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Marine Ecology Progress Series

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MEPS 157:295-302 (1997)  -  doi:10.3354/meps157295

Effects of water velocity on NH4 and PO4 uptake and nutrient-limited growth in the macroalga Dictyosphaeria cavernosa

Scott T. Larned*, M. J. Atkinson

Hawai'i Institute of Marine Biology, PO Box 1346, Kane'ohe, Hawai'i 96744, USA

Dictyosphaeria cavernosa is a spatially dominant macroalga on coral reefs in Kane'ohe Bay, Hawai'i, USA, and occupies a range of habitats from low energy reef slopes to a high energy barrier reef flat. Previous studies demonstrated that D. cavernosa growth is limited by the availability of dissolved inorganic nitrogen (DIN) in Kane'ohe Bay, and that, on protected reef flats and slopes, the rate at which DIN is supplied to thalli from the water column is too low for sustained growth. Under these conditions, DIN released from sediments into the water-filled chambers beneath thalli is used for growth. At exposed sites such as the barrier reef, nutrient-rich sediments do not accumulate but D. cavernosa is abundant, suggesting that high levels of water motion supply nutrients from the water column to thalli at rates high enough for sustained growth. To test the hypothesis that nutrient acquisition by D. cavernosa increases with increasing water velocity, rates of NH4 and PO4 uptake were measured at a range of water velocities within the range measured in D. cavernosa habitats (0.02 to 0.13 m s-1). Rates of uptake for both nutrients were positively correlated with velocity and with concentration. Results from the uptake experiments were used to construct a simple model to predict the combinations of nutrient concentration and water velocity at which the nitrogen and phosphorus requirements for growth can be met by uptake from the Kane'ohe Bay water column. The model predicts that, at sites in Kane'ohe Bay where average water velocities are higher than 0.05 m s-1, DIN supplied to thalli from the Bay water column can support the specific growth rate measured in the field. At sites where average water velocities are less than 0.05 m s-1, thalli must utilize DIN supplied from benthic sources in addition to water column nitrogen to maintain growth rates at field levels. Similarly, at sites where water velocities are higher than 0.01 m s-1, PO4 supplied to thalli from the water column can support growth at field levels, while thalli at sites with lower water velocities must utilize PO4 supplied from benthic sources.

Coral reef · Dictyosphaeria cavernosa · Growth · Macroalgae · Nitrogen · Phosphorus · Uptake · Water velocity

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