Estimating Biomass from the Cover of Gelidiella acerosa along the Coasts of Eastern Philippines

R. N. Rollon, M. S. Samson, M. Y. Roleda, K. G. Araño, M. W. B. Vergara, and W. Y. Licuanan


In three Gelidiella acerosa beds on the poorly-studied eastern coasts of the Philippines, cover and biomass of this commercially important agar-producing red alga were measured. The three selected G. acerosa beds represented low (mean cover < 5%), medium (approximately 10%) and high cover (> 20%) vegetation. In each of the sites, we estimated the cover of G. acerosa fronds within 10 x 10 cm grids (n ranged from 33–36) in various strata. All G. acerosa fronds within each grid were harvested, cleaned of attached epiphytes and sediments, and subsequently, oven-dried at 60 °C to constant weight. We applied regression analysis to establish empirical relationships between cover and biomass. All linear regression lines were highly significant (p < 0.001, R2 > 0.6) indicating that biomass could be confidently estimated from cover estimates: low cover (y = 0.0171x + 0.310;R2 = 0.67, p < 0.0001), medium cover (y = 0.0363x + 0.118, R2 = 0.83, p < 0.0001) and high cover (y = 0.0304x + 0.481, R2 = 0.80, p < 0.0001). Evidently though, sites with higher overall mean cover (also longer thalli) had significantly steeper regression slopes, which was most pronounced when comparing sites with the lowest (< 5%) and highest (> 20%) mean cover. Nevertheless, we subsequently projected a minimum annual yield from the known 51 ha along the eastern coasts amounting to, at a minimum, 23 t dw y−1. Such yield may translate to approximately 68.8 million pesos (US$ 1.38 million) in gross sales of agar.

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