EFFECT OF TEMPERATURE AND LIGHT ON THE GROWTH OF MICRO-PLANKTON NANO-PLANKTON AND PICO-PLANKTON - IMPACT ON ALGAL SUCCESSION

The influence of light and temperature on the phytoplankton succession in a temperate sea area was investigated in laboratory experiments with natural assemblages of micro-, nano-, and picoplankton collected from the northern Baltic Sea during 1989 and 1990. Respiration increased from 0 to 30 degrees C in all groups of phytoplankton, while gross photosynthesis stabilised at 10 to 15 degrees C. Light saturation occurred at 25 to 75 mu mol quanta m(-2) s(-1), indicating low light adaptation of the algae. Picoplankton showed the strongest temperature response, and at temperatures above 10 degrees C picoplankton obtained a higher biomass specific photosynthesis than that of the other groups. Different light treatments had no effect on the species composition in experiments with natural algal assemblages, while different temperature conditions had a marked effect. With a temperature increase from 0 to 10 degrees C, the algal community changed from a typical spring community, with diatoms and dinoflagellates, to a summer community, dominated by mixotrophic nanoflagellates. The small, or often non-existing, autumn bloom in the sea area studied, can be explained by short day lengths combined with relatively high temperatures, causing high community respiration rates and low gross photosynthesis, resulting in a negative energy balance. The net energy gain depends on a differential temperature effect on gross photosynthesis and endogenous respiration in various plankton groups. This gives the phytoplankton groups diverse competitive advantages during different seasons and thus may be an important factor in controlling algal succession.