1. Five oligotrophic clear-water lakes on the Faroe Islands were studied during August 2000. Algal and bacterial production rates, community respiration, and CO₂ saturation were determined. In addition, we examined the plankton community composition (phytoplankton and heterotrophic nanoflagellates) and measured the grazing pressure exerted by common mixotrophic species on bacteria.

2. High respiration to primary production (6.6-33.2) and supersaturation of CO₂ (830-2140 µatm) implied that the lakes were net heterotrophic and that the pelagic heterotrophic plankton were subsidised by allochthonous organic carbon. However, in spite of the apparent high level of net heterotrophy, primary production exceeded bacterial production and the food base for higher trophic levels appeared to be mainly autotrophic.

3. We suggest that the observed net heterotrophy in these lakes was a result of the oligotrophic conditions and hence low primary production in combination with an input of allochthonous C with a relatively high availability.

4. Mixotrophic phytoplankton (Cryptomonas spp., Dinobryon spp. and flagellates cf. Ochromonas spp.) constituted a large percentage of the plankton community (17-83%), possibly as a result of their capacity to exploit bacteria as a means of acquiring nutrients in these nutrient poor systems.