Light Stress Responses by the Eelgrass, Zostera marina (L)

Abstract

Zostera marina is the dominant seagrass species in the Northern Hemisphere where it grows in sheltered bays and estuaries. As a consequence of its distribution its conservation is commonly threatened by poor coastal water quality. The high minimum light requirements of seagrasses results in water quality degradation (high turbidity and eutrophication) being a significant risk. Bioindicators of light stress can be used to interpret seagrass responses to light limitation and therefore act as sentinels for conservation management. However, there exists limited experimental inter-comparison of the effectiveness of multiple individual bioindicator responses. Meta-analysis suggests that rhizome sugars, shoot C:N, shoot growth, and number of leaves per shoot provide the most consistent response variables to increasing light limitation in seagrass, but this premise remains largely untested at the plant level as a direct comparison of multiple bioindicators. The present study aimed to test the morphological, physiological, and photo-physiological bioindicator responses of Z. marina to light stress applied within controlled laboratory conditions. These bioindicators were used to assign minimum light thresholds. Growth rate and photophysiological parameters (alpha, E k , and ETR max) were rapidly (1st week) and drastically affected by low light shade treatments (20.12 µmol photons m −2 s −1 and lower). After 3 weeks at low light, significant reductions in maximum leaf length and leaf width were observed. Principal Component Analysis identified leaf length, shoot growth, shoot surface area, ETR max , E k , and alpha as having the strongest responses to reduced light. Shoot growth, ETR max , E k , and alpha were found to provide the best early warning of light limitation after 5-8 days. These results provide evidence for bioindicators of light stress in Z. marina and highlights the importance of understanding these responses for the successful management and conservation of this species.

Species-Specific Trait Responses of Three Tropical Seagrasses to Multiple Stressors: The Case of Increasing Temperature and Nutrient Enrichment

“…The study of the response of seagrass individual traits, namely biochemical, morphological, and physiological traits, serve as early indicators of environmental change (Roca et al, 2016) before population level responses, such as changes in shoot density, biomass and species composition or biodiversity loss, are detected. This approach has been used in a wide number of studies to detect rapid responses (within weeks) to different stressors (Lee et al, 2007;Roca et al, 2016;Bertelli and Unsworth, 2018, and previously cited references). One important limitation of trait-based responses is that different stressors could cause the same effect, for instance, lower rhizome carbohydrate content is both observed after nutrient over-enrichment or reduced light exposures (Roca et al, 2016).…”

“…One important limitation of trait-based responses is that different stressors could cause the same effect, for instance, lower rhizome carbohydrate content is both observed after nutrient over-enrichment or reduced light exposures (Roca et al, 2016). More recent studies that simultaneously assess different plant individual traits have highlighted the importance of testing the responses at different levels of organization in the plants (Bertelli and Unsworth, 2018;Mvungi and Pillay, 2019). Moreover, a deeper understanding of the species-specific responses to stressors and their interaction is important as different combinations of seagrass traits may sustain different ecological functions that upscale to the ecosystem level (Barbier et al, 2011).…”

Effect of depth gradient and temperature on Zostera marina vegetative and reproductive phenology in the first year of the 1997–1998 El Niño

“…We also recorded low vegetative biomass at +0.20 m (Table 2). Light is probably limiting the distribution of Z. marina at deeper sites (Cabello-Pasini et al 2002, Bertelli andUnsworth 2018), and desiccation is limiting its expansion to elevations above the tideline with longer air exposure (Nielsen et al 2002). Zostera marina confronts these conditions with high vegetative biomass and shoot density.…”

The response of the seagrass Halodule wrightii Ascherson to environmental stressors

“…It is expected that nutrient enrichment within the water-column results in increased turbidity and light attenuation, caused by an increase in epiphytic algae growth and eutrophication. Light limitation often results in a reduction of above-ground biomass exhibited by shorter, narrower leaves, with fewer leaves per shoot and becoming less dense at a meadow scale (Bertelli and Unsworth, 2018;Biber et al, 2005;Collier et al, 2012;Ochieng et al, 2010;Olesen and Sand-jensen, 1993;Yaakub et al, 2013). However, reduction in leaf length and elongation which is exhibited in many seagrass species as a response to light limitation does not always hold true for H. wrightii.…”