Production of epiphytic bacteria and bacterioplankton in three shallow lakes

Secondary production of epiphytic bacteria and bacterioplankton was measured from May to September in three temperate. shallow and eutrophic Danish lakes. The biomass of bacteria attached to the submerged angiosperms Ceratophyllum demersum and Potamogeton pectinatus averaged 4.3 and 3.0 mu g C cm(-2) and the production measured by thymidine incorporation averaged 0.28 and 0.40 mu g C cm(-2) h(-1). In Lake Dystrup. Ceratophyllum demersum covers about 95% of the lake area and the epiphytic bacterial production (thymidine method) per lake surface area from mid-July to early September aas twice the production of the bacterioplankton, With the leucine method the epiphytic bacterial production was seven-fold higher than the bacterioplankton production. A 20% area cover by Potamogeton pectinatus in Lake Stigsholm resulted in an epiphytic bacterial production equal to the bacterioplankton. The biofilms on macrophyte surfaces are "hotspots" with a bacterial per volume production 1000 times the bacterioplankton. Furthermore, the growth rates of the attached bacteria were from two to four times faster than in the bacterioplankton communities. A comparison of the twin lakes Dystrup and Ramten, respectively with and without submerged vegetation, showed bacterioplankton production of equal magnitude despite four-fold higher phytoplankton and bacterioplankton biomasses in Ramten. Higher bacterial growth rates in Dystrup almost compensated For the lower biomass. The total area production of the attached and Free living bacteria in Dystrup was three to five times the total area production in Ramten. It is suggested that the higher bacterioplankton growth rates in vegetated areas may be explained both by an enhanced grazing pressure and by more favorable growth conditions due to exudation of DOC From the macrophytes.