Multiple Interaction of Factors in the Distribution of Some Hawaiian Gelidiales (Rhodophyta)

Abstract

The biomass distribution of the three most common species of Gelidiales on three reefs of Oa'hu was found to form zones parallel to the shore correlated with the changing values of light intensity and water movement. Pterocladia caerulescens was restricted to the nearshore margin of reefs, tolerating intermediate intensities of water movement and some 30 to 100 percent of the incident light. Gelidiella acerosa occurred on the central part of the reefs, and while having similar light tolerances had a lower water movement optimum. Toward the seaward edge of the reef P. capillacea was restricted to areas with high water movement and much lower incident light (down to 6 percent). Thallus size and horizontal distribution of the two species of Pterocladia and biomass of all-three species had a seasonal cycle with a maximum during December and a minium in May. All the biological cycles correlated significantly with seasonal changes in light intensity and water movement but did not relate to the seasonal changes of water temperature and salinity.

Laboratory experiments tested: the' effects of five single factors and nine types of interactions on the growth and bleaching of the three species of Gelidiales. Results indicate that water movement and light intensity are indeed the factors regulating growth and bleaching of these algae in the field. Salinity and temperature attained statistically significant effects only at values exceeding those found in the field. In all experiments water enrichment compensated for water movement as all three species attained maximum growth at comparatively lower water movement intensities when grown in fertilizer-enriched media. The enhancement of diffusion resulting in the laboratory from higher water movement, frequent water renewal or greater enrichment counteracted the bleaching effects of high light intensity and high temperature. This multiple interaction seems to have major ecological influence regulating pigment concentration, growth rate, and distribution of Hawaiian Gelidiales.