Factors Influencing Seedling Establishment Rates in Zostera marina and Their Implications for Seagrass Restoration (original) (raw)
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Marine Ecology Progress Series, 2003
The influence of Zostera marina L. seed-density on germination and initial seedling success was investigated using seed-addition field experiments at 2 scales in the Chesapeake Bay region in 1999 and 2000. We first tested whether germination rates and initial seedling establishment were affected by initial seed-densities of 2.5, 25, 250, and 1250 seeds m -2 within 4 m 2 plots. We then tested whether plot size affects germination rates, following the hypothesis that rates of seed predation might be different in large and small plots. We broadcast seeds at a single density (500 seeds m -2 ) but at a much larger plot size (100 m 2 , or 25 times the size of the small plots). In the spring following seed broadcast, seedlings were present in most 4 m 2 plots (seedling densities of 0.6 to 15.4% of the number of seeds released in 1999, and 3.3 to 23.3% of those released in 2000) and in all 100 m 2 plots (4.3% to 13.9%). Seed-density effects were not significant in 1999 or 2000, while site effects were significant in both years. The percentages of seedlings in the larger plots were similar to those in the smaller plots. These results suggest that there were no density-dependent effects on germination and initial seedling establishment, and that within the size range of plots examined in this study, such processes are not likely to be scale-dependent. The significant differences among the sites may be related to micro-topographic complexities of the bottom caused by both biotic and abiotic factors that allow seeds to be retained close to where they settle. Our data, combined with previously published data on seed dispersal and patch dynamics, stress the importance of conserving existing beds, regardless of bed size and shoot density, since these are sources of seeds that may establish new patches. The data may also help in developing strategies for the restoration of denuded sites using seeds instead of adult plants.
Innovative Techniques for Large-scale Seagrass Restoration Using Zostera marina (eelgrass) Seeds
Restoration Ecology, 2008
The use of Zostera marina (eelgrass) seeds for seagrass restoration is increasingly recognized as an alternative to transplanting shoots as losses of seagrass habitat generate interest in large-scale restoration. We explored new techniques for efficient large-scale restoration of Z. marina using seeds by addressing the factors limiting seed collection, processing, survival, and distribution. We tested an existing mechanical harvesting system for expanding the scale of seed collections, and developed and evaluated two new experimental systems. A seeding technique using buoys holding reproductive shoots at restoration sites to eliminate seed storage was tested along with new techniques for reducing seed-processing labor. A series of experiments evaluated storage conditions that maintain viability of seeds during summer storage for fall planting. Finally, a new mechanical seed-planting technique appropriate for large scales was developed and tested.
Seedling establishment in eelgrass: seed burial effects on winter losses of developing seedlings
Marine Ecology Progress Series, 2012
Constraints on the transition of seeds to seedlings have the potential to control plant dispersal and persistence. We investigated the processes leading to low initial seedling establishment in eelgrass Zostera marina through a manipulative field experiment assessing the relative importance of germination failure and seedling loss during the winter. Seed plots were established in October at 3 unvegetated sites in the Chesapeake Bay (USA) region, with seeds either at the sediment surface or buried at 2 to 3 cm. Emerging seedlings were monitored at 6 wk intervals between December and April using a video camera, and seed germination was tracked in separate destructively-sampled plots. Sediment height change was measured, and sediment disturbance depth was estimated by deploying cores layered with tracer particles and examining tracer loss upon core retrieval. We found a low rate of seedling establishment 6 mo after seeding (1.2, 3.8, and 2.8% for surface seeds at the 3 sites) that was largely due to seed and seedling loss rather than to germination failure, with 90% of seeds retrieved after December having germinated. Seed burial significantly enhanced seedling establishment at 2 of 3 sites (40.4, 16.8, and 10.3% establishment for buried seeds). Seed loss occurred mostly within the first month of the experiment, and was most severe for seeds at the sediment surface. Indicator core results showed widespread disturbance of sediments to depths that could have dislodged early seedlings developing from surface seeds, and to a lesser degree seedlings from buried seeds. Our findings help identify the nature and timing of a substantial Z. marina seedling establishment bottleneck in our region, and show that some of the key processes pivotal to Z. marina recruitment dynamics and optimal restoration strategies involve physical sediment−seedling interactions rather than seed germination.
Buoy-deployed seeding: Demonstration of a new eelgrass (Zostera marina L.) planting method
Ecological Engineering, 2005
We describe an innovative method of dispersing Zostera marina L. (eelgrass) seed that has the potential to facilitate large-scale, citizen-based restoration programs. Mature reproductive shoots of eelgrass were collected during the second week of seed release and stocked into mesh pearl nets suspended from buoys set in 0.04 ha plots. As the seeds ripened, they were naturally released from the nets, fell to the bottom and germinated to form a distinct arc-shaped meadow under each buoy. A survey of seedling survival indicated that recruitment was at least 6.9% based on estimated seed abundance within each net. The advantages of this method are that (1) harvest and deployment of reproductive shoots takes place on the same day, eliminating the need to store reproductive shoots in an onshore storage facility, to obtain a sufficient number of seeds for large-scale restoration programs and (2) once trained, citizens can participate in both the collection and seeding phases, thereby, increasing awareness and value of Z. marina restoration programs. The technique presented allows for a low-cost, efficient, and simple method for successfully dispersing seed, which consequently has a significant impact on establishment of plants from seed. These attributes can also influence restoration programs for other species of aquatic plants for which the seeding of sites, historically degraded but now habitable, is possible.
