Showing papers by "Lars J. Tranvik published in 2007"

Plumbing the Global Carbon Cycle: Integrating Inland Waters into the Terrestrial Carbon Budget

Abstract: Because freshwater covers such a small fraction of the Earth’s surface area, inland freshwater ecosystems (particularly lakes, rivers, and reservoirs) have rarely been considered as potentially important quantitative components of the carbon cycle at either global or regional scales. By taking published estimates of gas exchange, sediment accumulation, and carbon transport for a variety of aquatic systems, we have constructed a budget for the role of inland water ecosystems in the global carbon cycle. Our analysis conservatively estimates that inland waters annually receive, from a combination of background and anthropogenically altered sources, on the order of 1.9 Pg C y−1 from the terrestrial landscape, of which about 0.2 is buried in aquatic sediments, at least 0.8 (possibly much more) is returned to the atmosphere as gas exchange while the remaining 0.9 Pg y−1 is delivered to the oceans, roughly equally as inorganic and organic carbon. Thus, roughly twice as much C enters inland aquatic systems from land as is exported from land to the sea. Over prolonged time net carbon fluxes in aquatic systems tend to be greater per unit area than in much of the surrounding land. Although their area is small, these freshwater aquatic systems can affect regional C balances. Further, the inclusion of inland, freshwater ecosystems provides useful insight about the storage, oxidation and transport of terrestrial C, and may warrant a revision of how the modern net C sink on land is described.

Patterns and regulation of dissolved organic carbon: An analysis of 7,500 widely distributed lakes

Abstract: Dissolved organic carbon (DOC) is a key parameter in lakes that can affect numerous features, including microbial metabolism, light climate, acidity, and primary production. In an attempt to understand the factors that regulate DOC in lakes, we assembled a large database (7,514 lakes from 6 continents) of DOC concentrations and other parameters that characterize the conditions in the lakes, the catchment, the soil, and the climate. DOC concentrations were in the range 0.1–332 mg L21, and the median was 5.71 mg L21. A partial least squares regression explained 48% of the variability in lake DOC and showed that altitude, mean annual runoff, and precipitation were negatively correlated with lake DOC, while conductivity, soil carbon density, and soil C : N ratio were positively related with lake DOC. A multiple linear regression using altitude, mean annual runoff, and soil carbon density as predictors explained 40% of the variability in lake DOC. While lake area and drainage ratio (catchment : lake area) were not correlated to lake DOC in the global data set, these two factors explained significant variation of the residuals of the multiple linear regression model in several regional subsets of data. These results suggest a hierarchical regulation of DOC in lakes, where climatic and topographic characteristics set the possible range of DOC concentrations of a certain region, and catchment and lake properties then regulate the DOC concentration in each individual lake. Dissolved organic carbon (DOC) is a major modulator of the structure and function of lake ecosystems. The DOC pool of lakes consists of both autochthonous DOC (i.e., produced in the lake) and allochthonous DOC (i.e., produced in the catchment), although allochthonous DOC is generally believed to represent the larger fraction of the total DOC in lakes. Due to the dark color of many DOC compounds, DOC affects the thermal structure and

Terrestrial carbon and intraspecific size-variation shape lake ecosystems

Abstract: Conceptual models of lake ecosystem structure and function have generally assumed that energy in pelagic systems is derived from in situ photosynthesis and that its use by higher trophic levels depends on the average properties of individuals in consumer populations. These views are challenged by evidence that allochthonous subsidies of organic carbon greatly influence energy mobilization and transfer and the trophic structure of pelagic food webs, and that size variation within consumer species has major ramifications for lake community dynamics and structure. These discoveries represent conceptual shifts that have yet to be integrated into current views on lake ecosystems. Here, we assess key aspects of energy mobilization and size-structured community dynamics, and show how these processes are intertwined in pelagic food webs.

Degradation rates of organic phosphorus in lake sediment

Abstract: Phosphorus (P) binding groups were identified in phytoplankton, settling particles, and sediment profiles by 31P NMR spectroscopy from the Swedish mesotrophic Lake Erken. The 31P NMR analysis revealed that polyphosphates and pyrophosphates were abundant in the water column, but rapidly mineralized in the sediment. Orthophosphate monoesters and teichoic acids degraded more slowly than DNA-P, polyphosphates, and P lipids. Humic acids and organic acids from phytoplankton were precipitated from the NaOH extract by acidification and identified by 31P NMR spectroscopy. The precipitated P was significantly more recalcitrant than the P compound groups remaining in solution, but does not constitute a major sink of P as it did not reach a stable concentration with depth, which indicates that it may eventually be degraded. Since P also precipitated from phytoplankton, the origin of humic-P can not be related solely to allochthonous P.

Importance of rare and abundant populations for the structure and functional potential of freshwater bacterial communities

Abstract: Lakewater microcosms were inoculated with freshwater bacterioplankton, to determine how the elimination of less abundant populations affects the structure and basic functional features (growth) of mi- crobial communities. The number of bacteria added to individual microcosms varied from <1 to 2.6 × 10 7 cells. Cultures amended with 11 mg C l -1 of either isolated hu- mic substances or phenol, as well as unamended con- trols, were studied in parallel. All cultures inoculated with 260 cells or more showed vigorous growth, whereas an inoculum size of 2.6 to 26 cells resulted in growth in the control and humic enrichment cultures only. All cul- tures were harvested at steady state within 14 d of inocu- lation. The biomass yield was only slightly affected by the dilution factor. The catechol 2,3-dioxygenase gene (encoding the enzyme responsible for starting the meta pathway of aromatic compound degradation) was de- tected in all phenol and in the least diluted humic en- richment cultures. Dominant members of the emerging bacterial communities were detected by terminal restric- tion fragment length polymorphism (T-RFLP) of PCR- amplified 16S rRNA genes. The number of detected community members was much higher in the humic treatment than in the phenol and control treatments. Based on the T-RFLP data, dilution of the inoculum sig- nificantly affected the resulting community composition (p < 0.0001). Rare, opportunistic populations were ap- parently able to exploit the humic enrichment cultures. Phenol appeared to be detrimental to the most abundant members of the original inoculum, but promoted the growth of relatively rare species carrying the catechol 2,3-dioxygenase gene. Thus, community functioning following an environmental perturbation can depend on the presence of rare as well as abundant species.

