Seasonal changes in metal and nutrient fluxes across the sediment-water interface in tropical mangrove creeks in the Amazon region

Abstract

Mangrove creeks are considered important routes between terrestrial and adjacent coastal waters regarding the transport of dissolved material to oceans. The present study assessed if Amazonian seasonal rainfall patterns affect the pore water biogeochemistry and the intensity and direction of nutrient (NH4+ and PO43−) and metal (Fe2+ and Mn2+) exchanges from intertidal creek mudflats fringed by pristine mangroves. The results indicate that mangrove-fringed mudflats are effective in retaining iron and nutrients in solid sediment phases compared to export to coastal waters, also potentially comprising a significant manganese contributor to coastal waters. However, nutrient and metal retention are lower during the wet season, as intense rainfall periods reduce pore water salinity and promote increased reducing sediment conditions. Such conditions enhance organic matter degradation and pore water NH4+, PO43−, Fe2+ and Mn2+ concentrations just below the sediment-water interface, generating higher effluxes during this period. Our findings demonstrate that seasonal variabilities drive substantial physicochemical property and pore water biogeochemistry changes, affecting the efficiency of mudflat sediments retaining and exporting nutrients and metals.