Nutrient-limited growth and sources of nutrients for coral reef macroalgae

Net productivity of benthic macroalgae is often high in coral reef systems, yet the rates at which nutrients are supplied from oligotrophic water columns may be insufficient for sustained growth. Eleven species of macroalgae from reefs in oligotrophic Kaneohe Bay, Hawaii were cultured at the same nutrient concentrations as the water column. Nine species did not sustain positive growth. Alternative hypotheses were proposed to explain this observation: (1) access to supplementary, benthic nutrient sources is required; (2) nutrients are supplied from the overlying water column in brief pulses of sufficient concentration and duration to sustain growth; (3) sufficient nutrients for growth are provided by the water column if average flow rates are high. These hypotheses were tested in lab and field experiments using the dominant macroalga in Kaneohe Bay, Dictyosphaeria cavernosa.; The first hypothesis was examined by measuring nutrient efflux from sediments and excretion by epifaunal invertebrates. Positive efflux of inorganic nitrogen and phosphorus was measured from sediment patches. Epifauna from D. cavernosa excreted inorganic nitrogen and phosphorus. Nutrient from these sources can enhance growth, as indicated by culture experiments. Results of a field experiment confirmed that nitrogen efflux from sediments beneath D. cavernosa thalli enhanced growth.; The second hypothesis was not supported. Analyses of a 40 month data set of inorganic nutrient concentrations in the water column near a Kaneohe Bay reef slope indicated that nutrient pulses rarely occurred. Results of experiments with D. cavernosa indicated that nitrogen pulses would have to occur at 3-4 day intervals to sustain growth.; To test the third hypothesis, measurements of nitrogen and phosphorus uptake by D. cavernosa at varied water flow rates were used to estimate flow rates in the field at which advection from the water column supplies sufficient nutrients for sustained growth. Flow rates above 5 cm/s were predicted to provide sufficient nitrogen, and flow rates above 1 cm/s, sufficient phosphorus. The contributions of nutrients from different sources appear to change with changing environmental conditions; at sites with high flow rates, nutrients are supplied from the water column at sufficient rates for sustained algal growth; at sites with low flow rates, sediment efflux and invertebrate excretion may be required in addition to advection from the water column.