Effect of nutrient inputs on growth, chlorophyll, and tissue nutrient concentration of Ova reticulata from a tropical habitat

Green tides caused by overgrowth of Ulva species are an increasing problem in tropical areas. The effect of dissolved nurtrients on uptake rates, growth, chlorophyll, and tissue nutrient concentration of Ulva reticulata was examined in laboratory experiments lasting up to 7 d. Sterile seawater was enriched with nitrate, ammonium, phosphate, ammonium + phosphate and nitrate + phosphate. U. reticulata expressed luxury uptake of both nitrogen (N) and phosphorus (P). The maximum N-uptake rate was found when ammonium was added alone. The maximum relative growth rate was about 15.1% per day but this was in the nitrate-fed algae not the ammonia-fed algae. N-enrichment resulted in an increase in chlorophyll concentration on day 4 and a decrease on day 7, probably as a result of cell division. P-enrichment resulted in an increase in chlorophyll concentration on day 4 and a decrease on day 7, probably as a result of cell division. P-enrichment had no significant effect on chlorophyll concentration. Treatments with added N, P or N+P showed significant increase in tissue N and P content on day 4. On day 7, N content in macroalgal tissue decreased while P content continued to increase. U. reticulata responded most strongly to added N; responses to P were much lower than for added N and there was little or no evidence for an additive effect of N+P. The N:P ratio of U. reticulata of control material suggested that N was the most limiting nutrient at the collection site (Paklok, Phuket, Thailand).