From: "Optimal nitrogen-to-phosphorus stoichiometry of phytoplankton"
http://preston.kbs.msu.edu/reprints/...al%202004a.pdf

"optimal N:P ratios will vary from 8.2 to 45.0, depending on the ecological conditions. Our results show that the canonical Redfield N:P ratio of 16 is not a universal biochemical optimum, but instead represents an average of species-specific N:P ratios"

"Laboratory studies show that phytoplankton are flexible in their overall stoichiometry, often matching their nutrient supply at low growth rates"

"The Redfield N:P ratio of 16 does not emerge as a universally optimal value from either our empirical survey or our theoretical results. Instead, it should be seen as merely the current average stoichiometry of phytoplankton in the ocean, weighted by the relative abundance of species with different structural N:P ratios. In turn, the relative abundance of species is determined by the ecological conditions under which species grow and compete. Competitive equilibrium selects high N:P ratios; exponential growth selects low N:P ratios. Real ecosystems experience a mix of equilibrium and exponential growth phases"