Lake Metabolism at North Temperate Lakes LTER 2000

Recent literature suggests that for many lakes and rivers, the respiratory breakdown of
organic matter (R) exceeds production of organic matter by photosynthesis (gross primary
production; GPP) within the water body. This metabolic balance (GPP less than R;
heterotrophy ) implies that allochthonous organic matter supports a portion of the aquatic
ecosystems respiration. Evidence that many lakes are heterotrophic comes from diverse
approaches, and debate remains over the circumstances in which heterotrophy exists. The
methods used to estimate GPP and R and the limited extent of lake types studied, especially
with respect to dissolved organic carbon (DOC) and total phosphorus (TP) concentrations, are
two reasons for differing conclusions. In this study, O2 and CO2 sondes were deployed during
July and August, 2000 to measure diel gas dynamics in the surface waters of 25 lakes in the
Northern Highland Lake district of Wisconsin and the Upper Peninsula of Michigan. The lakes
were chosen to span wide and orthogonal ranges in DOC and TP concentrations. From these
data, we calculated GPP, R and net ecosystem production (NEP=GPP-R). Over the broad range in
TP and DOC among the lakes, diel CO2 and O2 changed on a near 1:1 molar ratio. Metabolism
estimates from the two gases were comparable, except at high pH. Most lakes in our data set
had -NEP, but GPP and R appeared to be controlled by different factors. TP correlated
strongly with GPP, whereas DOC correlated with R. At low DOC concentrations, GPP and R were
nearly equal, but at higher DOC, GPP and R uncoupled and lakes had -NEP. Strong correlations
between lake metabolism and landscape related variables suggest that allochthonous carbon
influences lake metabolism. Sampling Frequency: Chemical parameters and physical properties
sampled from 1 to 4 times during the summer. Time series data step is 30 minutes. Number of
sites: Time series data for 25 lakes. Chemical and physical data from 31 lakes.