Stoichiometric theory extended to micronutrients: Comparison of the roles of essential fatty acids, carbon, and nitrogen in the nutrition of marine copepods

Conventional stoichiometric theory, which is used to study the limitation of zooplankton production by C, N, and other elements, is extended to include the essential polyunsaturated fatty acids (PUFAs) 20:5(n-3) and 22:6(n-3). Using typical biochemical compositions of consumer (Calanus helgolandicus) and algal food (hypothetical diatom-dinoflagellate mixtures), the analysis shows that PUFAs or macronutrients can be important in limiting zooplankton production, depending on the biochemical compositions of consumer and food and the efficiencies with which dietary components are used. Predicted limitation by fatty acids is strongest when zooplankton use a monospecific diet, indicating that such limitation may be of particular significance in laboratory zooplankton, which are often fed phytoplankton monocultures. The analysis illustrates several factors that operate to minimize the limiting potential of fatty acids to the extent that limitation by C or N could occur: selective grazing to obtain a nutritionally balanced diet, plasticity in consumer biochemical composition, and high C requirements for respiration. The possibility of macronutrient limitation is increased further if zooplankton are able to actively synthesize essential PUFAs, although this is not thought to be the case in most aquatic systems. The work highlights the need for complete data sets incorporating fatty acids and bulk elemental properties of consumers and prey, as well as an improved understanding of the roles and cycling of essential fatty acids, if we are to be able to provide a unified view of zooplankton nutrition.