Benthic macroalgae are often abundant and highly productive in oligotrophic coral reef systems. In these systems, the rates at which dissolved inorganic nutrients are supplied to macroalgae from the water column may be insufficient for long-term growth. Macroalgae from oligotrophic Kaneohe Bay, Hawaii were grown in culture at the same nutrient concentrations as the Kaneohe Bay water column, and did not sustain long-term positive growth. Alternative hypotheses were proposed to explain this observation. One is that nutrients are supplied from the water column in brief pulses that meet nutrient requirements of macroalgae for growth, but have little effect on long-term average nutrient levels. An alternative is that access to supplementary, benthic nutrient sources is required for growth. Nutrient efflux from reef sediments and excretion by benthic invertebrates may enrich the environments inhabited by benthic algae, compared to the overlying water column. A third alternative is that sufficient nutrients are provided by the water column if average flow rates are high. High water flow rates compensate for low nutrient concentrations by generating high advection rates (the product of water flow rates and nutrient concentrations). High flow rates also reduce the thickness of boundary layers surrounding algal thalli, reducing the resistance that boundary layers impose on nutrient transport. Therefore, the rates at which nutrients are acquired from the water column is expected to increase with water flow. Prior to testing the three alternative hypotheses, nutrient enrichment experiments were carried out to determine whether inorganic nitrogen (N) or phosphorus (P) limited the growth of 10 macroalgal species from Kaneohe Bay reefs. Growth rates in 9 of the 10 species were N-limited; the remaining species, Codium edule (Chlorophyta) was P-limited.

The first hypothesis, that nutrient pulses can sustain algal growth, was not supported. Pulses with concentrations >1 micro M N and >0.2 µM P rarely occur near Kaneohe Bay reefs, yet results of laboratory experiments with Dictyosphaeria cavernosa (Chlorophyta) indicated that such pulses would have to occur every 3 or 4 days for sustained growth.

Elevated N concentrations were measured within and beneath thalli of the mat-forming macroalgae D. cavernosa, Gracilaria salicornia (Rhodophyta) and Kappaphycus alvarezii (Rhodophyta). Elevated N and P concentrations were measured in the sediment porewater near the rhizophytic algae Caulerpa racemosa and C. seratularioides (Chlorophyta). N concentrations in these benthic microenvironments were high enough to sustain growth of the macroalgae that inhabit them, which lent support to the second hypothesis.

D. cavernosa is widely distributed in Kaneohe Bay, inhabiting environments ranging from patch reef slopes with low water motion (< 1 cm/s) to high energy reef flats (> 10 cm/s). D. cavernosa is therefore a useful species for examining the relationships between nutrient requirements, nutrient sources and water motion. In calm waters, D. cavernosa thalli cover sediment patches and slow the dissipation of nutrients released from sediments. Results of field experiments indicated that the efflux of nutrients from sediments beneath D. cavernosa thalli enhanced growth. Other benthic nutrient sources for D. cavernosa are the epifaunal invertebrates that colonize thalli. Epifaunal invertebrates released twice as much N per unit area of D. cavernosa thallus per day than sediment efflux, but only half as much P.

Fine, organic-matter rich sediments do not accumulate on the high energy reef flats inhabited by D. cavernosa, and thalli in these environments have lower epifaunal biomasses than at protected sites. Advection from the water column is probably the primary source of nutrients on high energy reef flats. N and P uptake rates by D. cavernosa measured at varied flow rates were used to estimate the flow rates in the field at which advection supplies sufficient nutrients for growth. Flow above 5 cm/s provided sufficient N, and flow above 1 cm/s provided sufficient P. Thus, the contribution of nutrients from different sources changes with changing environmental conditions; at protected sites with low flow rates, nutrients supplied from benthic sources are required for sustained growth, and at exposed sites with high flow rates, nutrient advection from the water column is sufficient.