The nitrogen: phosphorus ratio as a factor regulating phytoplankton community structure

Shifts in phytoplankton species composition following changes in N: P ratio have been observed in artificial laboratory microcosms and natural phytoplankton communities in vitro and in situ. The experiments reported and reviewed here have shown that high N: P weight ratios (20-50: 1) can favor the development of Chlorococcales, while a reduction of the N: P ratio to values of 5 to 10 frequently leads to a community dominated by Cyanophyta. Model calculations predict that the relative abundance of different phytoplankton species depends only on the relative amounts of N and P in the environment, so that the optimal N: P ratio for a given species is equal to the ratio of its minimum cell requirements for these elements. An empirical test of this hypothesis showed that for several species of Chlorococcales and Cyanophyta the ratios of their cellular requirements for N and P determined experimentally were close to their optimal (for growth) environmental concentration ratios. For instance, an experimental increase 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 increased abundance of Chlorococcales. The species shift was due mainly to Scenedesmus quadricauda, which has a high optimal N: P ratio for growth.