ABSTRACT

Bow Lake is a small alpine lake in western Alberta, Canada, mainly fed by glacial meltwater. Suspended sediment loads, high only during summer months, are transported through the lake dominantly by interflows within the epilimnion. Consideration of interflow velocities and particle settling rates suggests that most clay supplied by stream inflow should be transported out of the lake during summer when thermal stratification is best developed. Field observations, however, show that clay comprises a persistent and significant component of lake-bottom sediment deposited during summer.

This apparently anomalous clay-settling behavior can be attributed to production of fecal pellets found in both suspended sediment traps and bottom surficial deposits. The pellets, probably produced by pelagic copepods, are ovoidal with lengths typically ranging between 100 and 250 µm and are composed of clay and silt. Measured settling rates in water at 5°C ranged between 80 and 400 m/day.

Pellet production, then, accelerates the downward transfer of suspended detritus, resulting in deposition of much of the fine sediment that would otherwise escape the lake in interflows. This confuses patterns of bottom-sediment texture and mineralogy which would ordinarily be expected from settling behavior of unpelletized suspensates.