The effects of phosphorus (P) and nitrogen (N) enrichments, and N:P ratios on nutrient and carrageenan contents in Chondrus crispus Stackhouse were investigated using a factorial block experiment design. After a variable preconditioning period until they were markedly depleted in P and N, plants were cultivated for 5 weeks in seawater enriched with different concentrations of P (0, 1.5, 3, 4.5, 6 μM wk −1 ) and N (12.5, 20, 40 μM wk −1 ). Tissue total P content increased significantly with P enrichment up to 4.5 μM wk −1 . Variations in tissue total N content were more the reflection of seasonal variations of the initial nutrient content than due to experimental enrichment. Carrageenan production was optimized in the range of 20 μM N wk −1 and 1.5 to 3 μM P wk −1 , and of 40 μM N wk −1 and 1.5 μM P wk −1 , or higher, enrichments. No clear correlation was found between N:P ratio and carrageenan content. Dissolved inorganic N (DIN) in seawater remained low at all N and P enrichments; however, dissolved inorganic P (DIP) concentrations increased sharply at an enrichment of 4.5 μM P wk −1 and greater when algal tissues were becoming P saturated. Hence, determining the proper combinations of P and N enrichments, not N:P ratios, is of major importance for aquaculture systems for three reasons: 1) optimizing carrageenan production, 2) minimizing the direct cost of nutrients, and 3) minimizing the levels of DIP and DIN in effluents and, therefore, the indirect cost of nutrients due to their treatment to avoid excessive enrichment of coastal waters.