N influence on the bacterial C cycling of a large fluvial lake

English Abstract:

Carbon dynamics of aquatic ecosystems, particularly large rivers, are strongly influenced by bacterial respiration (BR), production (BP) and bacterial growth efficiency (BGE). Lake St-Pierre (LSP), a large fluvial lake in the St-Lawrence River, has a unique hydrology and can be separated into 3 general water masses with distinct chemical characteristics: 1) North, draining the Ottawa River, 2) Central, the Great Lakes and 3) South, 3 tributaries heavily impacted by agriculture. LSP is shallow and replete in P and organic C both of autochtonous and allochthonous origin, but spatially undergoes strong dissolved inorganic N depletion events over the course of the summer.

The aim of our study was to look at the impact of periodic N limitation on bacterial metabolism. We predicted that variation in C conversion efficiency would be influenced by N availability and result in important spatial and temporal variability in LSP. Sampling carried out over two years shows that BGE was positively related with BP (r² _adj= 0.31), although a stronger negative relationship with BR (r²_adj = 0.51, n=49) was observed. BR was negatively correlated to change in dissolved N (TDN) and nitrate concentration (r²_adj = 0.42 and 0.46 respectively). Given its effect on BR, enhanced TDN availability resulted in increased BGE (r²_adj = 0.29). BP did not show any strong relationship with P however BP was moderately related to TDN (r²_adj = 0.29). Multivariate analysis confirms that the temporal component significantly affected bacterial dynamics in LSP and suggests other factors like hydrology and temperature are involved.

Our study supports that, while P is probably a key element in the onset of eutrophying conditions in freshwater ecosystems, N can have important effects on the outcome of C cycling, be it retention or removal of organic C in aquatic ecosystems.