Fish stocks as phosphorus sources or sinks: Influenced by nutritional and metabolic variations, not solely by dietary content and stoichiometry.

The study provides a descriptive understanding of when fish (Cyprinus carpio model) are the source or sink of phosphorus. Dissolved reactive phosphorus (DRP; PO-P) losses (51.1 ± 5.9 % of intake-P) increase at excess of bioavailable P (>0.83 g 100 g dry matter, DM fed) or when food (digestible) N:P mass ratio (≤4.4:1) approaches organismal storage threshold (~4:1). This is known, however, even at a sub-threshold food P content (0.57 g 100 g DM) and food N:P mass ratio (7.3:1), DRP losses (57.8 ± 4.5 % of intake-P) may be extraordinary if two indispensable amino acids are biologically insufficient (lysine ≤1.43 g, methionine ≤0.39 g 100 g DM fed). Given that methionine and lysine are sufficient, DRP losses cease (≈0 %) and even some P from water is absorbed, given there is support from non-protein energy (NPE). Insufficient NPE (<180 kcal 100 g DM fed) may drive DRP losses (81.6 ± 4.3 % of intake-P) beyond expected levels (46-59 % of intake-P) at a given food P content (0.91 g 100 g DM). Natural food seldom fulfills low P, high lysine + methionine, and high NPE contents simultaneously, thus keeping fish in a perpetual P recycling for algae (scaleless carp > scaly carp). Such P recycling ceases only during basal metabolism. During feeding state, the richness of lysine + methionine bound N and lipid + carbohydrate bound C in the food base may enhance the fishes' threshold of P storage. P storage can be diminished when they are insufficient. We show that for fish, the decision of P recycling or not recycling (for algae) may change based on the supply of specific fractions of N or C from the food web or metabolic variations (basal metabolism, presence of scales). NOVELTY STATEMENT: The ecological stoichiometry theory is better connected to fish nutritional bioenergetics for better understanding and biomanipulation of eutrophication processes.