Pelagic bacteria-particle interactions and community-specific growth ratesin four lakes along a trophic gradient

The relationships between bacterial concentration, bacterial production, an d cell-specific activity of both free and attached bacteria and environment al factors such as suspended solids, nutrients, and temperature were examin ed in four lakes, two in New Zealand and two in Switzerland. Estimates of c ell-specific production were obtained by microautoradiographic counts of [H -3]thymidine-labeled cells. Bacteria attached to particles accounted for on ly 1.3 to 11.6% of the total bacterial abundance, but showed overall 20-fol d higher specific growth rates and were relatively more active than their f ree counterparts. On average, 80 to 100% of epibacteria were attached to or ganic particles. The abundance and production of free and attached bacteria were positively correlated; however, relationships between these fractions and some environmental variables differed. Cell-specific activities of act ive bacteria were not equivalent to mean cellular activities of the entire bacterial community and differed in their relationship to trophic state. [H -3]Thymidine-positive bacteria were more tightly linked to chlorophyll a th an were total bacteria. Our findings indicate that production by attached b acteria, fueled by phytoplankton carbon, supplies "new" free bacteria to th e bacterial community. Our results support the idea that particulate organi c matter acts as a source of dissolved nutrients to free bacteria. Bottom-u p control of bacterial biomass, as shown by regressions of biomass vs produ ction, appeared to be stronger in two ultraoligotrophic lakes than in two m ore eutrophic ones.