Autumn biomass, below-ground productivity, rhizome growth at bed edge and nitrogen content in seagrasses from Western Australia

Below-ground biomass and productivity were determined on one occasion in au tumn for four local meadow-forming species of seagrass from the genera Posi donia and Amphibolis (P. sinuosa Cambridge and Kuo, P. australis Hook. f., A. antarctica (Labill.) Sender and Aschers and A. griffithii (Black) den Ha rtog. In P. australis and P. sinuosa from Cockburn Sound, 59 and 48%, respe ctively, of living material was below-ground. Detritus within the sediment was of the same magnitude or exceeded the living material. In Amphibolis sp ecies, there was a greater proportion of living material above-ground than below (A. antarctica 82% and A. griffithii 72%). Tissue nitrogen concentrat ions in leaves, roots and rhizomes were 5-17 mg N g(-1) dry weight, and wer e higher in leaves of Posidonia than Amphibolis. The total nitrogen content , on a m(-2) basis, was dominated by leaves in all species. The percentage of below-ground to above-ground autumn productivity varied from 15% in Amph ibolis to 25% in Posidonia. Rhizome growth at the edge of the meadow was pr edicted to be 796+/-79 mm per year for P. australis, 576+/-42 mm per year f or P. sinuosa, 365+/-49 mm per year for A. antarctica and 146+/-24 mm per y ear for A. griffithii, the first record of rhizome growth rates of these sp ecies in Western Australia. (C) 2000 Elsevier Science B.V. All rights reser ved.