Beyond Duck Clubs: Suisun Marsh as a Network of Plankton Hotspots

By Alice Tung, Kyle Phillips, and John Durand

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Located in the center of the San Francisco Estuary, Suisun Marsh is an ecological hotspot in the intermediate zone between the freshwater Sacramento-San Joaquin Delta and salty San Francisco Bay. The Suisun Marsh landscape looks wildly different today compared to its historic past. In 1850, U.S. Congress passed the Swamp Land Act, which handed federal wetlands to states to encourage rapid reclamation for agriculture (Moyle et al. 2014). Wetland reclamation, i.e. the process of converting wetlands to drylands, converted tidal wetlands to drylands in Suisun Marsh into parcels and enclosed with dikes. However, agriculture succumbed to high salinity and, over time, was converted into seasonally managed wetlands for waterfowl habitat and recreational hunting. Now, managed wetlands comprise about 70% of Suisun Marsh and include both public and private lands mainly managed for waterfowl habitat (Figures 1 and 2).

Today, the landscape contains a complex network of tidal channels that connect to roughly 150 managed wetlands speckled throughout. These wetlands are typically managed to flood in the fall, provide habitat for migratory waterfowl in the winter, and then drain and dry through summer (Figure 3). Water-control structures are the main connection points between these managed wetlands on one side of the levee and tidal sloughs on the other as the source water. The flood schedule of these wetlands plays a large role in how much and what kind of life thrives in each of these systems. This intermittent seasonal connectivity provides opportunities to concentrate and then export plankton-rich water to adjacent sloughs.

A collage of six photographs shows a wetland landscape in changing water conditions, with reeds, ponds, distant hills, and people working in the marsh. At upper left, a calm channel reflects green reed beds beneath a broad blue sky. The upper middle image shows a small boat resting in shallow, plant-covered water, with dense reeds and low hills behind it. At upper right, a person in a light green shirt stands beside a wide expanse of cracked, dry mud under a cloud-streaked sky. At lower left, a dark-and-white wading bird stands in still water, its reflection visible beneath it; blurred tan reeds fill the background. The large lower-middle photograph shows shallow pools and marsh grasses extending toward green hills. At lower right, a person standing in water lifts a small piece of equipment overhead.

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Collage of wetland scenes, including reed-lined water, a stranded boat, cracked dry ground, a wading bird, marsh pools, and a person lifting equipment from the water. — **Figure 1**. **Top left**: Tules emerge from the water surrounding a serene, open-water pond; **top middle**: UC Davis field crew’s canoe rests atop freshly inundated sea purslane in the fall; **top right**: Dr. Kyle Phillips walking across the cracked, desiccated waterbed near the end of a managed wetland’s summer dry period; **bottom left**: Suisun Marsh provides habitat for numerous waterbird species, including this contemplative black-necked stilt; **bottom middle**: This shallow flooded habitat is the beginning of a transition between land and open water; **bottom right**: Lynette Williams Duman gracefully casts a net to collect a zooplankton sample. (Photo credits: Dr. Alice Tung)

A map of Suisun Marsh in the San Francisco Bay–Delta region, with a locator inset showing its position in California. Pale green shading marks the marsh, while gray areas show surrounding water and land; thin lines trace channels and marsh boundaries. Colored dots indicate sampling stations: gold for managed, teal for tidal, and purple for Wings Landing. Labeled sites include Wings Landing, Suisun Slough, Lower Joyce Island, Grizzly Canal, Grizzly King, Tule Red, Montezuma Slough, Meins Landing, Montezuma Wetlands, Potrero Pond, Luco Slough, and Denverton Duck Club and Slough. A north arrow and scale bars appear below the map.

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Map of Suisun Marsh showing sampling stations, labeled sites, marsh boundaries, and a California locator inset. — **Figure 2**. Map of the wetlands and reference sloughs in Suisun Marsh, California, sampled by the UC Davis Ponds Project team since inception of the project in 2017 (Phillips et al. 2026).

