Organic matter composition of gravel-stored sediments from salmon-bearing streams

Abstract

The objective of this project was to evaluate the changing composition and structure of the sediment-associated organic matter (OM) stored in the gravel bed of highly productive salmon-bearing streams and, determine if the OM changes affect the morphology and settling rates of the sediment. In July of 2001, a dozen infiltration gravel bags were buried in the channel bed of O'Ne-eil Creek in northern British Columbia (Canada) to collect fine sediment and the associated organic matter for chemical and morphological analysis. The bags were removed over a 10 week period which incorporated summer low flows, salmon spawning, salmon die-off and the onset of autumn low flow conditions. Our results indicate two visibly different structures in the organic matter film overlying the mineral material of the flocs. A web-like structure was noted during mid-spawn while a film-like covering was observed in pre-spawn and post-fish periods. The strength of the film-like covering is surmised to be associated with the larger gravel-stored floc sizes noted at these times. Chemical analysis of these biofilms indicated higher metal complexation properties during the spawning periods as opposed to before or after salmon were present. The changing OM contributions were associated with changes in floc size, density and settling rates. The physical disturbance to the gravels associated with spawning salmon was also correlated with altered characteristics of the gravel-stored flocs.