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

Abstract

Below-ground biomass and productivity were determined on one occasion in autumn for four local meadow-forming species of seagrass from the genera Posidonia and Amphibolis ( P. sinuosa Cambridge and Kuo, P. australis Hook. f., A. antarctica (Labill.) Sonder and Aschers and A. griffithii (Black) den Hartog. In P. australis and P. sinuosa from Cockburn Sound, 59 and 48%, respectively, of living material was below-ground. Detritus within the sediment was of the same magnitude or exceeded the living material. In Amphibolis species, there was a greater proportion of living material above-ground than below ( A. antarctica 82% and A. griffithii 72%). Tissue nitrogen concentrations in leaves, roots and rhizomes were 5–17 mg N g −1 dry weight, and were 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 Amphibolis to 25% in Posidonia. Rhizome growth at the edge of the meadow was predicted to be 796±79 mm per year for P. australis, 576±42 mm per year for P. sinuosa, 365±49 mm per year for A. antarctica and 146±24 mm per year for A. griffithii, the first record of rhizome growth rates of these species in Western Australia.