Different silicon forms sinalize an input of urbanized river and indicate the presence of phytoplankton with silicon structures in the tropical coastal area of Recife (PE-Brazil)

Abstract

The urbanized areas mark its presence in the hydrological systems with some biogeochemical signs including silicon inputs. Natural influence of leaching and erosion is done by silicate contribution and sometimes, the agriculturally soil contains microphytoliths and contributes with biogenic silica. Once in the hydrological system, the silicon can integrate the biogeochemical cycles showing a balance or not between inorganic and biological forms that reveals a tendency of processes that occur in each sector of the estuarine-marine interface. This study aims to evaluate the impact of a heavily polluted/urbanized river, whose input can be evidenced by the contribution of different forms of silicon that can evidence the terrestrial contribution, erosion process, the presence of phytoplankton species with siliceous structures, mainly associated to diatoms and also, to study the river plume spreading on the coastal area by undertaking silicon profiles in the region adjacent to the river mouth. The coastal region of Recife is under influence of fluvial inputs from Capibaribe and Beberibe rivers verified by observed low salinity (min. 33.93). The distribution of silicon forms showed sectorial variation, and the maximum values were dissimilar. Dissolved silicate reached 14.40 µmol L-1, lithogenic silicon (LSi) maximum of 25.76 µmol L-1 and biogenic silica (BSi) reaching 7.39 µmol L-1 near the river mouth toward the sea, evidencing the river influence coming from the west, achieving the port barrier at the end and propagating in the sea toward northeast. Due to higher nutrient availability in the stations nearest to the coast, primary production was encouraged with evidencing by high values of chlorophyll-a associated to silicate (r=0.71) and to BSi (r=0.44). In the stations more distant to the coast, the forms of silicon showed very low concentrations and BSi/Chlorophyll-a showed negative correlation associated with chlorophyll-a concentrations evidencing the participation of non-silicon actors in the primary production in medium depth in oligotrophic tropical waters.

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

“…Chlorophyll-a concentrations showed significant negative correlations with temperature and salinity, while positive correlations were observed with dissolved silicate and BSi in a study by Bastos and Braga (2018), confirming the link with siliceous organisms. Suspended particulate matter (SPM) exhibited significant correlations with silicate, BSi, and LSi, indicating their association.…”

“…For instance, Braga et al (2004) reported a benthic dissolved silicate anomaly (~4,000 m depth) in the Congo margin, indicating the transfer of Si from land to the ocean through the Congo River Canyon. Although studies in coastal tropical areas are scarce, Bastos and Braga (2018) observed dissolved silicate concentrations of up to 14.40 µmol L -1 , maximum LSi of 25.76 µmol L -1 , and BSi concentrations of 7.39 µmol L -1 in the coastal area of Recife, influenced by fluvial inputs from the Capibaribe and Beberibe rivers.…”