Benthic foraminifera and pore water carbonate chemistry on a tidal flat and salt marsh at Ria Formosa, Algarve, Portugal

Highlights • Foraminifera and halophytes showed a relationship with pore water properties. • Soil salinity and evaporation are the governing environmental factors. • Agglutinated foraminifera were rather related to pore water pCO2 than to submergence time or elevation. • Calcareous foraminifera specialised to tolerate carbonate-corrosive conditions prevailed at lowest saturation levels. Benthic foraminifera showed a vertical zonation in tidally influenced salt marshes, which has been used for sea level reconstructions. Growing evidence suggested that freshwater influx, salinity, or the pH of interstitial waters has also an impact on the foraminiferal distribution. A tidal flat and salt marsh transect was investigated in the north-western Ria Formosa coastal lagoon, Algarve, Portugal, to constrain the relationship of benthic foraminifera, halophytes, and pore water properties. The dominance of saltworts from the subfamily Salicornioideae and landward increasing soil salinities depicted evaporation as governing environmental factor. The carbonate chemistry from lagoonal and pore waters identified anoxic tidal flat sediments of as main source of total alkalinity. The alkalinity was lower in the salt marsh, where the pCO2 was extremely high. Salt marsh pore waters showed a high variability of carbonate system parameters, which mirrored small-scale spatial heterogeneities in the soil. The distribution of textulariid salt marsh foraminifera was confined to the vegetated zones, where their abundance increased with elevation. Calcareous species were frequent on the tidal flat and in the highest salt marsh. Many of them were specialised to high salinities or to extreme and variable environmental conditions. Two levels of faunal change in the salt marsh coincide with vegetation zonal boundaries, mean tide or mean high water levels. The two other faunal changes were related to changes in calcite saturation state or organic carbon concentrations. The proportion of textulariids showed a negative correlation with submergence time or elevation, and a significant correlation with pore water pCO2. The faunal distribution, pore water calcite saturation, and Ammonia dissolution patterns indicated that calcareous species specialised to tolerate carbonate-corrosive conditions prevailed even at lowest saturation levels