Effects of in situ nitrogen and phosphorus enrichment of the sediments on the seagrass Heterozostera tasmanica (Martens ex Aschers.) den Hartog in Western Port, Victoria, Australia

Abstract

The rhizosphere of Heterozostera tasmanica (Martens ex Aschers.) den Hartog was enriched in situ with ammonium and/or phosphate at a rate of 100 g N · m −2 and 20 g P · m −2 each month from August (winter) through February (late summer) in a 2 × 2 factorial experiment. Ammonium enrichment resulted in increased levels of total nitrogen in the rhizomes (from 0.83 to 1.03% of dry wt) and leaves (from 1.63 to 1.82%) and caused a 20% increase in leaf growth rate. Phosphate enrichment resulted in increased levels of total phosphorus in the rhizomes (from 0.16 to 0.18%), but no increase in total phosphorus in the leaves nor any change in the leaf growth rate. Neither ammonium nor phosphate enrichment caused a change in above sediment standing crop or shoot density. It is suggested that nitrogen limits the growth rate of H. tasmanica plants in northern Western Port during spring and early summer, but that an increase of 5 to 100 times the control level of nitrogen and phosphorus in the rhizosphere of these plants appears to have little direct effect on their growth or on standing crop during the first year of enrichment.