Aquatic Botany, 1999
Eelgrass (Zostera marina L.) restoration efforts have historically focused on the use of adult vegetative shoots because of generally low success using seeds, a propagule of potential, but littleknown utility, in restoration work. Previous work has shown that approximately 15% of seeds broadcast on unvegetated sediments survive to seedling stage, with losses in part resulting from predation, burial, or lateral transport. We conducted experiments using seeds in burlap bags under both laboratory and field settings to determine if protecting seeds increased survival or germination rates. Retention of seeds from preparation to initial sampling six months later was nearly 100%. Seedling survival at the field sites ranged from 41 to 56% in the burlap bag treatment, compared to 5-15% for seeds without burlap bag protection. Under laboratory conditions, seedling survival was identical in both treatments (50%). However, successful seedling growth noted in the protected treatment after 6 months was lost by 8 months because of significant sand accumulation over anchored seed bags. These preliminary results are encouraging for future restoration efforts that shift the focus to the use of seeds rather than adult plants, as greater survival of seeds in a protected environment can offer enhanced opportunities for addressing both basic and applied questions in restoration ecology. 0304-3770/99/$ -see front matter ©1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 -3 7 7 0 ( 9 9 ) 0 0 0 0 8 -X
Eelgrass (Zostera marina) restoration on the west coast of Sweden using seeds
MArine Ecology Progress Series, 2016
Along the northwest coast of Sweden, over 50% of Zostera marina L. (eelgrass) meadows have vanished since the 1980s. With the improvement of conditions, there is a growing interest to restore lost habitats, but methods are lacking for restoration of eelgrass beds at high latitudes where long winters create special challenges. We assessed if seed planting could be used for large-scale restoration, with the aim to identify the major causes of seed and seedling loss and to determine which planting method best increases restoration success. In the laboratory, we identified optimal conditions for long-term seed storage and demonstrated that eelgrass seeds can be successfully stored for 8 mo before being planted in the spring. However, field studies did not find an increased seedling establishment in seeds planted in the spring of 2013 compared to those planted in the fall of 2012. Field results suggest that the main causes for the seed loss were seed predation from crabs, seed transport by currents and bioturbation by lugworms, while the main processes affecting shoot development were light availability and physical disturbance. Covering the seeds with a layer of sand increased seedling establishment 2 to 6 times compared to uncovered seeds, probably through the reduction of seed predation and seed transport, and could potentially be developed as a method to reduce losses during restoration. In addition, positive feedback mechanisms (i.e. sediment resuspension and drifting macroalgae mats) may also prevent natural recovery and restoration success. However, high seed loss (on average 98.6%) and high shoot mortality pose a challenge that need to be addressed before restoration using seeds can be recommended for large-scale restoration.
Estuaries and Coasts, 2014
Seed germination and seedling establishment directly affect the resiliency of seagrasses to disturbance or environmental stress. The objectives of this study were to compare maximum seed germination, time to germination, nongerminated seed viability, and initial seedling biomass between mixed-annual and perennial Zostera marina seed populations in coarse (>90 % sand) and fine (<50 % sand) sediments and at shallow (1 cm) and deep (5 cm) burial depths. Perennial seeds collected from Virginia and North Carolina had greater maximum germination, shorter time to germination, and greater seedling biomass compared to mixed-annual seeds collected from North Carolina. For both mixed-annual and perennial seeds, maximum germination and seedling biomass were the greatest in shallow fine sediments. Mixed-annual seeds buried at 1 cm had a shorter time to germination than in the deep treatments; however, sediment type did not affect mean time to germination. Perennial seeds had a shorter time to germination in shallow compared to deep burial depths and in fine compared to coarse sediments. Cues for germination were present at the deeper depths; however, the cotyledon failed to emerge from the sediment surface resulting in mortality at depths of 5 cm. The greater performance of perennial compared to mixed-annual seeds and seedlings demonstrate the trade-offs which can occur between Z. marina reproductive strategies. Reduced germination of Z. marina seeds buried ≥5 cm and in coarse sediments may represent a possible bottleneck in successful sexual reproduction, feasibly affecting the resiliency to and recovery from disturbance for both perennial and mixed-annual Z. marina beds.
Seed addition facilitates eelgrass recovery in a coastal bay system
Marine Ecology Progress Series, 2012
Eleven years of eelgrass Zostera marina seed additions conducted in a coastal bay system where Z. marina had not been reported since 1933 have resulted in rapid Z. marina expansion beyond the initially seeded plots. From 1999 through 2010, 37.8 million viable seeds were added to 369 individual plots ranging in size from 0.01 to 2 ha totaling 125.2 ha in 4 coastal bays. Subsequent expansion from these initial plots to approximately 1700 ha of bay bottom populated with Z. marina through 2010 is attributable to seed export from the original plots and subsequent generations of seedlings originating from those exports. Estimates of annual patch vegetative expansion showed mean estimated diameter increasing at varying rates from 10 to 36 cm yr −1 , consistent with rhizome elongation rates reported for Z. marina. Water quality data collected over 7 yr by spatially intensive sampling, as well as fixed-location continuous monitoring, document conditions in all 4 bays that are adequate to support Z. marina growth. In particular, median chlorophyll levels for the entire sampling period were between 5 and 6 µg l −1 for each of the bays, and median turbidity levels, while exhibiting seasonal differences, were between 8 and 9 NTU. The recovery of Z. marina initiated in this coastal bay system may be unique in seagrass recovery studies because of how the recovery was initiated (seeds rather than adult plants), how rapidly it occurred (years rather than decades), and the explicit demonstration of how one meadow modulated water clarity and altered sediments as it developed and expanded. Our results offer a new perspective on the role seeds can play in recovery dynamics at large spatial scales.