Diel cycles of planktonic respiration rates in briefly incubated water samples from a fertile earthen pond

Planktonic community respiration rates were assessed every 30 min through two 48-h periods in near-surface water taken automatically from a fertilized carthen pond and incubated in a plastic chamber for 2 1 min of each sampling cycle. Parallel records of water temperature, air temperature, windspccd, and solar irradiance permitted calculation of gross and net primary production and photosynthesis-irradiance relationships. Nighttime respiration rates generally matched oxygen de-pletion rates in pond water, indicating that incubation-based rates were representative of a quickly darkened pond community throughout the day. Daytime rates averaged nearly 2 times the mean night rate and 58 % higher than the mean day rate determined by a typical interpolation used in free-water production calculations. Daily gross production ranged from 0.7 to 1.2 mmol 0, liter-l d I; respiration constituted 65-75 % of gross rates. Gross oxygen production per unit Chl a during sampling intervals was light saturated at irradiance values>600 PEinst m-2 s-l, with an asymptotic value of 1.58 pmol 0, (kg Chl a)-I h I. This system and method were capable ofresolving respiration and gross and net production when chlorophyll concentrations were near 40 pg liter-l. Daytime respiration in planktonic com