INFLUENCE OF AMMONIUM-N PULSE CONCENTRATIONS AND FREQUENCY, TANK CONDITION AND NITROGEN STARVATION ON GROWTH-RATE AND BIOCHEMICAL-COMPOSITION OF GRACILARIA-GRACILIS

The growth rate of Gracilaria gracilis maintained in tanks at an abalo ne farm near Port Elizabeth, South Africa, was examined under various tank conditions and NH4-N pulse frequencies and concentrations. This w as accompanied by analyses of the components of the internal nitrogen pool. A maximum growth rate of ca. 35% wk(-1) was obtained at 1200 mu M NH4-N. The alga was able to grow at non-nitrogen limited rates using only internal stored nitrogen to sustain growth for one week before t he growth rate decreased to ca. 17% per week. NH4-N pulse frequency di d not affect growth rate but one pulse per week led to a marked decrea se in total-N, protein, phycoerythrin and chlorophyll-a content. An in crease in pulse frequency to two pulses per week doubled the protein c ontent from 2.351 +/- 0.143 to 4.453 +/- 0.090% (per unit dry mass). C arbohydrate content was inversely related to nitrogen storage. The gro wth rate in fouled tanks was always lower than in clean tanks. It seem s likely that seaweeds and diatoms colonising the tank sides reduced l ight reflected off the inside of a tank, thereby reducing the growth r ate.