upper estuary, increasing its transfer to the ocean by 30%; the calculated saturation index showed a net undersaturation for aragonite and a slight undersaturation for calcite in the MTZ; and PIC decreased from 2.1% (% dry weight) in riverine suspension to 0.4% in the MTZ. A stoichiometric approach is used to assess the coupling between aerobic respiration and carbonate dissolution, where apparent oxygen utilisation, excess CO 2 , TAlk and dissolved inorganic carbon are compared quantitatively. About 20% of the CO 2 generated by respiration was involved in carbonate dissolution. The loss of PIC at the river-estuary transition quantitatively corresponds to the amount of authigenic calcite precipitated upstream in the highly eutrophic river. This suggests that CO 2 exchange with the atmosphere along the eutrophic river-estuary continuum is buffered by carbonate precipitation in the autotrophic river and its dissolution in the heterotrophic estuary.

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

“…Some other cases of net CO 2 uptake have been reported in some relatively polluted tropical coastal lagoons in Ivory Coast (Koné et al, 2009), in three temperate and marine-dominated Australian estuaries (Maher and Eyre, 2012), in temperate and tropical estuarine plumes either preserved (Körtzinger, 2003) or human-impacted (Cai, 2003;Zhai and Dai, 2009;Bozec et al, 2012), and in some pristine Arctic and subarctic fjords (Rysgaard et al, 2012;Ruiz-Halpern et al, 2010). In contrast, inner and low-salinity regions of most river-dominated, drowned-valley, "funneltype" estuaries, which are generally well-mixed and relatively turbid environments, have been documented as heterotrophic and CO 2 emitters under tropical , temperate and boreal (Silveneoieen et al, 2008) climates, regardless of the anthropogenic pressure (Abril et al, 2003(Abril et al, , 2004Zhai et al, 2007;Borges and Abril, 2011;Cai, 2011;Sarma et al, 2012).…”

“…The slope was not statistically different from 1 (p = 0.20) and the intercept was not significantly different from 0 (p = 0.86). The excess of DIC (E-DIC, µmol kg −1 ) was calculated as the difference between the in situ DIC (DIC in situ, µmol kg −1 ) and a theoretical DIC at atmospheric equilibrium (DIC equilibrium, µmol kg −1 ) according to Abril et al (2003). The DIC equilibrium was calculated from observed TA and the atmospheric pCO 2 measured in the bay.…”

Artificially induced migration of redox layers in a coastal sediment from the Northern Adriatic

“…Many estuaries where the residence time of water is supposedly relatively short also show severe hypoxia (Garnier et al, 2001;Rabalais et al, 2002;Hagy et al, 2004;Lanoux et al, 2013), sometimes leading to fish mortality; this includes estuaries only moderately impacted by anthropogenic eutrophication, such as the Loire River mouth (Abril et al, 2003). Furthermore, continental seas such as the Baltic Sea, Black Sea, and the Gulf of Mexico (Rabalais et al, 2002), show an increase in the occurrence and geographic extent of anoxic events.…”

Carbon Dioxide and Methane Emissions from Estuaries

“…This region often corresponds to the location of the ETM where and Oxygen (open squares%Saturation) in surface waters, showing the net antiparallelism between the two parameters. B. water-air CO 2 fluxes (floating chamber method) measured in three European estuaries studied during the BIOGEST project (Frankignoulle et al 1998;Abril et al 2003). Dotted lines are the atmospheric pCO 2 value of 365 µatm.…”