Abstract
Restoration of seagrass beds through seedlings is an alternative to the transplantation of adult plants that reduces the impact over donor areas and increases the genetic variability of restored meadows. To improve the use of Cymodocea nodosa seedlings, obtained from seeds germinated in vitro, in restoration programs, we investigated the ammonium and phosphate uptake rates of seedlings, and the synergistic effects of light levels (20 and 200 μmol quanta m-2 s-1) and different nitrogen to phosphorus molar ratios (40 μM N:10 μM P, 25 μM N:25 μM P, and 10 μM N:40 μM P) on the photosynthetic activity and growth of seedlings. The nutrient content of seedlings was also compared to the seed nutrient reserves to assess the relative importance of external nutrient uptake for seedling development. Eighty two percent of the seeds germinated after 48 days at a mean rate of 1.5 seeds per day. All seedlings under all treatments survived and grew during the 4 weeks of the experiment. Seedlings of C. nodosa acquired ammonium and phosphate from the incubation media while still attached to the seed, at rates of about twice of adult plants. The relevance of external nutrient uptake was further highlighted by the observation that seedlings’ tissues were richer in nitrogen and phosphorus than non-germinated seeds. The uptake of ammonium followed saturation kinetics with a half saturation constant of 32 μM whereas the uptake of phosphate increased linearly with nutrient concentration within the range tested (5 – 100 μM). Light was more important than the nutrient ratio of fertilization for the successful development of the young seedlings. The seedlings’ photosynthetic and growth rates were about 20% higher in the high light treatment, whereas different nitrogen to phosphorus ratios did not significantly affect growth. The photosynthetic responses of the seedlings to changes in the light level and their capacity to use external nutrient sources showed that seedlings of C. nodosa have the ability to rapidly acclimate to the surrounding light and nutrient environment while still attached to the seeds. C. nodosa seedlings experiencing fertilization under low light levels showed slightly enhanced growth if nourished with a balanced formulation, whereas a slight increase in growth was also observed with unbalanced formulations under a higher light level. Our results highlight the importance of high light availability at the seedling restoration sites.
Highlights
Seagrass meadows are invaluable marine habitats considering the myriad of goods and services they provide to the overall functioning of coastal systems
Our study showed that light was more important than water column nutrients for the successful development of young seedlings of C. nodosa
The use of seedlings to enhance the recovery or genetic diversity of C. nodosa meadows will be more efficient in shallow areas with high light levels
Summary
Seagrass meadows are invaluable marine habitats considering the myriad of goods and services they provide to the overall functioning of coastal systems. Compared to the transplantation of adult plants, the use of seeds (or in vitro germinated seedlings) in seagrass restoration constitutes an effective alternative, or complementary, re-plantation method that reduces the impact over donor areas, requires less labor and time, is less expensive, and increases the genetic variability within restored meadows (Ackerman, 2006; Ailstock and Shafer, 2006; Zarranz et al, 2010). Seed- and seedlingbased restoration has been considered more suitable for largescale seagrass restoration projects (Williams and Orth, 1998; Harwell and Orth, 1999; Granger et al, 2000; Orth et al, 2008) This restoration practice presents several bottlenecks (Statton et al, 2017), one of them being the low rate of seedling survival and establishment (less than 10% of the seeds placed in the field) (Orth and Moore, 1983; Orth et al, 2003), high rates of success have been observed (Balestri et al, 1998; Balestri and Lardicci, 2012). Due to the vulnerability of seedlings to environmental stressors during the initial phases of development, the successful development of viable beds depends upon several factors, including sediment burial, light, nutrients and initial patch size (Duarte and Sand-Jensen, 1990; Moore et al, 1993; Celdrán and Marín, 2013)
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