PULSE FEEDING WITH NITRATE AND PHOSPHATE IN RELATION TO TISSUE COMPOSITION AND NUTRIENT-UPTAKE BY MACROALGAE FROM THE RED-SEA AT GHARDAQA (EGYPT)

This investigation aimed at determining the effects of experimental en richment with either N or P or both elements on the tissue nutrient le vels, and on the internal N:P molar ratios of three macroalgal species collected in the vicinity of Ghardaqa, Red Sea, Egypt. The results ob tained showed that the rates of nutrient (nitrate and phosphate) uptak e were highly species dependent. Uptake rates of both nutrients were s ignificantly higher for Codium than for either Dictyota or Galaxaura. For the three algae, uptake, except nitrate by Galaxaura, was rapid du ring the first 6 h of immersion and declined during the rest of the ex periment. The observed increases in N levels after 6 h of enrichment d ecreased sharply after 24 h of enrichment. The fluctuations in tissue- N were accompanied by fluctuations in protein levels. Total carbohydra te content of the three macroalgae was decreased in seawater treatment s enriched with either N or N+P for 24 h compared with non-enriched pl ants or plants enriched with P only. For the three macroalgae, enrichm ent with P resulted in consistent increases in tissue-P levels over th e experimental period. The changes in the N:P ratios of the three plan ts were significantly correlated with the increases in tissue -P as co mpared to tissue -N. G. rugosa appeared to be the most strongly nutrie nt - limited of the three algal species studied and D. dichotoma was t he least.