The effects of herbivore exclusion and nutrient enrichment on growth and reproduction of Halimeda macroloba

Abstract

The questions on how Halimeda macroloba allocates resources to growth and sexual reproduction when encountering herbivores and elevated nutrient concentrations were experimentally examined in the natural subtidal zone and in water tanks. The field experimental design comprised 96 plots (50 × 50 cm2), with and without herbivores, and two nutrient levels. Cages were used to exclude fish herbivores: uncaged plots and fully caged plots; and 2 nutrient levels were manipulated: enriched and ambient levels in order to determine the effect of nutrients. Osmocote slow-release fertilizer was placed in each enriched plot. For the outdoor water tank experiments, mechanical clipping was used to simulate grazing on algae. Nutrient enrichment had a positive effect on the growth and reproduction of H. macroloba both in the field and water tank experiments, and herbivory or simulation of grazing did have a significant effect on the growth of algae in the water tank experiments. H. macroloba produced gametangia within 1-2 weeks after nutrient enrichment, to indicate that enrichment might be the trigger for reproduction of H. macroloba. From the tissue nutrient levels, the C:N, C:P, and N:P (18.9, 639, and 33.7) ratios indicated that the productivity of H. macroloba was phosphorus limited. Copyright © 2005 ScienceAsia.