Factors influencing the variability of Mg, Ca and K in waters of a mangrove creek in Braganca, North Brazil

Abstract

Sodium, potassium, magnesium, calcium and physico‐chemical parameters were monitored for one year in a mangrove tidal creek near Braganca, North Brazil, to determine their tidally induced and seasonal variations and the main parameters controlling the concentration of these cations. On a daily basis, cation concentrations showed a strong tidal rhythm due to the mixing of estuarine and mangrove waters. Mean concentrations were highest at the end of the dry season (December 1996) and lowest (April 1997) towards the end of the rainy season. Average values over one year were: Na+ = 329± 118mM, K+ = 6.9±2.5 mM, Mg++=37 ±14 mM and Ca++= 6.9±2.4 mM. Dissolved oxygen concentration was higher during the dry season due to enhanced aquatic primary production, with a maximum daily average value of 8.5 mg/L in July 1996, and a minimum value of 4 mg/L in June 1997. Cation concentrations were transformed relating them to the respective average values in ‘standard’ seawater at salinity 35. Although cation concentrations and salinity tightly correlated, this standardization showed that the concentrations of K+, Ca++ and Mg++ did not depend solely on salinity and reflected the seasonal variation in aquatic primary production. It also allowed the discrimination of their sources (marine, riverine and groundwater). Thermodynamic equilibrium calculations indicate that phytoplankton may be regulating the concentration of these cations in the water column indirectly by inducing precipitation through pH increase and directly through metabolic uptake.