Linking planktonic biomass and metabolism to net gas fluxes in northern temperate lakes

Plankton communities in oligotrophic waters are characteristically dominate d by the biomass of heterotrophs, including bacteria, micro-, and macrozoop lankton. It has been generally assumed that these inverted biomass pyramids are the direct result of high specific production rates of phytoplankton a nd a tight coupling between producers and consumers. There are, however, at least two alternative hypotheses: (1) heterotrophic biomass turnover is mu ch slower in oligotrophic than eutrophic systems; and (2) oligotrophic plan ktonic communities are significantly subsidized by allochthonous organic ma tter, In this study we assessed these hypotheses by establishing the relati onship between plankton biomass structure (partition between auto- and hete rotrophs), plankton function (plankton primary production and respiration) and whole-lake gas (O-2 and CO2) fluxes in 20 temperate lakes that span a l arge range in primary production. We show that the balance of phytoplankton production and community respirat ion (P/R ratio) is always below unity in unproductive lakes where heterotro phic biomass (H) is high relative to autotrophic biomass (A), suggesting th at these planktonic food webs function as heterotrophic systems and must be subsidized by allochthonous organic matter. Further, rates of phytoplankto n specific production are not highest in communities characterized by domin ance of heterotrophic biomass. All except the most productive lakes were su persaturated in CO2 and undersaturated in O-2. Our results support the hypo thesis that excess CO2 in lakes originates from the breakdown of terrestria l organic carbon by planktonic organisms. A simple model in which both allo chthonous organic matter and phytoplankton production support the metabolis m of heterotrophs reproduced the patterns and magnitudes of metabolism P/R ratio, biomass turnover time, and whole-system gas flux among lakes. These patterns of metabolism and structure suggest that inverted biomass pyramids in temperate lakes, and perhaps in other aquatic systems, reflect the hete rotrophic nature of these plankton communities rather than turnover rates o f autotrophs or heterotrophs.