San Francisco Estuary and Watershed Science

< Abstracts are not associated with Essays. -the SFEWS Editors. >

Tidal wetland restoration is integral to achieving the Delta coequal goals. Deeply subsided islands limit the potential for tidal wetland restoration. Floating peats may offer an opportunity to create tidal habitat in the subsided western and central Delta. We conducted a mesocosm experiment to assess the feasibility of floating peat blocks, and the potential food-web benefits, biomass production, carbon sequestration, methane emissions, and water-quality effects. We evaluated the effect of varying water residence time and initial peat-block density.

The peat blocks floated during the entire experiment, and accreted biomass at rates consistent with those reported for Delta non-tidal managed wetlands. Peat blocks placed in mesocosms with 45% open water expanded horizontally about 21% per year. We estimated average vertical accretion rates of 5.5 to 8.6 cm/yr  for all the mesocosms. Vertical and horizontal expansion increase floating peat-block stability.

We measured a 3-fold zooplankton population increase during the first year after deployment, relative to the Mokelumne River, which was the mesocosm’s water source. Measured and modeled methane emissions were lower than those reported in Delta non-tidal managed wetlands. Aqueous methane concentrations and methane fluxes were significantly lower for the shorter water-residence-time of about 5 days compared to longer residence times of about 11 days. Elevated dissolved oxygen (DO) concentrations generally corresponded with low methane concentrations. Our estimated net ecosystem carbon balance of – 820 +/– 137 g C m2/yr  indicates that the floating wetlands are potentially greater carbon sinks than Delta non-tidal wetlands. Nitrogen data indicated consumption by wetland plants, and denitrification and dissimilatory nitrate reduction in the mesocosms. Our preliminary results point to potential ecosystem benefits of floating peats on a larger scale.

Tidal wetland restoration to benefit at-risk fish species in the Sacramento-San Joaquin Delta and Suisun marsh has gained momentum over the past decade, much of it in response to mitigation requirements for the State Water Project and Central Valley Project. In fall 2023, the Department of Water Resources and the State Water Contractors convened a symposium, entitled Delta-Suisun Tidal Wetland Restoration Symposium: State of the Science and Future Directions, to discuss the latest wetland restoration research and future directions. The symposium was held 10 years after the 2013 symposium “Tidal Marshes and Native Fishes in the Delta: Will Restoration Make a Difference?”, so served as an opportunity to follow up on the progress that has been made over the past decade. This paper synthesizes the key findings from the 2023 workshop.

The paper begins with the historical context of wetland restoration in the Delta and Suisun marsh, then outlines the restoration process as it is currently implemented. It then describes the monitoring of tidal wetlands in terms of their capacity to support fish (capacity), the opportunity fish have to use the habitat (opportunity), and the realized functions provided when fish are actually using the site.

Finally, the paper identifies priority science actions to advance our understanding and management of tidal wetland restoration sites. These actions include further research into fish habitat utilization, improved monitoring techniques, and enhanced adaptive management strategies. This list of information needs is intended to inform future monitoring of restoration sites, scientific studies, funding, and prioritization of wetland research.

Recovery of endangered salmon species in the Central Valley of California amidst prolonged drought and climate change necessitates innovative water management actions that balance species recovery and California's water demands. We describe an individual-based ecological particle tracking model (ECO-PTM) that can be used to assess the efficacy of proposed actions. Based on a random walk theory, the model tracks individual particles’ travel time, routing and survival in a flow field simulated by the Delta Simulation Model 2 hydrodynamic module (DSM2 HYDRO). The random walk particles are parameterized to have fish-like swimming behaviors, including upstream/downstream swimming, probabilistic holding behaviors, and stochastic swimming velocities. Particle routing at key junctions is based on well-established statistical models, and route-specific survival is calculated using the XT mean free-path length model. Behavioral parameters were estimated by fitting several competing models to a multiyear dataset of travel times from acoustic tagged juvenile salmon. The model’s baseline simulations under historical flow conditions from 1991 to 2016 successfully replicated essential relationships between salmon outmigration survival and hydrodynamic conditions, consistent with previous studies and the STARS (Survival Travel Time and Routing Simulation) statistical simulation model. Simulation results for management scenarios revealed multifaceted influences on fish survival, including Delta flow, flow at key junctions, route alterations, seasons, and water availability characteristics. Importantly, these results highlight ECO-PTM’s potential to predict fish survival outcomes of proposed actions, serving as a foundation for informed future research, decision-making, and effective management strategies to enhance the survival prospects of outmigrating salmonids within the Sacramento-San Joaquin Delta ecosystem.

Genetic diversity is the fundamental building block of biodiversity and the necessary ingredient for adaptation. Specifically, the intra-specific diversity (biocomplexity) comprised of phenotypic and genetic variation partitioned within and among populations can determine the ability of a species to respond to changing environmental conditions. Here, we explore the biocomplexity of California’s Central Valley Chinook salmon (Oncorhynchus tshawytscha) population complex at the genomic level by quantifying population genomic diversity among and within migration life-history phenotypes. Notably, despite apparent gene flow among populations with the same migration (life history) phenotypes inhabiting different tributaries, each group is characterized by a distinct component of unique genomic diversity. While enumerating biodiversity contained within individual hierarchical levels is informative, it is important to consider inter- and intra-specific diversity simultaneously as there may be emergent properties at higher levels due to presence of diversity at lower ones. Our results emphasize the importance of formulating conservation goals focused to maintain biocomplexity at both the phenotypic and genotypic level. Doing so will preserve the species’ adaptive potential and increase the probability of persistence of the population complex despite changing environmental pressures.