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

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 12month 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. The protozoans of lotic ecosystems have received little attention. During the 196Os, extensive surveys of epibenthic stream protozoans addressed patterns of diversity and succession with great taxonomic detail (Cairns 1965, 197 1); however, protozoan population densities, biomass, or production were not estimated, and their importance in the trophic structure of rivers was not examined. Much of the ecological work on protozoans has been done in marine systems or on laboratory cultures of freshwater forms. Early work showing that much of the energy fixed by photosynthesis is used by protozoans (Pomeroy and Johannes 1968) led Pomeroy (1974) to suggest that, in addition to the classical algae-herbivore-carnivore food chain, there may be a “microbial loop” that funnels bacterial production back into the classical web via protozoans. Whether this microbial loop is truly a link or whether it acts as a sink, burning off carbon and Acknowledgments This research was supported by National Science Foundation grants BSR 84-0663 1 and BSR 87-05744 to J. L. Meyer. We are especially grateful to R. W. Sanders and R. T. Edwards for lab and field assistance. The manuscript benefited from the comments of K. G. Porter, L. R. Pomeroy, R. T. Edwards, G. M. Capriulo, and an anonymous reviewer. energy, hinges on the efficiency of the microorganisms, the number of microbial trophic levels involved, and their residence time in the water column. The question remains unanswered (Ducklow et al. 1986, 1987; Sherr et al. 1987). Before we can determine whether protozoans are a link between bacteria and higher trophic levels in stream ecosystems, we need to know their numbers and biomass in these environments. Here, the term “protozoan” includes Ciliophora (ciliates) and Sarcomastigophora (pigmented flagellates, colorless flagellates, and ameobae) and should not be taken to represent a true taxon (Lee et al. 1985). Our objective here was to quantify protozoan abundance in main-channel and backwater habitats of a blackwater river and examine the temporal variability on scales of 1 d, 2 weeks, and 1 yr. A microcosm experiment was also performed to estimate the potential protozoan production within a parcel of water as it flows downstream. Protozoan density and biomass from this river were compared to other, more frequently studied ecosystems. Our intent was to assess the potential importance of protozoans as a food resource for the abundant filter-feeding invertebrates in the river (Wallace et al. 1987; Benke and Meyer 1988)