Abstract
Abstract. The stable carbon (C) isotope variability of dissolved inorganic and organic C (DIC and DOC), particulate organic carbon (POC), glucose and polar-lipid derived fatty acids (PLFAs) was studied in a survey of 22 North American oligotrophic to eutrophic lakes. The δ13C of different PLFAs were used as proxy for phytoplankton producers and bacterial consumers. Lake pCO2 was primarily determined by autochthonous production (phytoplankton biomass), especially in eutrophic lakes, and governed the δ13C of DIC. All organic-carbon pools showed overall higher isotopic variability in eutrophic lakes (n = 11) compared to oligo-mesotrophic lakes (n = 11) because of the high variability in δ13C at the base of the food web (both autochthonous and allochthonous carbon). Phytoplankton δ13C was negatively related to lake pCO2 over all lakes and positively related to phytoplankton biomass in eutrophic lakes, which was also reflected in a large range in photosynthetic isotope fractionation (ϵCO2-phyto, 8–25‰). The carbon isotope ratio of allochthonous carbon in oligo-mesotrophic lakes was rather constant, while it varied in eutrophic lakes because of maize cultivation in the watershed.
Highlights
Studies suggest that lakes contribute significantly to the global carbon budget via organic matter burial and emission of CO2 to the atmosphere (Cole et al, 2007)
We explored patterns of isotopic variability in dissolved inorganic carbon (DIC), dissolved organic carbon (DOC),particulate organic carbon (POC), carbohydrates, phytoplankton, allochthonous carbon and heterotrophic bacteria, and their relationships
We examined the δ13C of fatty acids abundant in or specific to methanotrophs and these were not more depleted in 13C than other fatty acids
Summary
Studies suggest that lakes contribute significantly to the global carbon budget via organic matter burial and emission of CO2 to the atmosphere (Cole et al, 2007). The balance between primary production and external organic carbon input on the one hand and respiration and burial of organic carbon on the other governs whether individual lakes are sources or sinks of CO2. This metabolic balance can be disturbed by changes in nutrient or organic matter inputs to the lake. The loading of allochthonous (terrestrial) carbon is a key factor controlling community respiration of lakes. The metabolic balance of lakes is directly influenced by allochthonous organic carbon loading and trophic state (Del Giorgio and Peters, 1994; Hanson et al, 2003)
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