Sestonic bacteria as a food source for filtering invertebrates in two southeastern blackwater rivers1

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Abstract
Bacterial cell concentrations and biomass within seven size classes of seston in the Ogeechee River and Black Creek, two blackwater rivers in southeastern Georgia, were measured for 19 months. The proportion of sestonic carbon in bacteria was determined to assess the relative food. quality of the various fractions for filtering invertebrates. Cell concentrations were unusually high in both rivers, with values ranging from 2.03 × 109 to 1.10 × 1011 total cells liter−1 with a mean of 1.48 × 1010. Concentrations of attached cells varied from 1.20 × 108 to 1.70 × 109 cells liter−1. Most cells were small, coccoid forms not attached to particles. Mean mass per cell in the Ogeechee River was 1.86 × 10−14 g C and in Black Creek 8.28 × 10−15. Total bacterial biomass was also high, ranging from 19.1 to 1,105.0 mg C m−3. Most of the bacterial biomass was in the particle size range from 0.30 to 12 µm. The bacterial carbon composition of the seston was 12 µm. Percentages in the < 12‐µm fraction were much higher, ranging from 1.4 to 28.9% in Black Creek and 15.5 to 99.9% in the Ogeechee River. Cell concentrations and biomass varied with river discharge and were higher during periods of floodplain inundation. The high bacterial content of the smallest fraction of seston may be an important food source for filter‐feeding invertebrates such as blackflies and chironomids, which are abundant in these rivers.