ONLINE DETERMINATION OF RESPIRATION RATES OF AQUATIC ORGANISMS IN A MONO-PHASE OXYSTAT AT STEADY-STATE DISSOLVED-OXYGEN TENSIONS

On-line determination of respiration rates at steady-state dissolved o xygen tensions was performed in a thermostatted measuring chamber equi pped with a galvanic oxygen electrode. The measuring chamber is design ed to completely exclude gas bubbles from the liquid phase and is name d a mono-phase oxystat. Additions of oxygenated water to maintain stea dy-state dissolved oxygen tensions are controlled by a computer, and t he respiration rate is determined from the rate of these additions. An external loop with a flow cell in a spectrophotometer makes it possib le to determine and control the concentration of algal cells in the mo nophase oxystat and allowed simultaneous determination of respiration and filtration rates of filter-feeding invertebrates. Because the syst em maintains steady-state conditions the experiments do not suffer fro m time limitations. The response time of the system is negligible comp ared to the response time of other open systems.