River metabolism and carbon dynamics in response to flooding in a lowland river

Lowland riverine–floodplain systems often have significant but irregular inputs of allochthonous carbon. However, the importance of this carbon to riverine systems remains poorly understood. We assessed open water dissolved organic carbon (DOC) concentrations, metabolism and biofilm stable isotope (δ13C) signatures, upstream and downstream of an extensive floodplain forest on the Murray River, Australia, before and after a flood event. Prior to flooding, all sites had similar concentrations of DOC, rates of metabolism and biofilm δ13C signatures. During the flood DOC concentration increased up to three-fold downstream of the forest, gross primary production (GPP) increased at all sites, but community respiration (CR) increased only at the downstream sites, resulting in decreased in NPP downstream and a slight increase upstream. Biofilm δ13C signatures became depleted by between 4 and 7‰ downstream of the forest during the flood, reflecting a rapid incorporation of allochthonous carbon into the biofilm. These results indicate that flooding led to a substantial increase to the energy budget of the Murray River through the provisioning of large quantities of allochthonous carbon and that terrestrial carbon was processed within the river biofilms. Allochthonous carbon assimilation within biofilms during flooding provides a potential pathway for allochthonous carbon to be incorporated into the metazoan foodweb.

Stable carbon isotope biogeochemistry of lakes along a trophic gradient

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.

Stable carbon isotope biogeochemistry of lakes along a trophic gradient

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 (PLFA) were studied in a survey of 22 North American oligotrophic to eutrophic lakes. The δ13C of different PLFA 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 larger isotopic variability in eutrophic lakes compared to oligo-mesotrophic lakes 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.