Diel variations in photosynthetic activity of summer phytoplankton in Lindaspollene, western Norway

Abstract

Diel patterns in photosynthesis of natural phytoplankton were studied during a 5 d period during midsummer in Lindaspollene, a land-locked fjord of western Norway. Samples were taken from depths of 0.5, 5, 10 and 21 m at 3 to 4 h intervals. The natural light:dark cycle was 19:5 h. The coccolithophorid Emiliania huxleyi and ultraplankton ( lt 5 mu-m) flagellates were predominant in the upper 10 m, while diatoms and silicoflagellates were predominant at 21 m. No clear diel pattern was seen in the variations of chlorophyll a and cell numbers. The photosynthesis vs light relationship (P-I) was studied at 0.5 and 10 m, and the parameters P-max-B, alpha-B and I-K showed pronounced diel rhythms. Maximum and minimum values of alpha-B and P-max-B differed by a factor of 3 to 4 and occurred in the morning and evening periods, respectively. These parameters were linearly correlated at both 0.5 and 10 m, indicating a phased diel periodicity. I-K showed a clear diel variation at 0.5 m with minimum values at night, whereas no persistent diel rhythm could be seen at 10 m. P-max-B and I-K were correlated at 0.5 m but not at 10 m, while I-K and alpha-B were weakly correlated at both 0.5 and 10 m. In vivo fluorescence per unit chlorophyll a showed a diel variation pattern that was the opposite to that of P-max-B and alpha-B. Our results indicate that an endogenous rhythm is regulating the diel oscillations in photosynthesis. This endogenous mechanism is possibly entrained by environmental signals such as cycles in nutrient availability and light conditions.