Diel phytoplankton periodicity in Mikołajskie lake, Poland, as determined by different methods in parallel

Photosynthetic rates as measured by the oxygen light and dark bottle method were highly correlated with estimates using the 14 C technique. The high O 2 / 14 C ratios found are explained by algal respiration and extracellular release which are included in photosynthetic measurements by the oxygen technique, while the 14 C method yields values close to net photosynthesis. Separation of net‐ and nannoplankton using a 50 μm plankton net for filtration was not comparable to distinctions made by microscopic examination. Separation of both by filtration caused a significant decrease in the photosynthetic activity of nannoplankton in 24‐hour incubations, but had no detectable effect after 4 hours of exposure. “Bottle effects” in 24‐hour measurements of photosynthesis were similar using both methods. Asymmetric photosynthetic time‐curves as well as vertical phytoplankton migrations were the main reasons for errors in estimates of daily photosynthetic rates from part‐day incubations which were extrapolated to the entire day.