The effects of in situ light reduction on the growth of two subtropical seagrasses, Thalassia testudinum and Halodule wrightii

The effects of in situ light reductions on two species of subtropical seagrasses, Thalassia testudirum (reduced to 14% and 10% of surface irradiance; SI) and Halodule wrightii (reduced to 16% and 13% SI) were examined over a 10-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 warner 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−1 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 environment.

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