Correlation between Gracilaria parvispora (Rhodophyta) biomass production and water quality factors on a tropical reef in Hawaii

The factors controlling the growth of the edible, red seaweed, Gracilaria parv ispora Abbott (long ogo), on the south reef of Molokai, HI, were investigated to determine where productive new plantings could be located. Experiments were conducted in October, 1997, and March and June, 1998, in which G. parvispora biomass production was correlated with water quality factors measured at six sites over each 21-day experiment. Water motion, temperature, salinity, nitrate and phosphate varied among sites and experiments, but were not significantly ( P > 0.05) correlated with growth. A strong correlation, however, was found between biomass production and ammonia concentration ( r = 0.91, P < 0.001). Ammonia levels ranged from 0.2–4.0 mmol m –3 over sites and experiments but were skewed towards low values, as was biomass production. The transient nature of ammonia distribution on this reef explains the patchy distribution of locations at which G. parvispora is productive, noticed in previous experiments. Large-scale Gracilaria culture on such a reef would require adding an external source of fertilizer, which may disrupt the reef ecology. An alternative is to develop a dispersed form of cultivation at sites that receive ammonia enrichment from the land, in which case the crop can help absorb excess nutrients entering the reef.