A discrepancy between predictions of saturating nutrient uptake models and nitrogen-to-phosphorus stoichiometry in the surface ocean

Ballantyne IV, Ford, Duncan N. L. Menge, and Joshua S. Weitz

Limnol. Oceanogr., 55(3), 2010, 997-1008 | DOI: 10.4319/lo.2010.55.3.0997

ABSTRACT: We derive a simple prediction about euphotic zone N: P stoichiometry from a large class of models that use saturating nutrient uptake functions to characterize N and P acquisition by phytoplankton. The prediction is: At an ecological steady state, the ratio of phytoplankton N: P to inorganic N: P in the euphotic zone equals the ratio of phytoplankton maximum uptake rates of N and P. We estimate this predicted ratio using nutrient uptake parameters measured in laboratory growth experiments and compare the predicted ratio to empirical observations from long-term sampling in the Atlantic and Pacific oceans. The model predictions for the ratio of phytoplankton N: P to inorganic N: P are at odds with the majority of data from extensive long-term sampling in the Atlantic and the Pacific oceans. This discrepancy calls into question the scope of applicability of ecosystem models that explicitly describe phytoplankton growth as a function of N and P availability. We discuss efforts to resolve this discrepancy, including the need for performing more comprehensive N and P uptake experiments and by reexamining models of nutrient uptake.

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