Abstract
Reduced light availability is a leading cause of seagrass declines worldwide. Light deficiency can be chronic or episodic, where pulsed light stress is punctuated by periods of optimal light. Seagrass resilience to light stress is likely modulated by the durations of light stress and the level of light reduction, yet remains poorly understood. We used a laboratory experiment to examine the response and recovery of Zostera marina to 16 weeks of three types of light disturbance: (i) continuous (C; constant shade), (ii) episodic low (EL; cycles of 4 days shade:3 days no shade), and (iii) episodic high (EH; cycles of 12 days shade:2 days no shade), each implemented at three shade levels (40, 60, and 80% shade). Declines in shoot density and biomass began after 8 weeks and were highest in C and EH disturbance in 60 and 80% shade. Declines were minimal in 40% shade across all light disturbances and in EL across all shade levels. Sheath length responded mainly to shade level, initially increasing in 60 and 80% shade before decreasing. Rhizome carbohydrates initially declined in all light disturbances with 80% shade but did so earlier in C and EH disturbance, and then increased once shoot density decreased. When ambient light was restored, physiological measures recovered, but shoot density, biomass, and morphology did not. Our study showed that shade level strongly modulated Z. marina responses to light disturbance. Not only will chronic high light reduction have negative impacts but pulsed light disturbance can as well, especially when light-stress periods are long and light reduction high.
Highlights
Light availability is an important environmental factor that influences the distribution and performance of seagrasses, in part because of their high light requirement relative to other aquatic primary producers (Duarte 1991; Lee et al 2007)
Shoot density decreased significantly in all light disturbance regimes after 8 weeks relative to the control, which increased at 8 weeks before returning to initial density (GAMM regression: R2 = 0.708; smoother terms: df = 1–4.7, F = 4.4–61.2, p < 0.0001 to 0.03; Fig. 2a, b; Electronic supplement S1)
When shading was low (40%), declines in shoot density across all light disturbance types were of less magnitude than observed at higher shade levels (Fig. 2b), with density being lower in continuous disturbance than in episodic low or high disturbance
Summary
Light availability is an important environmental factor that influences the distribution and performance of seagrasses, in part because of their high light requirement relative to other aquatic primary producers (Duarte 1991; Lee et al 2007). Seagrasses employ several strategies to maintain carbon balance during low light conditions caused by both chronic and episodic light disturbance They can increase chlorophyll concentration and decrease chlorophyll a to b ratio and maximum electron transport rate to maintain photosynthesis (Ralph et al 2007; Ochieng et al 2010; McMahon et al 2013). They can maintain carbon balance by reducing growth, leaf length, number of leaves, and shoot density (Ralph et al 2007; Lee et al 2007; McMahon et al 2013) and by mobilizing stored non-structural carbohydrates (Alcoverro et al 1999)
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