THE EFFECTS OF IN-SITU LIGHT REDUCTION ON THE GROWTH OF 2 SUBTROPICAL SEAGRASSES, THALASSIA-TESTUDINUM AND HALODULE-WRIGHTII

The effects of in situ light reductions on two species of subtropical seagrasses, Thalassia testudinum (reduced to 14% and 10% of surface irradiance; SI) and Halodule wrightii (reduced to 16% and 13% SI) were examined over a IO-mo period (October 1992-September 1993) in relation to leaf elongation rates, sediment pore-water ammonium, and blade chlorophyll concentrations. No significant changes in pore-water ammonium levels were noted among treatments with time, but blade chlorophyll concentrations in both species were higher in plants exposed to the darkest treatments (10% and 13% SI) relative to controls exposed to 50% SI. In all treatments, blade chlorophyll concentrations were highest and chlorophyll a:b ratios lowest during the warmer months, coincident with higher water temperatures. Leaf elongation rates in T. testudinum plants decreased relative to unshaded controls after 1 mo of treatment in autumn, but no significant differences in leaf elongation were noted among treatments for H. wrightii in late autumn or winter when very low growth rates (<0.1 cm shoot(-1) d(-1)) were recorded. There were no differences between treatments during the spring growth period for T. testudinum (no data are available for H. wrightii), suggesting that growth (ca. 1 cm shoot-l d(-1)) was probably not related to available light but was supported by belowground reserves. After 10 mo of treatment, all H. wrightii plants at 13% SI (1,600 mol m(-2) yr(-1)) and 16% SI (2,000 mol m(-2) yr(-1)) disappeared from experimental plots; similarly, no T. testudinum plants exposed to 10% SI (1,300 mol m(-2) yr(-1)) remained, although 4% of the plants at 14% SI (1,800 mol m(-2) yr(-1)) survived nearly 12 mo of reduced irradiance. In neither species were leaf elongation rates, which showed little change among treatments, a reliable indicator of the underwater light