Seasonal and spatial variability of CO₂ emissions in a large tropical mangrove‐dominated delta

Abstract

This study quantified the seasonal and spatial variability of partial pressure of CO2 (pCO2) and water‐atmosphere CO2 fluxes in the Parnaíba River Delta, the largest delta in the Americas. It is a pristine equatorial, mangrove‐dominated environment located in a transitional between humid and semi‐arid climates, with marked seasonality in rainfall and river discharge. Major channels and bays were sampled during dry and wet seasons, with continuous measurements of pCO2, temperature, salinity, and wind velocity. Subsurface water samples were collected in discrete stations for pH, total alkalinity (TA), dissolved inorganic carbon (DIC), dissolved oxygen and chlorophyll a quantification. A significant positive correlation between carbonate system parameters with salinity was found in both periods, with salinity significantly higher in the dry season. Strong deviations of pCO2, TA, and DIC from two endmembers conservative mixing were found, particularly in mangrove‐dominated waters, due to organic matter degradation. The Delta showed high spatial variability of pCO2, with the highest values in mangrove‐dominated waters, moderate in the river‐dominated regions, and lowest in the high salinity areas, suggesting that pCO2 variability is likely controlled by a combination of river‐ocean mixing and biological processes (respiration and photosynthesis). The Delta outgasses about 20 times less CO2 in the dry season (9.06 ± 11.09 mmol m−2.d−1) than in the rainy season (209.68 ± 250.87 mmol m−2 d−1). Our results indicate this large mangrove‐dominated tropical delta is an important source of CO2 to the atmosphere, but a sharp decrease was observed during dry periods.

Changes in DIC/TA ratio by tidal asymmetry control <i>p</i>CO₂ over a spring-neap tidal cycle in a subtropical mangrove forest in Japan

“…Several studies have used the stoichiometric relationship between ΔDICM, ΔTAM, nDIC, and nTA to discuss the origin of mineralization pathways (Akhand et al, 2021b;Alongi, 2022;Borges et al, 2003;Bouillon et al, 2007a;Ho et al, 2017;Santos et al, 2019;Sippo et al, 2016;Volta et al, 2020;Chielle et al, 2023). However, as indicated in the Introduction, these stoichiometric relationships do not consider secondary redox reactions in water (Krumins et al, 2013;Reithmaier et al, 2021a); therefore, it is difficult to identify the origin of the mineralization pathways in mangrove soils.…”

“…However, as indicated in the Introduction, these stoichiometric relationships do not consider secondary redox reactions in water (Krumins et al, 2013;Reithmaier et al, 2021a); therefore, it is difficult to identify the origin of the mineralization pathways in mangrove soils. Although it is difficult to estimate the original mineralization process using the stoichiometric relationship of DIC and TA, the slope of ΔDICM and ΔTAM may represent the characteristics of the local mangrove environment as these slopes vary in each mangrove ecosystem (Akhand et al, 2021b;Alongi, 2022;Borges et al, 2003;Bouillon et al, 2007a;Ho et al, 2017;Santos et al, 2019;Sippo et al, 2016;Volta et al, 2020;Chielle et al, 2023). Owing to the repeated inundation of mangrove forests, the soil environment is expected to undergo substantial fluctuations within a brief timeframe.…”

“…Mangrove creeks are a net source of CO2 emissions (Borges et al, 2003;Maher et al, 2013;Chielle et al, 2023), which are markedly higher compared to that of other blue carbon ecosystems (Akhand et al, 2016;Akhand et al, 2021a;Santos et al, 2019). Organic matter deposited in mangrove soils is generally decomposed by sulfate reduction and aerobic respiration, although certain sites have demonstrated a higher contribution of denitrification and iron reduction (Alongi et al, 2005;Kristensen, 2007;Kristensen et al, 2008).…”

“…Each mineralization process contributes to a different DIC to the total alkalinity (TA) ratio (Krumins et al, 2013). Based on this theoretical stoichiometric relationship, the main mineralization pathways in mangrove creeks were discussed using the slope of the mangrove-derived DIC (ΔDICM) and TA (ΔTAM), or salinity normalized DIC (nDIC) and TA (nTA) (Akhand et al, 2021b;Alongi, 2022;Borges et al, 2003;Bouillon et al, 2007a;Ho et al, 2017;Santos et al, 2019;Sippo et al, 2016;Volta et al, 2020;Chielle et al, 2023). However, because secondary redox reactions with oxygen in the water were not considered, whether these DIC-to-TA ratios represent the mineralization processes that contribute to the decomposition of organic matter in soils remains questionable (Krumins et al, 2013) (Table S1).…”

“…Hence, the changes in DIC and TA owing to sulfate reduction and oxidation should be biased toward DIC (Krumins et al, 2013;Reithmaier et al, 2021a) (Table S1). Although estimating the original mineralization process using the stoichiometric relationship of DIC and TA is difficult, the slope of ΔDICM and ΔTAM may represent the characteristics of the local mangrove environment because the mangrove-derived DIC and TA have different slopes in each mangrove ecosystem (Akhand et al, 2021b;Alongi, 2022;Borges et al, 2003;Bouillon et al, 2007a;Ho et al, 2017;Santos et al, 2019;Sippo et al, 2016;Volta et al, 2020;Chielle et al, 2023). However, whether the slopes of ΔDICM and ΔTAM represent regional characteristics is unknown, including differences in the climate and topography, or differences in the water sampling environment, such as inundation or the exposure of mangrove swamps.…”