Annual primary production and nutrient dynamics of the seagrasses Posidonia sinuosa and Posidonia australis in south-western Australia

Abstract

Above-ground primary production and nutrient fluxes (N and P) were investigated for two species of seagrass, Posidonia sinuosa Cambridge et Kuo and P. australis Hook. f. from Warnbro and Cockburn Sounds over an annual cycle, at sites ranging in depth from 0.5–10 m where P. sinuosa formed either single-species stands or co-occurred with P. australis. Annual leaf primary production ranged from 600 to 900 g m −2 yr −1 in P. sinuosa and 900–1100 g m −2 yr −1 in P. australis, and epiphytes on the leaves produced 130–160 g m −2 yr −1. In some patches, flowering shoots and fruits also made a substantial contribution, up to 160 g m −2 yr −1. Annual above-ground productivity (dry weight production per unit ground area) of Posidonia spp. (600–1300 g m −2 yr −1) is similar to that of Amphibolis antarctica (Labill.) Sonder et Aschers. ex Aschers. and A. griffithii (Black) den Hartog, two species from the other genus of large seagrasses in south-western Australia, but only 30 to 50% of that of the kelp Ecklonia radiata (C. Ag.) J. Agardh. (3500 g m −2 yr −1). Nitrogen and phosphorus incorporated annually into new leaf tissue ranged from 9–17 g N and 1.1–1.7 g P m −2 yr −1, respectively, depending on species and site. Estimates of annual nutrient losses via leaf detritus ranged from 5–9 g N and 0.4–0.7 g P m −2 yr −1, compared to maximum losses of 1.2 g N and 0.4 g P m −2 yr −1 via the fruits at the highest density of flowering shoots (223 m −2). Thus, annual nutrient losses via leaf detritus represent a considerable proportion of the nutrients incorporated annually into new growth, indicating a lower degree of nutrient conservation than might be expected in a low nutrient environment.