Experimental analysis of the response and recovery of Zostera marina (L.) and Halodule wrightii (Ascher.) to repeated light-limitation stress

Seagrasses are considered important indicators of decline in water quality resulting in increased light attenuation that negatively influences their growth and survival. Chronic light-limitation interspersed with unpredictable acute attenuation events have had poorly understood effects on seagrass recovery dynamics. Zostera marina (eelgrass) and Halodule wrightii (shoalgrass) were subject to a matrix of light-deprivation events followed by recovery periods to mimic repeated acute shading events. Plant survival, morphology, biomass, chlorophyll content, and Fv/Fm were assessed over time to determine recovery. At the end of the experiment, all plants were harvested and species-specific treatment effects were determined. Significant differences due to treatments were noted in all parameters measured. In general, responses were similar for both life-stages and between species, suggesting similar physiological tolerance to repeated acute light-attenuation events. Only plants in treatments where light-deprivation was followed by a recovery interval of at least the same duration showed signs of long-term survival. Chlorophyll fluorescence (Fv/Fm) was an important metric for assessing recovery, but it failed to detect the onset of mortality in many plants. Other metrics of plant condition need to be assessed and coupled with chlorophyll fluorescence data to assess seagrass “health”. This is of particular importance in field studies, where the history of the plants is largely unknown.