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

Abstract

Planktonic community respiration rates were assessed every 30 min through two 48-h periods in near-surface water taken automatically from a fertilized earthen pond and incubated in a plastic chamber for 21 min of each sampling cycle. Parallel records of water temperature, air temperature, windspeed, and solar irradiance permitted calculation of gross and net primary production and photosynthesis-irradiance relationships. Nighttime respiration rates generally matched oxygen depletion 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 O2 liter−1 d−1; respiration constituted 65–75% of gross rates. Gross oxygen production per unit Chl a during sampling intervals was light saturated at irradiance values >600 µEinst m−2 s−1, with an asymptotic value of 1.58 µmol O2 (µg Chl a)−1 h−1. This system and method were capable of resolving respiration and gross and net production when chlorophyll concentrations were near 40 µg liter−1.