A side-by-side comparison shows the same wooden water-control structure in two seasons. In the left image, labeled “Fall,” the structure stands in a broad, high expanse of water, with ripples and turbulence near its base; low vegetation lines the distant shore. In the right image, labeled “Summer,” the water has receded to a shallow, irregular pool surrounded by exposed dry, brown ground and patches of green vegetation. The sky and a low, distant landscape appear behind both scenes.
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Side-by-side seasonal comparison of a wooden water-control structure: surrounded by broad water in fall and by exposed, mostly dry ground with a small pool in summer. — **Figure 3**. Water control structures connect managed wetlands and tidal sloughs and are operated to control flood and drain events. Managed wetlands in Suisun Marsh typically flood in fall (left) and drain in spring to remain throughout summer (right). (*Location: Potrero Duck Club; Photo credit: Dr. Alice Tung*)

Suisun Marsh as a network of productivity

Our research demonstrates that managed wetlands in Suisun Marsh tend to be notably more ecologically productive than adjacent tidal habitats. Comparisons between tidally restored marshes and nearby managed wetlands have shown greater phytoplankton biomass (Figures 4 and 5), zooplankton biomass (Phillips et al. 2026), and native fish representation (Platzer et al. 2026) within managed wetlands. When these wetlands drain into adjacent tidal channels, they potentially export some servings of “zoop soup” (Figure 6) for fish in otherwise food-limited waterways– even during the fall and winter, when production across the estuary is otherwise low. Juvenile salmon have been shown to grow significantly faster at the outfall of a managed wetland compared to tidal habitats nearby (Aha et al. 2021), suggesting outfalls may act as culinary hotspots. The importance of this contribution at the scale of the entire marsh remains largely unknown.

Two-panel box plot of ln(chlorophyll a) by wetland type in 2023 and 2024; managed wetlands have higher medians than tidal channels and tidally restored wetlands. — **Figure 4**. Comparison of phytoplankton biomass, measured using chlorophyll a concentrations across different wetland habitats in Suisun Marsh during 2023 and 2024. Managed wetlands consistently contained higher phytoplankton biomass than tidally restored wetlands and nearby tidal sloughs (Tung et al. 2026, *in prep*).

Scatter plot of chlorophyll a over time at eight wetlands, with points colored by wetland type and gray “DRY” periods marked at Luco, Meins, and Miramonte. — **Figure 5**. Phytoplankton biomass (using chlorophyll a as a proxy) across managed wetlands (blue), tidally restored wetlands (green), and each wetland’s adjacent tidal slough (black) from 2022 to 2024. Managed wetlands supported notably high biomass, with peaks often exceeding 100 ug/L in several wetlands, while tidal counterparts were substantially lower. Gray vertical bars illustrate the timing of dry periods between flood seasons in managed wetlands, where applicable. (Tung et al. 2026, *in prep*).

Figure 6. A delectable jar of “zoop soup” collected using a zooplankton tow net from a managed wetland in Suisun Marsh. This concentrated sample, composed primarily of Daphnia magna, shows the effects of how high phytoplankton production can support zooplankton populations in managed wetlands. (Zooplankton sample collected by Kyle Phillips; video credit to Alice Tung)

Lessons from nearby river floodplains

Why are managed wetlands so productive? One possible explanation lies in their resemblance to river floodplains, which are known for producing a feast of aquatic food in shallow, flood habitats and transporting that food to nearby waterways as they drain. For example, floodplain activation on the restored Consumnes watershed resulted in strong flood-pulse responses in phytoplankton biomass (Ahearn et al. 2006). Juvenile salmon from the Sacramento-San Joaquin River Delta were supported by zooplankton produced from upstream floodplains during wet years (Sturrock et al. 2022). Zooplankton production in the Yolo Bypass was found to produce 53 times more zooplankton than in the adjacent river, and juvenile salmon grown on the floodplain grew 5x faster than those grown in the adjacent river (Jeffres et al. 2020). Adding to this pool of research, we are conducting ongoing research to evaluate how Suisun managed wetlands may also contribute to the food web in adjacent waterways to support resident and migratory fishes.

