From limitation to excess: the consequences of substrate excess and stoichiometry for phytoplankton physiology, trophodynamics and biogeochemistry, and the implications for modeling

Current parameterization of several important physiological rates using rectangular hyperbolic saturation formulations is inadequate to capture our expanding understanding of the dynamic regulation of nutrients and energy at the primary producer level across all substrate levels, from limiting to super-saturating. Nutrient regulation by primary producers can affect chemical composition, in turn affecting predator–prey interactions and biogeochemical feedbacks in complex foodwebs. Anthropogenically altered nutrient loads are accentuating these challenges by altering nutrient stoichiometry. Using examples derived from the development of phytoplankton physiological dynamic regulation, the case is made that dynamic regulatory concepts are relevant at all levels of ecosystem regulation, that elemental stoichiometry must be considered in physiological, trophodynamic and biogeochemical constructs, and that the classical notion that nutrients and nutrient stoichiometry are only regulatory for physiology when at the limiting end of the spectrum must be laid to rest. Advancing models will require new emphasis on physiology including both dissipatory regulation and assimilatory regulation and the feed-back mechanisms between them.