Water level changes affect carbon turnover and microbial community composition in lake sediments

Abstract

Due to climate change, many lakes in Europe will be subject to higher variability of hydrological characteristics in their littoral zones. These different hydrological regimes might affect the use of allochthonous and autochthonous carbon sources. We used sandy sediment microcosms to examine the effects of different hydrological regimes (wet, desiccating, and wet-desiccation cycles) on carbon turnover. (13)C-labelled particulate organic carbon was used to trace and estimate carbon uptake into bacterial biomass (via phospholipid fatty acids) and respiration. Microbial community changes were monitored by combining DNA- and RNA-based real-time PCR quantification and terminal restriction fragment length polymorphism (T-RFLP) analysis of 16S rRNA. The shifting hydrological regimes in the sediment primarily caused two linked microbial effects: changes in the use of available organic carbon and community composition changes. Drying sediments yielded the highest CO2 emission rates, whereas hydrological shifts increased the uptake of allochthonous organic carbon for respiration. T-RFLP patterns demonstrated that only the most extreme hydrological changes induced a significant shift in the active and total bacterial communities. As current scenarios of climate change predict an increase of drought events, frequent variations of the hydrological regimes of many lake littoral zones in central Europe are anticipated. Based on the results of our study, this phenomenon may increase the intensity and amplitude in rates of allochthonous organic carbon uptake and CO2 emissions.

Keywords: carbon dioxide emission; keeling plot; phospholipid fatty acids; stable isotope; water level changes.

Figure 1.

(a) Carbon dioxide flux and (b) respired ¹³CO₂ for the three different treatments at the end of the experiment (day 66 and day 74). D, DRY; RW-dry, REWET-dry (Rewet cycle in the dry period); RW-wet, REWET-wet (Rewet cycle in the wet period); W, WET. The boundary of the box closest to zero indicates the 25th percentile, the line within the box marks the median, and the boundary of the box farthest from zero indicates the 75th percentile. Whiskers (error bars) above and below the box indicate the 90th and 10th percentiles, and circles indicate outlying points. Means with different letters are significantly different (P < 0.05).

Figure 2.

Stable carbon isotope ratios in PLFA extracted from the sediment at day 0 and the end of experiment (day 77) in DRY, WET and REWET treatments. The most abundant PLFAs are illustrated: (a) 14:0, (b) i15:0, a marker for heterotrophic bacteria, (c) 16:0 (Gram+ bacteria), (d) 18:1ω9, a proxy for fungi, and (e) 20:5ω3 a biomarker for diatoms.

Figure 3.

Changes of the relative abundance of Bacteria (diamonds), Actinobacteria (squares), and Archaea (triangles) based on 16S rRNA gene copy numbers under REWET conditions. A ratio of 1 represents the point where the rRNA gene copies peaked. The yellow part indicates the dry cycles.

Figure 4.

Abundance of (a) the metabolically active (RNA-based) and (b) the total (DNA-based) Bacteria, Actinobacteria, and Archaea as determined by quantification of the 16S rRNA copy numbers. The abundance is presented for the three treatments REWET (triangles), DRY (diamonds) and WET (circles). The yellow part indicates the dry cycle.

Figure 5.

Non-metric multidimensional scaling (NMS) ordination of the T-RFLP patterns of (a, b) the active and (c, d) the total bacterial community obtained from sediments on day 74 (a, c: REWET-dry) and day 77 (b, d: REWET-wet). The vectors indicate the strength and direction of the correlation with abiotic and biotic factors.