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

Plankton communities in oligotrophic waters are characteristically dominated by the biomass of heterotrophs, including bacteria, micro-, and macrozooplankton. It has been generally assumed that these inverted biomass pyramids are the direct result of high specific production rates of phytoplankton and a tight coupling between producers and consumers. There are, however, at least two alternative hypotheses: (1) heterotrophic biomass turnover is much slower in oligotrophic than eutrophic systems; and (2) oligotrophic planktonic communities are significantly subsidized by allochthonous organic matter, In this study we assessed these hypotheses by establishing the relationship between plankton biomass structure (partition between auto- and heterotrophs), plankton function (plankton primary production and respiration) and whole-lake gas (O-2 and CO2) fluxes in 20 temperate lakes that span a large range in primary production. We show that the balance of phytoplankton production and community respiration (P/R ratio) is always below unity in unproductive lakes where heterotrophic biomass (H) is high relative to autotrophic biomass (A), suggesting that these planktonic food webs function as heterotrophic systems and must be subsidized by allochthonous organic matter. Further, rates of phytoplankton specific production are not highest in communities characterized by dominance of heterotrophic biomass. All except the most productive lakes were supersaturated in CO2 and undersaturated in O-2. Our results support the hypothesis that excess CO2 in lakes originates from the breakdown of terrestrial organic carbon by planktonic organisms. A simple model in which both allochthonous organic matter and phytoplankton production support the metabolism 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 heterotrophic nature of these plankton communities rather than turnover rates of autotrophs or heterotrophs.