| Summary: | Total annual production of Amphibolis antarctica (Labill.) Sonder ex Aschers. and Posidonia australis Hook.f. were measured for monospecific meadows at adjoining sites in Shark Bay during 1982–1983 and values of 1.6 and 1.0 kg dry weight m−2, respectively, were obtained. Amphibolis antarctica had maximum production in summer, coincident with high light intensities and temperatures, whereas P. australis showed no clear seasonal pattern. Both species had maximum specific growth rates in summer. There were fluctuations in plant tissue concentrations of nitrogen and phosphorus, but standing-stock values for total nitrogen and total phosphorus were relatively constant in the meadows. Amounts of nutrients incorporated into new growth were 49.7 mg total nitrogen and 3.6 mg total phosphorus m−2 day−1 for A. antarctica, and 25.6 mg total nitrogen and 4.3 mg total phosphorus m−2 day−1 for P. australis. The dominance of A. antarctica in Shark Bay, a nutrient-poor environment, is reflected in its higher productivity and this may be related to its more efficient utilization of phosphorus.
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