Protozoans in two southeastern blackwater rivers and their importance to trophic transfer

Protozoan population densities in the sixth#x2010;order Ogeechee River and fourth#x2010;order Black Creek ranged from 6 × 10 4 to 11 × 10 6 flagellates liter −1 and 0 to 1.5 × 10 5 ciliates liter −1 over a 12#x2010;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 × 10 5 to 9 × 10 6 flagellates liter −1 and 7 × 10 3 to 4 × 10 5 ciliates liter −1 in 2 d, representing an estimated net protozoan production of 600 µ g C liter −1 d −1 . 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#x2010;feeding macroinvertebrates are very high in this river. Many of the filter#x2010;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 be a trophic link between the metazoans and otherwise unavailable carbon sources in blackwater rivers.