Anthropogenic influence on silicon behavior in an estuary member of the Biosphere Reserve in Southeastern Brazil

Abstract

The Cananéia-Iguape estuarine-lagoon complex (CIELC), located in the State of São Paulo, Brazil, is considered an area of the Biosphere Reserve. However, an artificial channel built in the northern sector of the system (Iguape) has promoted an unnatural input of fresh water into the system, leading to enhanced drainage of terrestrial material into the estuary and influencing silicon biogeochemistry within the complex. This study used the distribution of different fractions of silicon along the system as a proxy for the freshwater input. The samples were collected during August, 2012 (winter) and February 2013 (summer) and analyzed for dissolved silicate, biogenic silica, and lithogenic silica. Dissolved silicate reached values above 200 µmol L -1 in the two seasonal periods, indicating the constant supply of terrestrial material in the northern sector of the system associated with fresh water. Southward, where the system is more preserved (Cananéia-Ararapira), the concentration decreased dramatically. Lithogenic silica (LSi) reached extremely high values of ~700 µmol L -1. Biogenic silica (BSi) showed concentrations up to 120 µmol L -1 with distribution more dependent on assimilation processes and the nature of the biological communities than on the source of fresh water. The availability of different forms of Si revealed remineralization, deposition, adsorption, and uptake mechanisms with different dynamics in the northern and southern sectors of the system, evidencing the environmental impact by the Valo Grande Channel.