Shifts in phytoplankton speeies eomposition following ehanges in N P ratio have been observed in artifieial laboratory mieroeosms and natural phytoplank ton eommunities in vitro and in situ. The experiments reported and reviewed here have shown that high N P weight ratios (20-50 1) ean favor the development of Chloroeoeeales, while a reduetion of the N P ratio to values of 5 to 10 frequently leads to a eommunity dominated by Cyanophyta. Model ealculations prediet that the relative abundanee of different phytoplankton speeies depends only on the relative amounts of N and P in the environment, so that the optimal N : P ratio for a given spe eies is equal to the ratio of its minimum eell requirements for these elements. An empirieal test of this hypothesis showed that for several speeies of Chloroeoeeales and Cyanophyta the ratios of their eellular requirements for N and P determined ex perimentally were dose to their optimal (for growth) environmental eoneentration ra tios. For instanee, an experimental inerease in the N P ratio from a value of 4 1 to 25 -50 : 1 by mass in the water of fish-breeding ponds led to an inereased abundanee of Chloroeoeeales. The speeies shift was due mainly to Scenedesmus quadricauda, whieh has a high optimal N: P ratio for growth.

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