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.