Seasonal growth of the coral reef macroalga Dictyosphaeria cavernosa (Forskål) Børgesen and the effects of nutrient availability, temperature and herbivory on growth rate

Abstract

The green alga Dictyosphaeria cavernosa (Forskål) Børgesen competes with corals for space on the slopes of patch and fringing reefs in Kane'ohe Bay, O'ahu, Hawai'i. Macroscopic algae rarely occur on pristine coral reef slopes, but are increasingly common at sites of anthropogenic nutrient input and overfishing. A field study of D. cavernosa growth rates indicated that, in the absence of grazing, an annual cycle occurs with maximum growth rates in fall and minimum growth rates in spring. This pattern corresponded more closely to annual changes in water temperature than to annual changes in irradiance or water column nutrient concentrations, yet laboratory experiments showed that growth rates do not vary with temperature within the annual range for Kane'ohe Bay reef habitats. Enrichment experiments conducted in outdoor laboratory tanks supplied with running seawater indicated that D. cavernosa growth is nitrogen-limited at nutrient levels measured in Kane'ohe Bay. Inorganic nitrogen and phosphorus levels are significantly higher in enclosed spaces or chambers created by the growth of D. cavernosa thalli than in the adjacent water column. It is hypothesized that nutrient regeneration from sediments beneath thalli, and/or excretion by animals inhabiting these chambers contribute to the elevated nutrient levels. Nutrient regeneration rates appear to be temperature dependent, and seasonal changes in nutrient availability beneath thalli may explain the annual growth pattern observed in D. cavernosa.