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San Francisco Bay Fund Inventory of Projects

San Francisco Bay Fund Inventory of Projects

Richardson Bay Eelgrass Restoration
Eelgrass and Native Oyster Restoration Project Results

Eelgrass Field Experiment

We transplanted adult eelgrass plants and set up seed buoys in experimental plots located adjacent to and outside of existing eelgrass patches to determine if the presence of existing eelgrass beds improves the survival and growth of transplanted eelgrass. Eelgrass seeds placed adjacent to and isolated from existing eelgrass patches did not germinate. However, adult eelgrass shoot survival was 60 percent higher for transplants located adjacent to existing eelgrass beds compared to survival in open mud flats after the first two weeks of transplanting. Despite the initial differences in shoot survival, shoot survival did not differ between plots in subsequent monitoring events in November 2009 and May 2010.

Transplanting eelgrass shoots adjacent to existing eelgrass patches may initially provide some protection for newly planted shoots, but it may not provide significant protection for longer-term survival. However, we observed dense algal mats (Gracilaria sp., Ulva sp., and diatoms) on the experimental plots which may have negatively impacted transplanted shoots. Algal mats have been detrimental to young eelgrass shoots in other areas due to a reduction in light which reduces survival.

To reduce the likelihood of algal mat formation, we recommend that future restoration efforts avoid using structures such as pvc poles to mark plots edges because these structures likely act to trap algae. Although long-term shoot survival was highly variable in plots adjacent to and isolated from existing eelgrass patches, transplanting adult shoots appears to be the most effective means of restoring eelgrass in the Bay.

Environmental Stress on Eelgrass

We measured rate of water flow, sedimentation, and water clarity to determine if these environmental factors affect the establishment, growth, and survival of transplanted eelgrass in plots located adjacent to and isolated from existing eelgrass beds. Although we observed small differences in water flow, sedimentation, and water clarity between experimental plots, these differences were not significant. This suggests that the small to medium patches of eelgrass in Richardson Bay used for the experiment were not large enough to ameliorate the environmental stressors (flow, turbidity, deposition) that may affect eelgrass survival. We did, however, detect significant differences in water flow, sedimentation, and water quality in different parts of Richardson Bay, which may affect eelgrass survival and warrants further study.

Native Oyster Settlement

The multiple benefits and ecosystem services provided by native oysters have motivated restoration efforts in San Francisco Bay over the last decade. Researchers and resource managers are working together to develop and refine methods to increase oyster abundance and to choose restoration sites that offer the highest probability of success. Because much of the hard substrate has been removed to facilitate shipping channels or has been buried under soft sediments that washed into San Francisco Bay during the Gold Rush, placing hard substrate at appropriate tidal depths has increased the abundance of oysters in some areas.

Previous research in Richardson Bay demonstrated that artificial reefs made of clean oyster shells were colonized by naturally occurring oyster larvae, confirming that reefs are a reliable way to provide hard substrate as well as a settlement cue for larvae. We examined if additional structure provided by eelgrass facilitates native oyster recruitment on reefs made of clean oyster shells to further identify restoration sites with the highest probability of success. We hypothesized that the physical structure of eelgrass may facilitate oyster recruitment because eelgrass blades extend up into the water column, buffer wave action, and reduce turbidity.

We examined oyster settlement on artificial shell reefs in plots located inside and outside of eelgrass beds from August to September 2009. We also examined oyster settlement in plots located inside and outside of eelgrass beds without the artificial shell reefs. Settlement by native oysters was two times greater on plots with artificial shell reefs compared to plots without artificial shell reefs regardless of location (inside eelgrass vs. outside eelgrass). Although we did not find significant differences in oyster recruitment between artificial shell reefs located inside and outside of eelgrass patches, eelgrass may affect oyster survival in other ways. For example eelgrass may affect oyster feeding rates and/or alter predation rates on oysters by birds or other wildlife.

Our results indicate that creating reefs out of clean oyster shell can be a valuable restoration tool that increases native oyster settlement, and thereby increases the local population density. In addition, we found that placing artificial reefs can increase settlement in as little as one year.

Waterbirds

We conducted 11 waterbird surveys from October 2009 to March 2010 comparing wintering waterbird use within the Richardson Bay Audubon Sanctuary, which contains eelgrass, to use outside of the Sanctuary which does not contain eelgrass. We observed the highest density of waterbirds inside Sanctuary boundaries compared to areas outside the Sanctuary. However, waterbird density was not similar across all parts of the Sanctuary. Waterbird density was slightly higher in areas that border the navigational channel compared to the northeast corner of the Sanctuary, which is furthest from the channel. In the northeast corner we observed a greater proportion of waterbirds resting (as opposed to foraging or flying), which is a critical part of their winter activities. The more energy waterbirds can conserve by resting during the winter season improves their physical condition for spring migration and breeding, which will likely improve populations.

Although we noted increased density of waterbirds in areas with eelgrass (within the Sanctuary), we cannot conclude that eelgrass is responsible for increased density. Disturbance associated with boat traffic could also have affected waterbird use. Areas within the sanctuary are closed to boat traffic during the winter months but the area we surveyed outside the sanctuary is open to boat traffic which may have increased disturbance and lead to fewer detections.

We recommend that additional studies examine waterbird food resources in patches of eelgrass as well as monitor disturbance caused by boat traffic to determine areas within SF Bay that are critical for wintering waterbirds and warrant protection.

Quick Links

Project Photos

Map of eelgrass restoration site
Richardson Bay Audubon Center
Eelgrass Restoration Project, 2009-10

Audubon staff and a volunteer deploying an oyster shell sampling bag in the eelgrass bed. Other sampling bags were placed in mudflat areas free of eelgrass to determine which location resulted in higher oyster settlement.

Eelgrass in Richardson Bay.

Last modified: 4/19/2012 9:59 AM by S. Haren

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