Effects of drift macroalgae and light attenuation on chlorophyll fluorescence and sediment sulfides in the seagrass Thalassia testudinum

We examined fluorescence parameters, chlorophyll indices, and sediment sulfides in the leaves of the subtropical seagrass Thalassia testudinum in relation to shading by drift macroalgae and artificial light reduction in a shallow meadow near Port Aransas, Texas, USA. Experiments conducted during a 5-week period in autumn 2003 included three cages in 0.7 m water depths covered with neutral density screening to achieve four light treatments: a control at full insolation (Io) and at 10%, 33%, and 50% Io. Seagrasses in six remaining cages were covered with a 50% cover of drift macroalgae similar to that observed in situ. Effective quantum yield (Y), non-photochemical quenching (NPQ), and minimal fluorescence (Fo) at the apex and base portions of six leaves were measured under the four different light regimes and under the drift algal treatment once a week for four weeks. Values of fluorescence quantum yield in older distal segments were lower than in proximal younger segments in all treatments, likely reflecting differences in tissue age. Y decreased at the leaf apex in both the light reduction and drift macroalgae treatments following the first week of the study. At the end of two weeks Y had also decreased at the leaf base and continued to decrease in both leaf areas over the course of the experiment in all treatments relative to the control. Shading by drift algae and artificial light reduction also induced increases in NPQ. The increase was highest at the 10% Io treatment. Fo values were stable beneath the drift macroalgae treatment but increased in plants subjected to artificial light reduction. Leaf chlorophyll (chl) a content, measured in each treatment at the end of the study, ranged from 4 to 5.2 mg chl g dry weight− 1. Chl a increased at 50% and 33% Io, showed no change in the drift macroalgal treatment, but decreased significantly at 10% Io. Chl a : b ratios at 10% Io (1.3) were also significantly lower than all other treatments (range 1.9 to 2.2), including the control. The changes in pigment concentrations under various regimes of light reduction suggest various degrees of photoacclimation relative to controls and plants shaded by macroalgae. Coincident with the changes in Y and NPQ, sediment porewater sulfide concentrations increased in drift macroalgal and artificial light reduction treatments, and were inversely correlated to light. Highest sulfide values were measured in the 10% Io treatment but were significantly different at each experimental light level. These preliminary results suggest that chlorophyll fluorescence is a useful and non-destructive indicator of seagrass condition that has potential as a monitoring tool for management of an important but threatened coastal resource.