Sommer U, Lengfellner K, Lewandowska A
Plankton, spring bloom, irradiance, mesocosm, Kiel indoor, Kiel, Germany, 1.4 m3
Through the use of mesocosm experiments, we show that an unusually early springphytoplankton bloom can be induced by intermittent high-light periods. We performed mesocosmexperiments where plankton assemblages from Kiel Bight (Western Baltic Sea) received a lightregime based on the natural seasonal irradiance dimmed to 43% of surface irradiance of cloudlessdays, starting with irradiance levels of mid-January (6 mesocosms) and mid-February (6 mesocosms).After 6 d, half of the mesocosms received a ca. 2-fold increase in irradiance. In the Januarymesocosms, a phytoplankton bloom developed only in the treatments with the high-light episode,whereas in the February mesocosms a phytoplankton bloom also developed in the controls. Phytoplanktonnet growth rates, pro duction:biomass ratios and biomass at the end of the high irradianceepisodes were positively correlated to the daily light dose. The relative biomass of diatomsincreased with increasing light, whereas the relative biomass of cryptophytes decreased. A bottom-up transmission to mesozooplankton (mainly copepods of the genera Acartia and Oithona)was evident by increased densities of copepod nauplii and egg production under higher light conditions,whereas copepodids and adults showed no responses during the experimental period. Thetaxonomic composition of the nauplii was shifted to the advantage of Acartia/Centropages (notdistinguished at the naupliar stage) under higher light conditions.