Biogeochemical, physical and anthropogenic transformations in the Daugava River estuary and plume, and the open Gulf of Riga (Baltic Sea) indicated by major and trace elements

This work was based on suspended major (N, P, Al, Fe, Mn, Ca) and trace (Cr, Ni, Cu, Zn, Cd, Pb) element analyses, and is focussed on the trace metal biogeochemical transitions and transport at the land–sea interface. Samples were collected in the Daugava River and the Gulf of Riga during nine cruises in 2001–2004. Biogenic material contributed to the metal transfer between the phases in the Daugava estuary, and the trophogenic layer and also depths of the Gulf of Riga. Input from a terrestrial source to the particulate organic matter pool and supposedly to the surface-related reactions in the study area was suggested. Flocculation of clay colloids, Fe and Mn hydroxyoxides, and also precipitation of Ca carbonates seemed to provide supplemental inorganic sorption surfaces to the clay suspended minerals, which were major inorganic conveyers for trace metals. Depending on the Daugava discharge regimes, clay-related trace metal behaviour gave diverse phase-exchange signals for seasons and also for individual elements. Mn hydroxyoxides at elevated concentrations were suggested to be efficient phosphate sorbents competitive to Fe hydroxyoxides. Apparently, the in-gulf processes substantially transform the size and geochemical structure of the river-borne suspended particle population. Enrichment factors indicate that the human-made trace metal pollution in the Daugava and at the surface of the Gulf is highest for Cd, Pb and Zn. Compared to the average values for world rivers, the Daugava estuary and plume also show higher contamination by Cd, Pb and Zn, but similar or lower contents of Cu, Ni and Cr.