Colloid dynamics and transport of major elements through a boreal river - brackish bay mixing zone

A range of biogeochemical methodologies were applied to investigate how agg regation processes affected the phase distribution and mixing of Fe, Si, an d organic carbon between the Kalix River and the Bothnic Bay, northernmost Baltic Sea (salinity less than or equal to 3, the low-salinity zone (LSZ) w as stretching over 60 km in the spring). During the dynamic springflood con ditions studied, small U-238-Th-234 disequilibria, low sediment trap fluxes , laboratory mixing experiments, as well as results from an independent two -box, two-dimensional mixing model combine to suggest that no significant r emoval of Fe, Si, or organic C was occurring in the highly-resolved LSZ. Wh ile no conclusions may be drawn based solely on property-salinity plots ove r narrow salinity ranges, apparently linear graphs for Fe and Si over 3 sep arate years also suggest minimal removal in this regime. At the same time, size distributions both of elements - from cross-flow ultrafiltration - and of bulk suspended solids - from light scattering (photon correlation spect roscopy [PCS]) - indicated that significant aggregation was taking place. The aggregation-without-significant-settling scenario in this low-salinity mixing regime, with a geochemistry similar to that of neighboring Russian A rctic rivers, is hypothesized to result from a comparatively high organic-t o-detrital matter characteristic of the aggregates. While first principles would indeed suggest that decreasing electrostatic repulsion during mixing lead to aggregation, a low specific density of mineral-poor amorphous organ ic aggregates may lead to transport of these authigenic particles further a way from the river mouth. The role of detrital "sinkers" on vertical remova l of suspended organic matter is discussed in the wider context of scavengi ng mechanisms in the ocean. (C) 2000 Elsevier Science B.V. All rights reser ved.