Microbial carbon processing in oligotrophic Lake Lucerne (Switzerland): results of in situ 13C-labelling studies

Abstract

Although lakes play a major role in the storage of organic carbon, processes involved are not yet very well characterized, especially for oligotrophic lakes. Whether a lake functions as a net source or sink for carbon depends on relative rates of primary production, inputs of terrestrial organic matter and respiration. The microbial community will affect the efficiency of carbon cycling and thereby potential carbon storage. Because the organic matter fluxes are smaller in oligotrophic lakes they have been studied less intensively with respect to their carbon cycling compared to eutrophic lakes. Whether they play an appreciable role in freshwater carbon cycling relies on unraveling primary and secondary production. Here we present the results from such a study in oligotrophic Lake Lucerne, Switzerland. Based on in situ carbon isotopic labelling experiments using dark, glucose-labelled and transparent, DIC-labelled bottles positioned at different depths in the water column, we conclude that even though the photic zone was very deep, integrated primary productivity was consistently low. The carbon processing efficiency of the heterotrophic producers was such that photosynthesized organic matter was fully consumed, even during times of maximum productivity. This implies that the heterotrophic producers were well adapted to rapidly respond to a temporary increase in primary productivity, which is in line with calculated bacterial growth efficiencies in the surface water layer. Highest glucose-based productivity, as a measure of the heterotrophic potential, was observed in the deepest parts of the water column. Chemoautotrophy was shown at 60 m water depth and is of relatively minor importance for overall fluxes. Mixotrophy was recognized as a strategy to keep up production when light conditions become less favorable for autotrophic growth. A mesocosm experiment earlier in the year indicated lower primary production, which agrees well with the timing of this experiment preceding the annual spring bloom. During the low-productivity season the coupling between phytoplankton and bacterial production was much weaker and potentially more organic matter could escape recycling at that time, although quantitatively fluxes remained very low.

Water Depth Underpins the Relative Roles and Fates of Nitrogen and Phosphorus in Lakes

“…Importantly, N is lost to the atmosphere in gaseous forms (e.g., N 2 ) via denitrification under biogeochemically reduced conditions. Generally, the efficiency of N removal from the water-sediment system via denitrification is determined by availabilities of nitrate, labile organic matter, 28 and alternating anaerobic/aerobic conditions. 29 Hypolimnetic redox conditions are stable in deep lakes, 30 but in shallow lakes, windinduced mixing in the water column transports oxygen frequently to the lake bottom and favors facultative (oxic/ anoxic) conditions 29 at the water-sediment interface (Figure S3).…”

Estimation of spatial and temporal changes in net primary production based on Carnegie Ames Stanford Approach (CASA) model in semi-arid rangelands of Semirom County, Iran

“…Solar radiation is used in photosynthesis to convert carbon dioxide and water into carbon compounds including glucose and cellulose (Lammers et al, 2017). Solar radiation is thus effectively stored in rangeland plants as the first trophic level of the food chain (Sainte-Marie et al, 2012).…”

Deciphering Eutrophication-Limiting Nutrients in Lakes: A Multiscale Analysis of Chinese Waters

“…N is also lost from the surface through sedimentation like P, but a substantial portion of N is retained in a biologically available dissolved form, resulting in a lower removal efficiency. Any unrecycled P typically remains in a particulate form. − The large-scale Yangtze River watershed had severe water quality issues owing to the high population density and intensive agricultural and industrial development . Consequently, the TN/TP ratio was high at both the small and large scales.…”