Humic substances cycling in a tropical oxbow lagoon (São Paulo, Brazil)

This study aimed at describing the cycling of humic substances (HS) in a tropical oxbow lagoon (São Paulo, Brazil). Humic substances were obtained from Oxycaryum cubense leachate submitted to degradation and fractionated into fulvic acid (FA) and humic acid (HA). Incubations were prepared using FA and HA and water from Infernão lagoon and maintained under high dissolved oxygen (HDO) and low dissolved oxygen (LDO) conditions at 16 °C, 22 °C and 26 °C. FA and HA decay was monitored (on a carbon basis) for 40 days. The results were fitted to first-order kinetics model and the effect of temperature and oxygen availability on mineralization of FA and HA was described and used on simulations of humic substance cycling. The simulations allowed us to infer that in the Infernão lagoon HS mineralization is faster under LDO than under HDO conditions, with higher mineralization occurring in February (summer) and HA presented lower mineralization coefficients. For the Infernão lagoon, HS decay is a slow process and these compounds are expected to be extensively incorporated into the sediments of this environment.