Effect of in situ light reduction on the maintenance, growth and partitioning of carbon resources in Thalassia testudinum banks ex König

Abstract

The effects of in situ light reduction on the subtropical seagrass, Thalassia testudinum Banks ex König, in Corpus Christi Bay, Texas were examined from April 1993 to August 1994. The annual quantum flux at the seagrass canopy level was 5207 mol · m −2 · yr −1 or 46% of surface irradiance (SI) compared with two manipulated treatments that reduced underwater light to 1628 mol · m −2 · yr −1 (14% SI) and 864 mol · m −2 · yr −1 (5% SI). All plants subjected to 5% SI died after 200 days and over 99% of the plants receiving 14% SI died by the end of the experiment (490 days). Blade widths of plants in the control groups ranged from 6.4 to 7.0 mm, but decreased within months to 4.7 mm in both treatment group as a result of light reduction. Leaf production rates were significantly higher in control plants compared to plants within the 14 and 5% SI treatment groups, with all plants showing a seasonal trend of high productivity in July and low productivity in April. Blade chlorophyll (chl) concentrations increased, while the chl a- b ratio decreased with reduced light level. Soluble carbohydrate carbon content of controls was highest in rhizomes (102–152 mg C · g −1 dry wt.) and was relatively low in leaves (50–66 mg C · g −1 dry wt.) and in roots (57–74 mg C · g −1 dry wt.). In both light treatment groups, rhizome carbohydrate carbon content was 50% lower and leaf carbohydrate carbon content was about 15% lower than in controls, while the root carbon content did not differ significantly between the treatment groups and the controls. Ammonium and sulfide concentrations of pore water in the shaded cages were significantly higher than in control cages. We conclude that indices of shoot density, blade width, leaf growth, chl a: b ratio and blade chlorophyll content may be important early warning indicators of chronic underwater light stress in T. testudinum. A quantum flux of 1628 mol · m −2 · yr −1 (14% SI) was insufficient to maintain a positive carbon balance in T. testudinum in this bay system.