Water-Air Exchanges In The Lower Estuary Of The Patos Lagoon: Seasonal Variability, Drivers, And Sources Of CO2

Cíntia Albuquerque (  cintia_albuquerque@furg.br ) Instituto de Oceanogra a, Universidade Federal do Rio Grande FURG https://orcid.org/0000-00023185-328X Rodrigo Kerr Instituto de Oceanogra a, Universidade Federal do Rio Grande FURG Thiago Monteiro Instituto de Oceanogra a, Universidade Federal do Rio Grande FURG Eunice da Costa Machado Universidade Federal do Rio Grande FURG, Av. Itália km 8 s/n Andréa da Consolação de Oliveira Carvalho Instituto de Oceanogra a, Universidade Federal do Rio Grande FURG Carlos Rafael Borges Mendes Instituto de Oceanogra a, Universidade Federal do Rio Grande FURG


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Estuaries are known to be large sources of carbon dioxide (CO2) to the atmosphere (e.g., in terms of CO2 saturation with the atmosphere were found. As a larger body of water in 164 the Southern Hemisphere, a better understanding of the regional behavior of the CO2 165 fluxes in the PLE is mandatory for its inclusion in a global CO2 analysis. Therefore, in 166 this study, we present the first overview of the behavior of the water-air CO2 exchanges     The total scale was chosen and further used for pH at in situ temperature.

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The pCO2 and other CO2-carbonate variables not directly measured were estimated  2.5 CO 2 estuarine concentration estimates 290 We followed the approach described in Jiang et al. (2008) to determine the CT change 291 caused by river-ocean mixing, using CTmr (Equation 9) to estimate the riverine water input 292 and CTmix (Equation 10) to estimate ocean mixing at each pier-fixed station (i): where CTr, Sr, CToc and Soc are the CT and salinity river and ocean end-members, 296 respectively, and Si is the salinity at station i.

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As there was no continuous sampling during the study period at the mouth of the river or

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When there is no river influence, the CT at station i can be calculated as follows: where CTmo is CT due to water mixing; CToc and Soc are CT and salinity at the ocean end-306 member, respectively; and Si is the salinity at station i. 307 Then, produced/consumed CT due to estuarine-biogeochemical processes (CT est ) can be 308 calculated as follows: where CTi is CT at station i and CTm is CT due the mixing of river and ocean and can be 311 calculated from Equations 9-11. Following the same approach, ATm and AT est can be   than that found near the more sea-exposed zone (average of 2 ± 31 mmol m -2 d -1 ). In June to September in the mouth of the estuary (Figure 3a and b).

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Despite the high monthly variability and the marked seasonal cycle of the CO2 exchanges 372 observed in the PLE (Figures 3 and 4), during the 4-year period analyzed, the inner inlet

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The orange diamonds depict the annual averages (note that 2017 and 2021 do not consider all the seasons).

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The FCO 2 annual average and standard deviation for each estuarine station (color indicated by the legend) where the freshwater discharge was low, the input of CO2 with marine sources was 417 intense.

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Despite the small difference, the inner inlet zone produced more CO2 (11.8  12.6 µmol 419 kg -1 ) than the exposed area   December and May) and CO2 outgassing (between June and November), mainly in 507 protected areas along the estuary (Figure 3b). CO2 exchanges are modulated by winds picoplanktonic species, which helps to balance the carbon concentration. The production 536 of CO2 was lower in the more sea-exposed area than in the inner inlet station, suggesting that the abundance of diatoms in the region closest to the coast is higher due to local 538 hydrodynamics, mainly at the end of winter and spring. Moreover, in spring (October and 539 November), the blooms caused by biological activity combined with vertical mixing due 540 to the higher wind intensity lead to highly variable water-air CO2 fluxes in the lower zone