Date Published: April 11, 2019
Publisher: Public Library of Science
Author(s): Martin Søndergaard Jørgensen, Rodrigo Labouriau, Birgit Olesen, Silvia Mazzuca.
Seed burial in the sediment is critical for successful seedling establishment in seagrasses because it protects from predation and dispersal into unsuitable sites, and it may enhance germination by exposing the seeds to suitable germination stimuli. However, relatively little is known about the fate of buried seeds and their ability to emerge from greater depths. The goal of this study was to determine seed survival in the sediment, seedling emergence success and initial seedling biomass of Zostera marina in relation to burial depth and to evaluate if large seeds, having larger energy reserves, are more tolerant to burial than small seeds. Seeds from a perennial Z. marina population were buried at 7 different sediment depths (0.1–8 cm), and seeds sorted by size (large and small) were buried at depths of 2, 4 and 6 cm in outdoor mesocosms. Total seedling emergence after 2 months was significantly affected by seed burial depth, with maximum values in the top 2 cm of the sediment (48.1–56.7% of planted seeds), and a marked decline below 4 cm depth to only 5% seedling emergence at the deepest burial depth of 8 cm. Moreover, seeds had shorter time to emergence from shallow compared to deep burial depths. At all burial depths, a small fraction of seeds (<10%) died after germination but before emerging, and 15–30% remained viable after 6 months. Seed mortality was the major limitation to seedling recruitment from the deeper burial depths. The effect of seed size on seedling emergence success and time was not clear, but heavier seeds displayed greater longevity and gave rise to seedlings of significantly higher biomass, indicating that the mobilization of metabolic reserves may be important during initial seedling development.
Zostera marina (eelgrass) is a widespread and often dominant seagrass species in coastal and estuarine ecosystems across the northern hemisphere, where it fulfils important roles as ecosystem engineer and service provider [1,2]. Widespread seagrass declines have led to increased awareness of the need for conservation and restoration of these vulnerable ecosystems and their capacity to recover from impacts associated with environmental pressures [3–5].
Like most seagrass species, Z. marina is regularly found at highly disturbed sites, where sediment dynamics caused by hydrodynamic forces or activity of burrowing animals may influence adult plant performance and re-establishment from seeds [12,28,52,53]. In this study, experimental burial of seeds indicated that seedling recruitment from Z. marina seed banks may occur from a sediment depth of 8 cm, which is substantially deeper than observed for seeds of other submerged angiosperm species (1–3 cm) [41,54]. However, only a small fraction of seeds below 4 cm depth emerged successfully, and increased seed mortality suggests that a part of the seed bank may be lost permanently due to burial. The optimum depth for successful recruitment was within the top 2 cm, which is in accordance with previous studies [20,21]. While moderate burial generally is considered advantageous for seedling recruitment, seeds located on the sediment surface germinate slower  and are more vulnerable to losses caused by export to unfavourable sites, uprooting due to poor anchorage  and predation [26,27].