Transformation of terrestrial dissolved and colloidal material into suspended particles in estuaries is a critical process that impacts the global biogeochemical cycles of carbon and trace metals. These flocculation and aggregation processes occur due to the increasing salinity and pH towards the open sea. The role of pH is particularly relevant in the context of acid sulphate soils that can generate extremely low pH conditions and high soluble trace metal concentrations in river waters. We examined the flocculation dynamics with a series of experiments where natural river water draining acid sulphate soils was mixed with artificial seawater to simulate freshwater-seawater gradient in estuaries. The study focused on the changes in particle size distribution, optical properties of dissolved organic matter and behavior major metal elements. The humic-like DOM fluorescence showed inverse relationships with concentrations of organic-bound particulate Al and Fe, and particulate organic carbon, indicating co-precipitation of humic-like organic matter and dissolved metals. We also identified two major, distinct particle pools pertinent to the flocculation process: the finer classes (9-11 µm) showed low initial volume concentrations until salinity 1, while the coarsest classes (>150 µm) showed a strong initial increase that leveled off and turned to decrease at salinity 2. Our findings underline the importance of low-salinity flocculation and aggregation processes as crucial part of the coastal filter, regulating the transport of organic matter and metal elements from land to sea.

Primary Presenter: Eero Asmala, Geological Survey of Finland (eero.asmala@gtk.fi)

Authors:

Peter Österholm, Åbo Akademi  (Peter.Osterholm@abo.fi)

Joonas Virtasalo, Geological Survey of Finland  (joonas.virtasalo@gtk.fi)