The regulation of surface water pCO2 was studied in a set of 33 unproductive boreal lakes of different humic content, situated along a latitudinal gradient (57°N to 64°N) in Sweden. The lakes were sampled four times during one year, and analyzed on a wide variety of water chemistry parameters. With only one exception, all lakes were supersaturated with CO2 with respect to the atmosphere at all sampling occasions. pCO2 was closely related to the DOC concentration in lakes, which in turn was mainly regulated by catchment characteristics. This pattern was similar along the latitudinal gradient and at different seasons of the year, indicating that it is valid for a variety of climatic conditions within the boreal forest zone. We suggest that landscape characteristics determine the accumulation and subsequent supply of allochthonous organic matter from boreal catchments to lakes, which in turn results in boreal lakes becoming net sources of atmospheric CO2.

We calculated the carbon loss (mineralization plus sedimentation) and net CO2 escape to the atmosphere for 79 536 lakes and total running water in 21 major Scandinavian catchments (size range 437–48 263 km2). Between 30% and 80% of the total organic carbon that entered the freshwater ecosystems was lost in lakes. Mineralization in lakes and subsequent CO2 emission to the atmosphere was by far the most important carbon loss process. The withdrawal capacity of lakes on the catchment scale was closely correlated to the mean residence time of surface water in the catchment, and to some extent to the annual mean temperature represented by latitude. This result implies that variation of the hydrology can be a more important determinant of CO2 emission from lakes than temperature fluctuations. Mineralization of terrestrially derived organic carbon in lakes is an important regulator of organic carbon export to the sea and may affect the net exchange of CO2 between the atmosphere and the boreal landscape.

No abstract

Abstract-We investigated the importance of a regional factor for bacterial communities in lakes. External factors dominated the control of community structures in lakes with retention times up to 200 d, most likely as a result of bacterial import. Because these lakes are numerous in the boreal zone, regional processes can be of great importance for bacterial communities in general. Consequently, we propose that lakes function more like flow through systems, as opposed to the classical ''lake as microcosm '' concept. During the last decade, our knowledge about the diversity of bacteria in nature has increased enormously; however, little is understood about which factors are shaping bacterial communities (DeLong and Pace 2001;Zwart et al. 2002). Central to understanding patterns in community structure is knowledge about the relative importance of local versus regional processes (Chase 2003;Cottenie and De Meester 2004). It can be assumed that the degree of isolation, and thereby the rate of exchange of cells and genes between communities, should have consequences for which forces shape local microbial communities (Curtis and Sloan 2004;Papke and Ward 2004). In nature, a gradient in isolation of communities should exist from endosymbionts to those exhibiting a cosmopolitan distribution (Papke and Ward 2004).Early on, lakes and their ecosystems were assigned as isolated units in the ''lake as microcosm '' concept (Forbes 1887). This view of the lake was later revised, for instance, because of the discovery of the importance of allochthonous carbon for lake ecosystem function (Hessen and Tranvik 1998;Pace et al. 2004). Thus, lakes today are more often ecologically regarded as a part of a larger unit, i.e., the drainage basin (Soranno et al. 1999). Still, efforts in lake microbial ecology and diversity have largely focused on within-lake selective forces, rather than external influences on community structure and diversity, although it has been shown that lake (Crump et al. 2003;Masin et al. 2003; Lindström and Bergström 2004) and estuarine (Crump et al. 2004) community compositions can be largely influenced by inflowing bacteria. It can be assumed that the degree of isolation a lake bacterioplankton community experiences, and thereby the degree of influence by inflowing bacteria on local community structure, depends on the hydrological retention time of the lakes. Therefore, we used a range of lakes of different hydrological retention times to determine the magnitude of external influence on bacterial community structure in lakes. Our hypothesis was that lakes with short hydrological retention time should have bacterial communities that are more similar to those of the inflowing water than lakes with longer hydrological retention times because of the larger amount of imported bacteria to the former type of lake. Twelve relatively unproductive Swedish lakes with different hydrological retention times were studied (Table 1). We used denaturing gel electrophoresis (DGGE) of polymerase chain reaction (PCR)-amplified...

Summary 1. The aim of this study was to assess the effects of different nitrogen (N) to phosphorus (P) loading ratios on phytoplankton community composition and primary production in a naturally eutrophic lake. Furthermore, the sources of N fuelling primary production were estimated using 15N stable isotope tracers. 2. A mesocosm experiment was performed with the same amount of P added to all mesocosms (similar to internal loading rates) but with a range of N additions (0–86 μm N), resulting in a gradient of N : P supply ratios. 3. Low N : P supply ratios resulted in a significant shift in the phytoplankton assemblage to a community dominated by N‐fixing cyanobacteria and a supply of atmospheric N2 estimated to be up to 60% of total supply. 4. The N : P loading ratio had no significant effect on primary production, total nitrogen (TN) concentration or particulate N concentration. 5. Our results imply that a reduced N : P ratio of the nutrient load does not necessarily result in a lower TN concentration and downstream N export due to compensation by N‐fixing cyanobacteria.