Effect of Nitrogen and Phosphorus Supply on Growth and Tissue Composition of Ulva Fenestrata and Enteromorpha Intestinalis (Ulvales, Chlorophyta)¹

The chlorophyte macroalgae Ulva fenestrata (Postels and Ruprecht) and Enteromorpha intestinalis (Linnaeus) Link. were grown under various nutrient regimes in indoor semi‐continuous and batch cultures. Tissue nitrogen contents ranged from 1.3–5.4% N (dry wt), whereas tissue P ranged from 0.21–0.56% P (dry wt). Growth in low nitrogen medium resulted in N:P ratios of 5–8, whereas growth in high nitrogen medium resulted in N:P ratios of 21–44. For U. fenestrata , tissue N:P < 16 was indicative of N‐limitation. Tissue N:P 16–24 was optimal for growth and tissue N:P > 24 was indicative of P‐limitation. Growth of U. fenestrata was hyperbolically related to tissue N but linearly related to tissue P. Phosphorus‐limited U. fenestrata maintained high levels of tissue N, but N‐limited algae became depleted of P. For E. intestinalis , tissue N remained at maximum levels during P‐limitation whereas tissue P decreased to about 85% of maximal levels during N‐limitation. Growth rates for U. fenestrata decreased faster during P‐limitation than during N‐limitation. Simultaneously, tissue P was depleted faster than tissue N. Our results suggest that comparing tissue N and P of macroalage grown in batch cultures is useful for monitoring the nutritional status of macroalgae.