Toward a marsh-scale understanding

Floodplain management can provide multiple ecosystem services for rice growers, waterfowl, recreation, and fishes. Likewise, managed wetlands may offer similar benefits to fishes and aquatic food webs in addition to waterfowl and recreation. In contrast to the vast scales of floodplains, managed wetlands comprise a system of separately operated “plankton farms” that together form a much larger network. Tidal waterways connect the network and provide pathways for the movement of plankton-enriched waters throughout the marsh. Viewed through this lens, managed wetlands represent a network of productive brackish floodplains capable of supporting aquatic food webs outside of their boundaries. Understanding the magnitude of these subsidies, and their contribution to marsh-scale productivity, remains an important area of ongoing and future research, including current studies led by CA Dept Water Resources and supported by UC Davis, US Geological Survey, Suisun Resource Conservation District, and Inter-City Fund.

About the Authors

Dr. Alice Tung and Dr. Kyle Phillips are postdoctoral researchers in the Durand Lab at UC Davis’s Center for Watershed Sciences. Collectively, their research focuses on wetland ecology, aquatic food webs, and the effects of water management on ecosystem productivity in the San Francisco Estuary.

Further Reading

Aha, Nicole M., Peter B. Moyle, Nann A. Fangue, Andrew L. Rypel, and John R. Durand. 2021. “Managed Wetlands Can Benefit Juvenile Chinook Salmon in a Tidal Marsh.” Estuaries and Coasts 44 (5): 1440–53. https://doi.org/10.1007/s12237-020-00880-4.

Ahearn, Dylan S., Joshua H. Viers, Jeffrey F. Mount, and Randy A. Dahlgren. 2006. “Priming the Productivity Pump: Flood Pulse Driven Trends in Suspended Algal Biomass Distribution across a Restored Floodplain.” Freshwater Biology 51 (8): 1417–33. https://doi.org/10.1111/j.1365-2427.2006.01580.x.

Jeffres, Carson A., Eric J. Holmes, Ted R. Sommer, and Jacob V. E. Katz. 2020. “Detrital Food Web Contributes to Aquatic Ecosystem Productivity and Rapid Salmon Growth in a Managed Floodplain.” PLOS ONE 15 (9): e0216019. https://doi.org/10.1371/journal.pone.0216019.

Moyle, Peter B., Amber D. Manfree, and Peggy L. Fiedler. 2014. Suisun Marsh: Ecological History and Possible Futures. Berkeley: University of California Press. https://doi.org/10.1525/9780520957329.

Phillips, Kyle A., Alice M. Tung, Rachel M. McConnell, et al. 2024. “Forbs from Seasonal Managed Wetlands Boost Plankton Production More than Emergent Graminoids by Supplying Novel Labile Detritus.” Freshwater Biology 69 (11): 1686–701. https://doi.org/10.1111/fwb.14336.

Phillips, Kyle A., Alice M. Tung, Elsie Platzer, Teejay A. O’Rear, Sharon P. Lawler, and John R. Durand. 2026. “Pulsed Zooplankton Production in Seasonally Managed Wetlands Could Supplement Food Availability for a Threatened Fishery in the San Francisco Estuary.” Estuaries and Coasts 49 (3): 69. https://doi.org/10.1007/s12237-026-01686-6.

Platzer, Elsie, Kyle Phillips, Teejay O’Rear, and John Durand. 2026. “Silverside Gold Mine? Restored Tidal Habitats Host Abundant Invasive Fishes in a Novel California Marsh (U.S.A.).” Restoration Ecology, June 1, e70452. https://doi.org/10.1111/rec.70452.

Stompe, Dylan K., Peter B. Moyle, Kiva L. Oken, James A. Hobbs, and John R. Durand. 2023. “A Spatiotemporal History of Key Pelagic Fish Species in the San Francisco Estuary, CA.” Estuaries and Coasts 46 (4): 1067–82. https://doi.org/10.1007/s12237-023-01189-8.

Sturrock, Anna M., Mollie Ogaz, Kelly Neal, et al. 2022. “Floodplain Trophic Subsidies in a Modified River Network: Managed Foodscapes of the Future?” Landscape Ecology 37 (12): 2991–3009. https://doi.org/10.1007/s10980-022-01526-5.

Tung, Alice M., et al. 2026. “Managed Wetlands are Phytoplankton Biomass Hotspots in the San Francisco Estuary.” Manuscript in preparation.