Carbonate system in an Amazonian mangrove creek

Introduction

Mangrove forests are coastal environments extremely effective at accumulating carbon and are scientifically targeted due to the role they can potentially play facing climate changes^[1]. Mangroves fix atmospheric CO₂ through photosynthesis and introduce in their ecosystem organic carbon as forest biomass, root biomass and exudation, and litter fall. This organic carbon is intensively recycled by heterotrophic organisms in the mangrove soils and waters. Indeed, one main source of greenhouse gases in mangroves is soil respiration, that is, the remineralization of carbon through microbial decomposition of organic matter and the roots respiration^[2]. As in intertidal ecosystems, mangroves gas fluxes vary with the tide and the emersion and immersion times. During high tide, gases from soil respiration are solubilized and hardly escape to the atmosphere due to limited gas exchange. They can be transported to the tidal channel (lateral export). Since CO₂ is an extremely soluble acidic oxide, the lateral export of dissolved inorganic carbon (DIC) occurs in two distinct chemical forms: dissolved CO₂ (which rapidly returns to the atmosphere) and alkalinity, that represents a long-term carbon storage and increases the buffer capacity of coastal waters^[3].

This study aims to describe the evolution of the carbonate system throughout tidal cycles at the creek of a macrotidal amazonian mangrove during the dry season. We evaluated the proportion of alkalinity and gaseous CO₂ among the DIC as well as the rate of CO₂ evasion at the water-air interface. Concomitant measurements of CO₂ fluxes and CO₂ partial pressure allow the quantification of the gas transfer velocities (K₆₀₀) in the creek^[4]. This study was conducted in a mangrove at the Marapanim river estuary (Pará, Brazil), which is part of the largest continuous strip of mangrove forest in the world.

Experimental

We solved the carbonate system by measuring 3 parameters: dissolved CO₂ (pCO₂), pH and total alkalinity (TA) during a complete tidal cycle (25 hours) at the confluence of mangrove’s drainage creeks. Water pCO₂ were measured every minute using an equilibrator connected to a LI-COR LI-830 CO₂ Gas Analyzer^[5]. The water pH, salinity and temperature were simultaneously measured with a HANNA HI98194 Multiparameter Probe. At last, water samples were collected every hour, filtered through a glass microfiber filter (Whatman GF/F 0.7 µm) and stored in plastic vials for further analysis of TA with a METTLER TOLEDO T50 automatic titrator using HCl 0.1 mol.L^-1.

The water pH during the cycle was also calculated as a function of salinity, temperature, pCO₂ and TA using a MS Excel program developed for CO₂ system calculations^[6]. The pH values were compared providing an equation to relate measured pH with a corrected pH consistent with the most reliable variables (pCO₂ and TA). The same program was used to calculate the water pCO₂ from the corrected pH, as the two pCO₂ values (measured and calculated) were compared to validate the method^[7]. Once validated, the program was applied to calculate the TA for every minute along the cycle.

For the water-air CO₂ fluxes, separate experiments were conducted. The fluxes were determined using a dark custom-built floating chamber (1800 cm²; 16200 cm³) connected to a LI-COR LI-830 CO₂ Gas Analyzer^[4]. The air pCO₂ inside the chamber was registered every minute during 10 minutes. The experiment was also accompanied by water sampling (for TA) and physical-chemical parameters monitoring (pH, temperature and salinity). The measured pH values were corrected using the equation obtained previously. This way it was possible to derive a precise estimation for the water pCO₂ during the chamber measurements, calculated from the corrected pH and TA. At last, the fluxes and K₆₀₀ were calculated according to equations described by Wanninkhof et al (2014)^[8].

Results and Discussion

The DIC along the 25 hours cycle was strongly related with the tide. The highest values of both total alkalinity and water pCO₂ occurred at low tide (Figure 1). During the first low tide pCO₂ reached values around 6500 ppm and 8000 ppm at the second. At high tide pCO₂ values were much lower, reaching 1370 ppm. The TA showed a similar behavior, also reaching the highest measured (3.493 mmol.Kg^-1) and calculated (3.515 mmol.Kg^-1) values at low tide, which is about 1.48 to 1.58 times higher the typical values observed in ocean waters. At high tides TA oscillated around 2.3 mmol.Kg^-1 (the same as seawater). The pH also varied according to the tide. At low tides the water was less basic, reaching pH values around 7.3 during the first low tide and 7.2 at the second. This acidification is fully explained by the concentration of carbonic acid and the carbonate buffering capacity. At high tide, pH reached values around 7,7. Salinity barely varied, ranging between 35 to 37 PSU during the cycle. Beside the small range, the highest and the lowest values were observed respectively at high and low tides. The salinity was slightly lower at the second low tide, highlighting the peak of the spring tide, which is corroborated by pCO₂ and pH data.

Regarding the method validation, the measured and calculated pCO₂ values showed a good correlation, with an angular coefficient of 0.9762 and a R² of 0.9915. From the flux experiments it was found an average CO₂ flux of 12,50 ± 8,28 mmol.m^-2.h^-1 and an average K₆₀₀ value of 18,80 ± 8,06 cm.h^-1. The studied creek is wide, has abundant wind and strong currents between the tides, which explains the high gas transfer velocities observed. With such high K₆₀₀ values it’s likely that most of dissolved CO₂ is lost to the atmosphere before reaching the ocean.

Figure 1. Carbonate system behavior during a spring tide cycle in a mangrove creek part of Marapanim river estuary (Pará, Brazil).

Conclusions

The results reinforce the hypothesis of a pronounced carbon lateral export in the form of DIC driven the flooding cycles inside the mangrove. The peak of total alkalinity and water pCO₂ during low tide reveals the occurrence of tidal pumping of mangrove porewaters DIC to the creek. Despite the high pCO₂ values, the high K₆₀₀ and fluxes observed and the much lower pCO₂ at high tide, indicate that dissolved CO₂ rapidly escapes to the atmosphere. In contrast, Amazonian mangrove proved to be a significant source of alkalinity export to the ocean.

Acknowledgements

The authors thank the Centre National de la Recherche Scientifique (CNRS) responsible for financing TROPECOS project^[9], which this work is part of.