EFFECTS OF PROLONGED SHADING STRESS ON GROWTH AND SURVIVAL OF SEAGRASS POSIDONIA-AUSTRALIS IN JERVIS-BAY, NEW-SOUTH-WALES, AUSTRALIA

Experimental shading reduced the light reaching a shallow Posidonia australis meadow in Jervis Bay, New South Wales, Australia, to less than 10% of incident light. Shaded seagrass had significantly lower leaf growth rate, shoot density, shoot weight and epiphyte weight than seagrass in control plots. The structure of the epiphyte community also changed under shade as the fleshy macroalgae disappeared during the first month of shading and the leaves were dominated by encrusting invertebrates. The magnitude of the decrease in shoot number and leaf growth varied among 3 experiments at different times of year. Shading in early summer had a more severe effect on the seagrass than shading at the end of summer. There was no significant recovery of P. australis in the shaded plots during 17 mo following removal of the shades.