Microbial colonization of seston and free bacteria in an impounded river

Abstract

A headwater river-reservoir system was studied from March–November, 1982 to determine effects of impoundment on seston quality. We used epifluorescent microscopy to compate and contrast microbial colonization of seston and abundance of free microbes. There was a significant relationship between colonization density and particle size at all sites. Smaller particles were colonized more densely by bacteria than larger particles. Total counts of bacteria (free plus attached) did not differ significantly beween sites. However there were significantly more free bacteria (# ⋅ ml−1) in the reservoir and 5 km below the dam than upstream of the impoundment. Density of attached bacteria (# ⋅ µm−2) was similar upstream and downstream of the reservoir but slightly higher at lentic sites. Proportionally, attached bacteria were more important upstream (mean 42–45% of total counts) than in the reservoir and downstream (19–31%). We concluded that reservoir seston was of higher quality than riverine seston and major downstream effects included decreases in seston concentration (due to sedimentation) and shifts in proportional abundance of free and attached bacteria. Seston consumers capable of using ultrafine particles (<25 µm), including free bacteria, had higher quality food resources below the dam than in upstream areas.