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The macroalga Dictyosphaena cavernosa grows profusely on the reef slopes of Kane'ohe Bay, Hawai'i (USA), despite low inorganic nutrient concentrations in the water column. When cultured outdoors in laboratory contamers supplied with flowing seawater from the Kane'ohe Bay water column, the growth of thalli was limited by inorganic nitrogen availability, even though water motion in the containers was greater than in the field. Approximately 1 pM DIN (dissolved inorganic nitrogen) was required to sustain growth during 10 to 12 d experimental runs, but Kane'ohe Bay water column DIN concentratlolls are usually less than 0.5 pM, too low to sustain growth in the laborator){. The unusual morphology of D. cavernosa thalli suggested a possible mechani.sm for the acquisition of sufficient DIN for sustained growth in the field. Thalli are anchored to living or dead coral colonies, and form thick canopies spanning the substratum. The enclosed spaces or chambers beneath thalli isolate volumes of water over sediment patches, and prevent sediment-derived nutrients from mixing into the overlying water column Ammonium concentrations are nearly twlce as high, and nitrate+nitrite concentrations are 10 times hlgher within D. cavernosa chambers than in the adjacent water column. The DINenriched water within chambers exceeds the minimum DIN requirement of the alga for sustained growth. Results from a field experiment indicated that growth rates of D. cavernosa thalli exposed to both DIN released from sediments and DIN from the water column were significantly higher than growth rates of thalli exposed only to DIN from the water column. Experimental D. cavernosa thalli utilized ammonium and, to a lesser degree, nitrate for growth, but organic nitrogen (urea) enrichment had no effect on growth. Results from tissue nutrient analyses suggested that the nitrogen storage capacity of D. cavernosa is very low; the alga appears to require a continual supply of DIN, at a concentration greater than that of the water column, to sustain growth.

Nitrogen-limited growth in the coral reef chlorophyte Dictyosphaeria cavernosa, and the effect of exposure to sediment-derived nitrogen on growth