Mesocosm tracer studies. 2. The fate of primary production and the role of consumers in the pelagic carbon cycle of a mesotrophic lake

Abstract

The fate of primary production in the pelagic food web in mesotrophic Svinsjoen, southeastern Norway, was studied in a 10-d mesocosm experiment with 14C addition. Specific assimilation rates for all the main compartments in the system were estimated by applying tracer models and further combined to construct a balanced C budget by means of mathematical inverse methods. The specific carbon assimilation rates for the phytoplankton and bacteria were 0.24 and 0.49 d-l, respectively, and 70% of the bacterial production was fueled by phytoplankton exudates. The C budget shows that roughly half of the gross primary production was lost as exudates and 10% was respired. Only 20% of phytoplankton net production was assimilated by consumers. The rest of the net primary production was lost as dissolved and particulate detritus or by sedimentation. Of the net bacterial production, a third accumulated as bacterial biomass, a third was assimilated by bacterivores, and a third was lost as detritus. The coupling between the microbial loop and the classical grazer food web was insignificant, probably due to low Daphnia biomass. The microbial food web thus acted as a sink rather than as a source of C to the higher consumers. Altogether, <2% of gross primary production was incorporated into consumer biomass.

Phytoplankton-bacteria coupling under elevated CO&lt;sub&gt;2&lt;/sub&gt; levels: a stable isotope labelling study

“…At the end of their incubations, isotope ratios in bacteria were lower than those of POC, indicating that bacteria relied partly on not freshly produced material. A similar estimation of fluxes has been reported by Lyche et al (1996) who traced 14 C in different size fractions as probes for primary and secondary production in a mesocosm study with lake communities. Their values were slightly different from ours (Table 2), with bacteria assimilation rates of 0.485 d −1 and a fraction of 0.704 derived from the phytoplankton.…”

“…The drawback of these methods is that net processes are measured and that temporal and spatial decoupling and grazing cannot be quantified. The use of carbon isotope tracers ( 13 C, 14 C) provides the possibility to directly quantify the flux of carbon from dissolved inorganic carbon (DIC) to phytoplankton and subsequently bacteria and this method has been successfully used in previous mesocosm experiments (Norrman et al, 1995;Lyche et al, 1996) and in whole lake isotope tracer addition experiments (Kritzberg et al, 2004;Pace et al, 2007). Since it is difficult to physically separate phytoplankton from bulk particulate organic matter (POM), the isotope signal of POM has often been used as a representative for phytoplankton, which can lead to an underestimation of phytoplankton carbon uptake.…”

Regulation of the relationship between phytoplankton Scenedesmus acutus and heterotrophic bacteria by the balance of light and nutrients

“…A number of studies showed that bacteria can use dissolved organic carbon (DOC) originated from phytoplankton as an energy source (Coveney 1982, Pomeroy & Wiebe 1982, Riemann & Serndergaard 1986, Bjornsen et al 1989, Baines & Pace 1991, Lyche et al 1996. Bacterial production and/or biomass covary with primary production or phytoplankton biomass over various aquatic systems (Cole et al 1988, White et al 1991, Ducklow & Carlson 1992, Egli 1995.…”

Carbon‐nitrogen coupling and algal‐bacterial interactions during an experimental bloom: Modeling a 13C tracer experiment

“…This is because biomass changes are the net result of many processes that operate at the same time. The deliberate introduction of a tracer such as 13 C-labeled dissolved inorganic carbon (DIC) under controlled conditions, and its consequent tracking into the various components, provides valuable extra constraints (Norrman et al 1995;Lyche et al 1996;Cole et al 2002). It allows one to pinpoint which pathways are significant and to identify the main players of the ecosystem.…”