Seagrass and sediment nutrients: Species comparison and fertilisation responses of P. australis at Rottnest Island, Western Australia
Seagrasses, marine angiospenns with high rates of primacy productivity, are often hmned by the supply of nutrients or light. We investigated the ambient sedirncnt nutrient availability, biornass, growth and physiological characteristics of five seagrass species common around Rottnest lsland (P. australis, P. sinuosa, A. antarctica, A. griffithii and H. ovalis). Interstitial nutrient concentrations were relatively consistent between species and low compared to seagrass sediments around the world, ranging from 8.9 to 15.9 µM NH4+ and 2.0 to 3.4 µM PO;'. The majority of nutrients wen: adsorbed to sediment particles with concentrations ranging from 40 to 95 µmol NH; L I sediment and from 210 to 329 µmol PO4-3 L·' sediment. There was considerable variability between the plant parameters of the various seagrass species. Shoot density and total biomass varied between species by factors of 10 and 34, respectively, with H. ovalis having the largest shoot density and smallest leaf biomass. Leaf tissue nutrient content was highest in H. ovalis (3.0 %N, 0.23 %P) and lowest in P. sinuosa (1.4 %N, 0.13 %P). The two Amphibolis species had 3-4 times more total free amino acids than the two Posidonia species,with asparagine being the major amino acid in all species and fertilisation treatments. Addition of fertiliser to the sediment of P. australis had no effect on the seagrass canopy height, shoot density, biomass or growth. However, the %N and amino acid concentration in the leaves increased in response to nutrient additions (%N, 1.8%-2.1%; amino acids 1 µmol g-1 fresh wt- 7 µmo! g-' fresh wt.). Epiphyte biomass on P. australis leaves also increased in response to sediment nutrient additions. The results from the fertilization experiment suggest that nitrogen and phosphorous availability is not lirmtmg seagrass growth on Rottnest Island and suggest that any increase in the nutnent availability will stimulate epiphyte growth on the seagrass leaves and lead to decreased seagrass growth.