| Abstract: | A strong biomass increase of two Anabaena species was observed in natural plankton community enclosed into nine large mesocosms (51 m3) and manipulated with mineral nutrients and an organic carbon source during a 3‐week period in the coastal Baltic Sea. The water column and settled material from the bottom of the mesocosms were sampled at 2‐day intervals. Planktonic populations of Anabaena lemmermannii Richter and A. cylindrica Lemmermann and sedimentation rates of akinetes to the bottom were quantified. Comparing mesocosms with artificially induced nitrogen and phosphorus limitation, we found that during the third week of the experiment, the population size of A. lemmermannii was clearly higher in nitrogen‐limited units (by a factor of 2.4), whereas the production rate of akinetes was higher in the phosphorus‐limited units (by a factor of 2.5). Input of freshly produced A. lemmermannii akinetes to the benthos was on average 15 × 106 and 6 × 106 cells· m?2·d?1 in the P? and N? limited mesocosms, respectively. Our estimates of specific akinete production rate of A. lemmermannii in P? and N? limited mesocosms revealed an even larger divergence (a factor of 5.5), being on average 2.4 and 0.4 akinetes·10?3 vegetative cells?1·d?1, respectively. The phosphorus addition effectively reduced akinete production of A. lemmermannii. Differences in the nutrient manipulation had no apparent effect on the biomass and akinete production of A. cylindrica. The akinete production pattern of A. cylindrica revealed a 1‐week delay compared with the vegetative population peak, whereas such a delay was not obvious in A. lemmermannii. |
