NITROGEN AND PHOSPHORUS UPTAKE IN SEEDLINGS OF THE SEAGRASS AMPHIBOLIS-ANTARCTICA IN WESTERN-AUSTRALIA

The uptake of nitrate, ammonium and phosphate was examined in vitro in seedlings of the seagrass Amphibolis antarctica ((Labill.) Sonder ex Aschers.). Uptake of all three nutrients was significantly correlated with external concentration up to 800 mu g l-1. The uptake of nitrate (0-200 mu g NO3 -N g dry wt-1 h-1) was significantly lower than the uptake of ammonium (0-500 mu g NH4 -N g dry wt-1 h-1), suggesting that the seedlings have a higher affinity for this form of nitrogen in the water column. Data were in general agreement with uptake rates recorded for other seagrasses, notably Zostera marina. In comparison to the dominant macroalgae for the same region, seedlings had either similar or higher uptake rates in relation to external concentration, lending support to the hypothesis that seedlings, which do not possess roots, behave like macroalgae in terms of nutrient acquisition from the water column. A comparison with literature data on adult seagrass, however, that seagrasses show lower uptake rates than macroalgae suggesting that the macroalgae, which are totally reliant on the water column for nutrients, are more efficient at uptake than seagrasses which may potentially use the sediment for a nutrient source.