Long-term trophic state responses of a large tropical coastal lagoon to land use changes and nutrient transport

Abstract

The watershed of a large tropical coastal lagoon, historically hosting large sugarcanecrops, which has been reduced during the last decades. It suggests that land use andland cover changes (LULCC) would lower the trophic state of the coastal lagoon dueto less fertilization application and consequently reducing nutrient transfer into thelagoon. In this context, this study assessed the influence of LULCC, represented bythe reduction of the sugarcane crops on the trophic state of a large tropical coastallagoon. This influence was assessed by means of information about the long-termoverall mass-balance budget, involving the estimation of the N and P loads from thesugarcane crops, other anthropogenic sources and natural processes; measurementsof the river N and P fluxes into the lagoon; its seaward outflow and the long-termlacustrine nutrient retention. The results indicate that sugarcane crop reductionrepresented a decrease of 686 t of N and 51 t of P being emitted to the lagoonwatershed. However, N and P loads from sugarcane crops were partially replaced byother anthropogenic activities, but even with the reduction of the nutrient fluxes, thelong-term supereutrophic responses of the coastal lagoon was maintained. Thesediment-related C/N and isotopic composition of the lagoon was sensitive toLULCC, indicating changes on the organic matter inputs into the lagoon. The trophicstate was also influenced by the soil N and P accumulation and the long-termnutrient retention within the lagoon imposed by its restricted water connection tothe sea.