phosphorus addition experiment in the
Nurit Kress, Ph.D.
Israel Oceanographic and Limnological Research
Wednesday, May 12, 2004
Pacific Forum – 3:00 p.m.
The CYCLOPS Program (Cycling of Phosphorus in the Eastern Mediterranean) was set up to directly prove (or refute) the hypothesis that the productivity of the eastern Mediterranean is limited by the availability of phosphorus. Until CYCLOPS, only indirect and partial evidence pointed to this hypothesis. The strategy used was similar to the iron addition experiments performed prior to CYCLOPS microcosm experiments and in situ addition experiments, following the changes in the phytoplankton, bacterial, and small plankton communities. The phosphorus "trail" was also followed to develop a quantitative understanding of its cycle in the oligotrophic area.
The results from the second in situ addition experiment performed in May 2002, showed a complexity of response not anticipated by the microcosm experiments. The added phosphate was taken up rapidly by the bacterial community which showed a significant increase in activity but not in biomass. Dissolved organic nitrogen, which decreased, probably provided them with the needed nitrogen. By contrast, the phytoplankton took up phosphorus but did not grow. These changes caused an increase in grazers and in grazing rates, which caused the bacterial biomass to remain stable and caused a decrease in the phytoplankton chlorophyll. It was concluded that the bacteria are phosphorus limited while the phytoplankton are nitrogen and phosphorus co-limited. Microbial "grazers" are present within the system and seem to be 'hungry' and able to rapidly take advantage of short term changes in the food supply.