Tidal and seasonal variation in carbonate chemistry, pH and salinity, for a mineral-acidified tropical estuarine system

Highlights

• This paper describes tidal fluxes and seasonal patterns in pH, salinity and carbonate chemistry attributes for a tropical Southeast Asian estuary, when heterotrophically generated CO2 and mineral acidification (through Acid Sulphate Soil (ASS) discharge) are likely to be of greater or lesser importance.
• Heavy daily downpours had little effect on the estuarine water salinity and pH, whereas accumulative rainfall (and anticipated elevated ASS discharge) during the monsoon period distinctly lowered these parameters baselines across the system.
• Remarkably low pH (relative to salinity), extraordinary pCO2 super-saturation, and carbonate under-saturation occurred extensively across our study system most of the time.
• In addition to modifying pH, mineral acidification is implicated in shifting the carbonate equilibrium and elevating the already high pCO2 levels.

Abstract

Estuarine acidification and carbonate chemistry derive from multiple biogeochemical processes. Other than biogenic CO2−-acidification, estuaries can be acidified allochthonously through non-carbonate sources originating in freshwater and land ecosystems. The present study considered the carbonate chemistry of a nutrified, turbid, tropical mangrove estuary, influenced by acidic groundwater discharge from pyritic soils (Acid Sulphate Soils, ASS). We studied the spatial and temporal variation of the surface water pH, salinity, total alkalinity (TA), partial pressure carbon dioxide (pCO2), dissolved inorganic carbon (DIC), and calcite (Ωcal) and aragonite (Ωara) saturation in the Brunei Estuarine System (BES), Borneo, Southeast Asia. pH and salinity for tidal to seasonal timeframes were determined from data collected half-hourly, logged at three stations (upper, middle and lower estuary); these data were correlated with rainfall incidence and intensity. Carbonate parameters were calculated from TA using discrete samples collected from six stations. pH (6.8–7.9) and salinity (4.2–28.2) increased expectedly seawards, due to tidal forcing and freshwater dilution at opposite ends of the estuary; amplitudes within a tidal cycle became expanded landwards and during spring tides. While the overall effect of heavy daily downpours on estuarine salinity and pH was muted, cumulative rainfall during the monsoon season distinctly lowered parameter baselines; the response was again more pronounced in the upper estuary. In the mid-to-upper estuary, we observed a remarkably low pH relative to salinity, extraordinary pCO2super-saturation (13031 ± 4412 μatm) and carbonate undersaturation (Ωcal and Ωara were 0.006-1.431 and 0.004-0.928, respectively). Although the relative contributions of heterotrophic metabolism and ASS-discharge to the estuarine pH and pCO2 were not determined, both processes are implicated in increasing both acidity and CO2 levels. This study contributes to the understanding of carbonate fluxes in mineral-acidified estuaries.

Proum S., Santos J. H., Lim L. H. & Marshall D. J., in press. Tidal and seasonal variation in carbonate chemistry, pH and salinity, for a mineral-acidified tropical estuarine system. Regional Studies in Marine Science. Article (subscription required).

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