Growing food for ducks and fish in seasonally flooded managed wetlands

By Kyle Phillips, Alice Tung, Teejay O’Rear, John Durand

In a recent publication in Freshwater Biology, UC Davis researchers found that waterfowl food-plants in managed wetlands of Suisun Marsh can boost plankton production by an order of magnitude compared to plants in open tidal habitats. Suisun Marsh is one of the largest tidal wetlands on the west coast of North America and is critical habitat in the San Francisco Estuary (SFE). While managed wetlands in the marsh are operated to promote waterfowl, this study supports other research showing how fishes can benefit from waterfowl management actions (see here and here). This work has direct implications for managing food availability for native and desirable fishes, especially those that rely on plankton in the SFE. This is another example of a reconciliation ecology approach for California water.

What are managed wetlands?

Managed wetlands are diked, shallow ponds fitted with gated culverts that control water exchange with neighboring tidal channels. Most managed wetlands in Suisun Marsh were constructed in the early 1900’s for recreational waterfowl hunting and, today, still comprise most of Suisun Marsh’s acreage. To expand winter habitat for migratory ducks, most managers flood ponds in early fall and drain them in late spring. During the dry season, after most ducks migrate north, annual terrestrial plants grow that set seed in fall, providing an attractive food source for returning waterfowl. The active management of water flows on these wetlands simulates floodplains, vernal pools, and marsh upland pans that were part of the region’s historical habitat used by waterfowl and other fauna. Ducks are well studied in managed wetlands, but until recently little was known about how other animals (including mammals, reptiles, and fish) use these novel man-made habitats.

**Figure 1.** *Left:* flooded sea purslane, a common forb in seasonal managed wetlands. *Right:* tule wrack floating in a tidal slough.

What is different about vegetation in managed wetlands?

Tidal habitats in Suisun Marsh are dominated by flood-tolerant emergent plants such as tules (Schoenoplectus californicus), cattails (Typha domingensis), and common reed (Phragmites australis). Seasonal managed wetlands support emergent plants, but also support terrestrial “leafy green” forbs during the dry phase, including fat hen (Atriplex triangularis) and sea purslane (Sesuvium portulacastrum) which are also favored by wintering waterfowl (Figure 1). Both plant types supply detritus (i.e. dead plant matter) for the aquatic food web, but researchers found that forb detritus released more nitrogen and phosphorus, which are key macronutrients for plants and animals. When managed wetlands flood, forbs rapidly decay and release a pulse of nutrients which kickstarts phytoplankton growth which then feed zooplankton (Figures 2 and 3). This phenomenon is akin to the flood pulse concept in natural floodplains, whereby floods promote decomposition of terrestrial plant material which fuels productivity of the aquatic ecosystem. Emergent plants, on the other hand, are deciduous and can re-absorb nutrients into their perennial root structures before shedding leaves or stems–similar to deciduous trees before dropping their colorful autumn leaves. As a result, leaves shed by emergent plants are comparatively low in nutrients and, in the study, provided little benefit for plankton even compared to control treatments without added detritus. Deciduousness makes emergent plants useful for building up peat in tidal marshes, and for naturally cleaning waterways of excess nutrients, but ultimately makes them less suitable for growing plankton than other sources of detritus.

**Figure 2**. Phytoplankton response to different types and species of plant detritus in a mesocosm study. Researchers found decaying emergent plants (cooler colors) released fewer nutrients and produced much less phytoplankton than annual forbs (warm colors). Points represent observed chlorophyll-α concentrations, while trend lines depict LOESS regressions for each group.

**Figure 3.** Zooplankton response to different functional types of detritus in a mesocosm study. Researchers found decaying emergent plants (blue) resulted in fewer zooplankton than did terrestrial forbs (orange).

But aren’t nutrient inputs harmful to aquatic ecosystems?

Nutrients are necessary for ecosystem productivity but, like with most things, moderation is key. Excessive nutrients create bacterial or algal blooms that can deplete waterways of dissolved oxygen for a time. Low dissolved oxygen is sometimes observed in managed wetlands following initial flood up which can indeed pose a risk to resident fishes, but this risk is mitigated by windy days and via coordinated floods and water circulation among wetland managers. The study found decaying forb detritus depleted oxygen levels initially, but stabilized as phytoplankton consumed nutrients and produced oxygen, even without the aid of water mixing from wind or circulation.

In general, the SFE is not nutrient-depleted, but opportunities for nutrients to transform into phytoplankton biomass are increasingly scarce. For example: (1) Introduced aquatic weeds can compete with phytoplankton for nutrients in much of the estuary; (2) Introduced clams are highly efficient grazers that feed on phytoplankton and can severely suppress their numbers, and (3) Intensive channelization and high water exports decrease water residence times (i.e. the time between when water enters and leaves a system), which constrains the window of time for plankton to grow and accumulate. By definition, plankton are free-floating organisms that cannot swim against currents. They go wherever the water takes them, so once water is exported, any suspended plankton are also lost.

Managed wetlands likely provide some measure of control against these problems. The seasonal dry period may reduce impacts from invasive aquatic weeds and clams which can take several months to recolonize newly flooded wetlands, compared to plankton which can establish and grow exponentially within a few days. During the flooded period, managed wetlands can also slow the exchange of water enough to allow plankton accumulation. On top of that, forbs that quickly decay upon flooding in managed wetlands can jumpstart plankton production, so long as low-oxygen risks are mitigated. Plankton often flourish in these conditions until they either deplete available nutrients or attract large numbers of predators, such as larval fish.

Where could managed wetlands fit with future restoration?

The SFE has many ongoing efforts to tidally restore wetlands to increase accessible habitats for fishes and boost plankton production. However, converting managed wetlands to tidal wetlands might not meet this goal. While they may provide habitat, including refuge, they may not be very productive due to loss of the seasonal flood-pulse from managed wetlands. In addition, tidal restorations might lack controls against the introduced species and water exports that undercut plankton production in the first place.

Future habitat restoration and enhancement projects might be more successful if they incorporate features of managed wetlands in their designs or include ways to import plankton from nearby managed wetlands (Figure 4). Restoration of functional ecosystems often requires a mosaic of habitat types that provide different resources to a range of organisms. Managed wetlands may prove an important piece of the SFE mosaic and remain an excellent example of how humans can use infrastructure for multi-species benefits.

**Figure 4.** An example of where plankton inputs from managed wetlands could hypothetically benefit tidally restored ecosystems, if implemented. *Left:* Plankton monitoring locations in the Tule Red restoration site (blue) and adjacent Grizzly King managed wetland (orange). *Right:* boxplot depicting copepod densities in Tule Red and Grizzly King sampled in the 2023 water year, with densities in Grizzly King often exceeding those in Tule Red. Figure is from a presentation (Phillips *et al*) given at the Bay Delta Science Conference in September 2024.

Kyle Phillips and Alice Tung are PhD Candidates studying plankton productivity at the Center for Watershed Sciences. Teejay O’Rear is a fish ecologist at the Center for Watershed Sciences. John Durand is a senior researcher specializing in estuarine ecology and restoration at the Center for Watershed Sciences.