Does ecosystem size determine aquatic bacterial richness? Comment.

Abstract: In their paper ‘‘Does ecosystem size determine aquatic bacterial richness?’’ Reche et al. (2005) observed a significant correlation between lake surface area and lake bacterial OTU (operational taxonomic unit) richness in 32 lakes. The authors propose that this relationship corroborates one of the predictions of the island-biogeography theory, i.e., that larger islands support more species than smaller islands (MacArthur and Wilson 1967). The results of Reche et al. (2005) have already been cited in support of a positive relationship between habitat size and bacterial taxonomic richness (Bell et al. 2005, Dolan 2005). We argue that the study by Reche et al. (2005) does not provide support of a causal relationship between bacterial richness and habitat size, since their conclusions are biased by incorrect merging of data sets that are not comparable and because the methods used to determine bacterial richness are not adequate. The significant correlation between lake area and bacterial OTU numbers obtained by Reche et al. (2005) was based on data from three separate papers (Lindström and Leskinen 2002, Zwart et al. 2002, Reche et al. 2005). Treated separately, the data from Lindström and Leskinen (2002) and Zwart et al. (2002) show no significant correlation between lake area and number of OTUs detected (P 1⁄4 0.40 and P 1⁄4 0.20 respectively; linear correlations of log-transformed data) while the data from Reche et al. (2005) are almost significantly correlated (P 1⁄4 0.072). When these three data sets are merged, the correlation becomes significant (P , 0.001), as reported by Reche et al (2005). In two of the data sets (Lindström and Leskinen 2002, Reche et al. 2005), OTU richness was determined by counting the number of bands formed in denaturing gradient gel electrophoresis (DGGE). This method is commonly used in microbial ecology to obtain an image of microbial community structures (e.g., Forney et al. 2004, Loisel et al. 2006). Some of the limitations of DGGE are briefly discussed by Reche et al. (2005), for instance they acknowledge that DGGE in the best case only reflects the most dominant taxa. However, the consequence of bacterial community structures being skewed with few abundant and many rare taxa, a quite likely scenario (Acinas et al. 2004, Venter et al. 2004, Gans et al. 2005), is not addressed. If a low fraction of the present populations was detected, a change in the number of DGGE bands could reflect a change in rankabundance of populations (i.e., in the number of populations above the threshold of detection) rather than a change in richness (Forney et al. 2004). Thus, the number of DGGE bands may provide a biased estimate of richness, since it also depends on the evenness of a community. Furthermore, a recent study combining numerical simulations with laboratory experiments (Loisel et al. 2006) demonstrates that the number of bands or peaks obtained using DGGE and similar methods is not related to the richness of communities. Instead, the number of OTUs detected saturates around 35. Thus, the number of DGGE bands is a poor estimator of community richness. In the third data subset used by Reche et al. (2005), i.e., the data from Zwart et al. (2002), OTU richness is represented by the number of unique bacterial 16S rRNA sequences obtained from seven different studies screening clone libraries from nine lakes. Bacterial OTU numbers as determined by DGGE band numbers appears to saturate around 35 as reported by Loisel et al. (2006). In contrast, the number of unique sequences obtained from clone libraries from similar communities can be much greater. In the data set compiled by Zwart et al. (2002) the number of unique sequences per lake ranged from six to 125, the average being 60. Therefore clone library data are not comparable with DGGE data, and accordingly these two types of data should not be merged. Further, the available data (six lakes) from the original references in Zwart et al. (2002) show that the clone libraries screened range largely in size, at least from 45 to 350 clones per lake. Since the number of OTUs, or unique bacterial 16S rRNA sequences, from each lake was not corrected for sample size (i.e., number of clones picked) we suspect that the reported OTU richness reflects the effort spent by the respective researcher rather than community richness. Therefore, these data are not suitable for analysis of how lake surface area or other parameters are related to bacterial richness. Manuscript received 10April 2006; revised 20 January 2006; accepted 20 June 2006. Corresponding Editor: S. Findlay. 1 Limnology/Department of Ecology and Evolution, Evolutionary Biology Centre, Uppsala University, Sweden. 2 E-mail: Eva.Lindstrom@ebc.uu.se

First evidence for a bipolar distribution of dominant freshwater lake bacterioplankton

Abstract: As a result of the recent application of DNA based technology to the investigation of maritime Antarctic freshwater lakes, patterns have begun to emerge in the bacterioplankton communities that dominate these systems. In this study, the bacterioplankton communities of five Antarctic and five Arctic freshwater lakes were assessed and compared with existing data in the literature, to determine whether emerging patterns in Antarctic lakes also applied to Arctic systems. Such a bipolar comparison is particularly timely, given the current interest in biogeography, the global distribution of microorganisms and the controversy over the global ubiquity hypothesis. In addition, it has recently been discovered that commonly encountered bacterial sequences, often originating from uncultivated bacteria obtained on different continents, form coherent phylogenetic freshwater clusters. In this study we encountered both identical sequences and sequences with a high degree of similarity among the bacterioplankton in lake water from both poles. In addition, Arctic freshwater lakes appeared to be dominated by some of the same groups of bacterioplankton thought to be dominant in Antarctic lakes, the vast majority of which represented uncultivated groups.