Seasonal Response and Recovery of Eelgrass (Zostera marina) to Short-Term Reductions in Light Availability

Anthropogenic activities in the marine nearshore, such as dredging and construction, can significantly reduce light availability for seagrasses through increased turbidity. Seagrass resilience to such shading depends on the seasonal timing of the event, as water temperature and photoperiod strongly influence plant response to disturbance. We examined the response and recovery of Zostera marina to 9 weeks of low and moderate light reduction (35 and 64% reduction) implemented in situ during the spring/mid-summer growth and late summer/fall senescence periods. Fall shading had the most severe impacts, with shoot densities of both low and moderate shade plants decreasing faster to lower densities relative to controls (unshaded plants), than observed during spring shading. Moderate shade plants in the fall also had high leaf loss, reduced growth and chlorophyll content, and utilized stored rhizome water-soluble carbohydrates, whereas in the spring, they had lower leaf loss, increased growth and chlorophyll content, and did not utilize stored carbohydrates. Although most physiological aspects of both spring and fall shaded plants recovered rapidly with restoration of ambient light, shoot density did not, remaining lower than unshaded plants into the next growing season. The stronger impact of fall shading resulted from the decreased photoperiod and daily light saturation, which caused plants to drastically alter their morphology and density to maintain carbon balance. Our study shows that Z. marina resilience to light reduction depends on its seasonal timing, suggesting that nearshore activities affecting light availability should be conducted during periods of maximum plant resilience.