Sixteen subarctic lakes in northern Sweden were studied regarding bacterioplankton numbers and production (BP), phytoplankton primary production (PP), zooplankton carbon nutrition, and carbon dioxide saturation. An additional set of lakes were studied regarding the regional variation in the concentration of dissolved organic carbon (DOC, 75 lakes) and CO2 saturation (35 lakes). The lakes were situated along an altitude gradient from 270 to 1140 m a.s.l, corresponding to an air temperature gradient of approximately 5°C. The climatic differences between the lakes also reflected in the physical and chemical characteristics of the lake water. There were a general trend of decreasing summer mean lake water temperatures (range 1999: 4.9 to 14.0 °C), and DOC (1.9–9.0 mg L-1) and total nitrogen (62 to 420 micro-g L-1) concentrations with increasing altitude. The total phosphorus concentrations were also low (2.9 to 11.3 micro-g L-1), but showed no clear trend with altitude.

Both the PP (0.2 to 7.7 micro-g C L-1 d-1) and BP (0.4 to 7.7 micro-g C L-1 d-1) were low resulting in a PP:BP ratio that were below unity (0.1 to 4.0) in 10 of the 16 lakes. It was evident that, in a majority of the lakes, phytoplankton produced carbon could not support BP. Instead BP was supported by allochthonous carbon, which availability in relation to nutrients were high enough to uncouple bacteria from phytoplankton dependence and indirectly depress PP by monopolizing a large share of available nutrients (N, P) in the lake water. The crustacean zooplankton in the lakes contained carbon of both allochthonous and autochthonous origin. The relative share of allochthonous carbon in zooplankton was related to the proportions between the energy mobilization by bakterioplankton (based on allochthonous carbon) and phytoplankton available for zooplankton. Supersaturation of CO2 was found in most lakes across the entire DOC gradient, which was indicated to result from allochthonous DOC input and heterotrophic respiration in the lakes. Thus, several sets of data showed that allochthonous organic matter to a large extent was metabolized within the lakes, forming a heterotrophic base for secondary production and net heterotrophic lakes that were supersaturated with CO2.