Abstract
We studied temporal dynamics and regulation of oxygen metabolism in the upper mixed layer of a nutrientrich
shallow Danish lake by continuous measurements of oxygen, irradiance, wind, and temperature and frequent
measurements of algal chlorophyll, organic pools, and inorganic nutrients. Chlorophyll, algal growth rate, and
mean irradiance (Emean) in the mixed surface layer were calculated daily from continuous measurements of
irradiance and temperature with depth. There were three to four distinct maxima in gross primary production
(GPP) and community respiration (R) during the summer season and minima from fall to spring after broad-scale
changes in irradiance, temperature, mixing depth, and biomass and growth rate of the algal community and
concentrations of inorganic nutrients. Lake metabolism was annually balanced (mean GPP :R 1.04 in 2003 and
1.01 in 2004), with net autotrophy occurring mainly from mid-May to mid-September (mean GPP:R 1.14 in 2003
and 1.10 in 2004), and net heterotrophy outside this period (mean GPP:R 0.60 in 2003 and 0.81 in 2004).
However, GPP :R varied two- to threefold from day to day because lower surface irradiance, higher mixing
depth, and thus lower Emean significantly reduced GPP. Normalizing GPP to chlorophyll provided an index of
algal growth potential (GPPB), which followed a hyperbolic relationship to Emean, and both parameters were
related to blooms and collapses of algal biomass. Metabolic rates were much more variable from day to day than
algal biomass, which integrates growth and loss processes over longer periods. The continuous approach to lake
metabolism provides better data and can provide a more accurate picture than averages of a few discrete
measurements. Weekly averages reflected the characteristic seasonal peaks and troughs also observed for algal
biomass, whereas monthly averages did not. Daily measurements of lake metabolism, therefore, can provide the
optimal background for evaluating temporal changes and regulation of algal biomass and organic pools in
nutrient-rich shallow lakes.
| Original language | English |
|---|---|
| Journal | Limnology and Oceanography |
| Volume | 52 |
| Issue number | 1 |
| Pages (from-to) | 108–120 |
| ISSN | 0024-3590 |
| Publication status | Published - 2007 |