ONLINE ISSN 1821-7249Kappaphycus alvarezii and Eucheuma denticulatum were cultivated under two treatments: with & without additional nutrients in Uroa, Zanzibar. The seaweeds were cultivated for 8 weeks (total of 15 weeks) in contrast to the 4-6 weeks normally used by farmers. Water column nutrients were 8-17μM total ammonia nitrogen & 2-38 μM soluble reactive phosphate. There were no significant differences in specific growth rates of fertilised & unfertilised seaweeds (P>0.05). However, during heavy rains & high surface seawater temperatures there were significant differences between the two treatments for both species (P<0.05). Whereas seaweed growth rates seemed to be affected by protracted periods of rain and a hot season of 31 0C maximum, fertilised K. alvarezii was more affected by the hot season than the unfertilised plant. For short rains and cooler months (27 0C maximum) both treatments showed similar rates. No significant differences (P>0.05) were found in carrageenan yield, dry matter, iota carrageenan percentage, and viscosity between fertilised and unfertilised treatments. Seasonal variations showed lower carrageenan yield & dry matter during heavy rains and higher values at the end of rainy season. Seaweed farmers in Tanzania could use a lower stocking density for higher seaweed biomass production & cultivate seaweeds for longer periods to satisfy customers in the carrageenan industry if so desired.