Protozoan population densities in the sixth-order Ogeechee River and fourth-order Black Creek ranged from 6 x lo4 to 11 x 10" flagellates liter-l and 0 to 1.5 x lo5 ciliates liter-' over a 12-month period. These numbers approximate those reported for various marine and freshwater habitats. A microcosm study with Ogeechee River water showed a density increase from 4 x lo5 to 9 x lo6 flagellates liter -I and 7 x 10) to 4 x lo5 ciliates liter-' in 2 d, representing an estimated net protozoan production of 600 pg C liter -I d-l. This production was mainly attributed to bacterial growth on DOC and subsequent grazing by the protozoans. Although problems always accompany container studies, this indicates an immense potential for protozoan production in the river. Daily transport of protozoan carbon ranged from 1.08 to 1,360 g, and annual estimated transport downstream was 60 t of protozoan carbon in the Ogeechee River. Densities of filter-feeding macroinvertebrates are very high in this river. Many of the filter-feeding species are known to feed primarily on amorphous material in the seston, within which protozoans consitute about 4% by carbon content. Since protozoans can be assimilated at relatively high efficiency (-50%), they appear to bc a trophic link between the metazoans and otherwise unavailable carbon sources in blackwater rivers.
As part of a study examining the trophic base of secondary production in the Ogeechee River, Georgia, bacterial grazing by sestonic protists was measured directly with fluorescently labeled bacteria. For the period April 1988 through September 1989, flagellates and ciliates ingested an average of 18.6 (range, 1.7-43) and 314 (range, 25-1,140) bacteria individuall' h-'. These grazing rates are comparable to those of protists in marine and lentic habitats. When combined with bacterial and protist densities in the river, an average of 15.6% (range, 0.2-590/o) of the water column was cleared by protists per day. On the basis of literature values of microbial C content and assimilation efficiencies, this percentage translates to a production rate of 11.7 fig C liter-' d-l. Protist bacterivory can facilitate carbon flow to higher trophic levels by turning bacteria into larger parcels of carbon and nutrients that are then available to a greater portion of the filter-feeding community. The microbial loop in this lotic system differs from pelagic systems; bacteria are derived from external sources, and bacteria and protists are directly consumed by macroinvertebrates.
SUMMARY
1. The role of suspended organic aggregates in aquatic systems has received much attention, but their origin and function vary between environments and conditions.
2. In the Ogeechee River (Georgia, U.S.A.) particles are constructed of clay, organic debris, and microorganisms in a matrix of mucopolysaccharide fibrils. Although these fibrils are probably of bacterial origin, microcosm studies suggest that synthesis of particles from dissolved substances can occur in the absence of microbial processing.
3. Processing of organic material and production of amorphous aggregates occurs through a series of microbial events and follows a well‐defined successional cycle that results in a high concentration of protists in sestonic particles.
4. Chemical analyses show that the relatively high protein and fat content of this amorphous material is a valuable food resource for filter‐feeding macroinvertebrates that are dense in this system.
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