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.