Abstract:
Suspended particulate matter (SPM) in the Columbia River is a mixture of particles of
several origins having varying physical and biogeochemical properties. The relative
abundances of these freshwater particles changes with season and apparently also with
tide. Prior investigation has quantified seasonal variation of organic material in both the
Columbia and Willamette rivers. In this investigation, seasonal variability in the abundace
of manganese in the mineral fraction (Mn/Al) is confirmed in both rivers. Mn/Al hovers
around 0.01 (crustal average = 0.011, Whetten et al., 1969) during the winter and early
spring, rising in the spring and summer to 0.04 and 0.06 in the Columbia and Willamette
Rivers, respectively. Results from particle settling experiments and electron microprobe
photos of SPM indicate that seasonal Mn/Al variability is a result of addition of Mn-oxides
to SPM in the summer months, and that these oxides exist as discrete, slow settling
particles.
A short-term (12 hour) variability of Mn/Al, as well as Chl a/POC, is observed at mid-depth
in the Columbia River. Owen Tube particle settling experiments revealed that with
respect to geochemical properties, SPM could be divided into two settling classes: slow
(<0.11 cm/s) and fast (>0.11 cm/s). Particles in the slow settling class were typically
enriched in Mn and Chl a , while fast settling particles were more characteristic of detrital
material. One consequence of this geochemical separation is that it leads to differential
particle settling and resuspension in the tidal freshwater portions of the river, resulting in
the observed 12-hour variability of SPM character. Such variability could have further
reaching consequences with regard to chemical transport and dispersal.
