Net autotrophy in an oligotrophic lake rich in dissolved organic carbon and with high benthic primary production

Biomass and production of microbiota -primary producers as well as heterotrophic bacteria-were studied both in the pelagial and in the benthic habitat over 2 yr in the shallow oligotrophic Lake Eckarfjarden, Sweden. Both biomass and production of microbiota were concentrated in the benthic habitat. Despite a high dissolved organic carbon (DOC) concentration of about 25 mg C l(-1) in the water, the total bacterial production was lower than the total primary production. Moreover, measurements of DOC concentrations in the in- and outflow, and CO2-saturation measurements, indicate that the system is net autotrophic. Generally, low-productive systems (< 100 mu g C l(-1) d(-1)) tend to be net heterotrophic. In contrast, we found a low-productive (55 mu g C l(-1) d(-1)) but net autotrophic system, the conditions of which were largely influenced by benthic production. Many lakes in the world are shallow and may provide substantial benthic areas suitable for primary production. Hence, it is important to include this habitat when evaluating whether lakes are autotrophic or heterotrophic systems.