Importance of eelgrass early life history stages in response to oyster aquaculture disturbance (original) (raw)
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Journal of Shellfish …, 2009
The presence of bivalves and bivalve aquaculture can have positive and negative impacts on seagrass and associated benthic communities. Some oyster (Crassostrea gigas) aquaculture methods recently have been restricted to reduce benthic disturbance and protect native eelgrass (Zostera marina) in West coast (USA) estuaries. We argue that aquaculture, like all food production systems, involves tradeoffs with natural systems, but that the magnitude of those tradeoffs depends on the ecological details of the production system. Capitalizing on oyster aquaculture farms as large scale ''manipulations'' in Willapa Bay, WA (USA), we explored three different oyster aquaculture methods (mechanical harvest or ''dredged'' on-bottom, hand picked on-bottom and long line off-bottom). We found that both the biological (oyster-eelgrass interactions) and physical (disturbance or structure) components of aquaculture led to changes in the eelgrass population. Eelgrass density declined with oyster density in all aquaculture areas, likely as a result of direct competition for space. Eelgrass relative growth rate, plant size, and production did not change with oyster density. However, all eelgrass measures were affected by aquaculture, and the type and magnitude of impacts varied among eelgrass measures and aquaculture methods. Throughout the bay, eelgrass in long line areas occurred at densities indistinguishable from nearby uncultivated areas, but in 2004, eelgrass in long line areas was smaller (32%) and had lower production per area (70%). Cultivating oysters in dredged or hand picked beds increased eelgrass growth rates slightly, but led to lower eelgrass density (70% and 30%, respectively), plant size (32%, both cases), and production (70%, both cases). In a large scale simulated mechanical harvest experiment, the temporal response of eelgrass density varied dramatically by site, ranging from 1 to >4 y. If eelgrass impact reduction, rather than avoidance, is identified as the management goal, the degree of tradeoff between eelgrass habitat and oyster production can be minimized by managing aquaculture methods or oyster planting densities, depending on the eelgrass measure of interest. Explicit management goals and appropriate eelgrass habitat indicators must be developed before our findings can be used to suggest best management practices for intertidal aquaculture in the Pacific Northwest.
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.
Marine Ecology Progress Series, 2007
Transplantation of eelgrass Zostera marina has become a promising restoration tool since natural recolonisation during the last century failed after massive mortality, due to a combination of a wasting disease outbreak and a sequence of human impacts. We studied the interactive effects of planting density and hydrodynamic exposure on the survival of transplants of an annual population of intertidal eelgrass. Accordingly, eelgrass seedlings were planted in high density (HD: 14 plants m-2) and low density (LD: 5 plants m-2) units at 3 locations with varied wave and current exposures. We also tested the potential of blue mussel beds (Mytilus edulis) to facilitate eelgrass survival. Transplant survival decreased as hydrodynamic exposure increased. Survival was high (75% after 7 wk) at the low exposure location. The intermediate exposure location had slightly lower overall survival (60% after 7 wk), and lowest overall survival rate was at the most exposed location (20% after 7 wk). Facilitation existed among eelgrass plants. Survival was significantly higher in the HD units than in the LD units at both high and intermediate exposure locations. Planting density had no effect on survival at the low exposure location. Hence, there was an interactive effect of planting density, hydrodynamic exposure and shelter. Eelgrass planted in open spaces within a mussel bed survived significantly better than transplants situated 60 m seaward of the mussel bed. Thus, mussel beds facilitate eelgrass survival. The insights into the processes affecting transplantation success will be of use in eelgrass restoration around the world.
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.
Aquaculture and eelgrass Zostera marina interactions in temperate ecosystems
Aquaculture Environment Interactions, 2021
This paper reviews the impacts of shellfish and finfish aquaculture on eelgrass Zostera marina, the most widely distributed seagrass species in the northern hemisphere. Shellfish aquaculture can have positive, neutral, and negative effects on eelgrass. Positive interactions can be generated by the filtering activity of cultured bivalves, which may improve water quality and reduce epiphyte loads, and shellfish biodeposits may provide more nutrients to eelgrass and other vegetation. However, negative responses are more commonly reported and can be caused by shading and sedimentation. These negative effects tend to occur directly under and immediately surrounding shellfish farms and rapidly diminish with increasing distance. In contrast to shellfish aquaculture, only one field study has investigated the effects of finfish aquaculture on eelgrass in a temperate setting, and the results were inconclusive. However, many studies have investigated the effects of Mediterranean finfish farms ...
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