We quantified the greenhouse-gas (GHG) emission and economic implications of alternative crop and wetland mosaics on a Sacramento–San Joaquin Delta island: Staten Island. Using existing GHG fluxes measurements for the Delta and biogeochemical models, we estimated GHG emissions for a range of scenarios, including the status quo, modified groundwater management, and incorporating rice and managed wetlands. For current land uses, emissions were predicted to vary greatly (48,000 to 105,000 t CO₂-e yr^{− 1}) with varying groundwater depth. GHG emissions were highest when water depth was 120 cm, the typical depth for a Delta island, and lowest if water table depth was shallowest (60 cm). In the alternate land-use scenarios, we simulated wetlands and rice cultivation in areas of highest organic-matter soils, greatest subsidence, and GHG emissions. For each scenario, we analyzed economic implications for the land-owner by determining profit changes relative to the status quo. We spatially assigned areas for rice and wetlands, and then allowed the Delta Agricultural Production (DAP) model to optimize the allocation of other crops to maximize profit. The scenario that included wetlands decreased profits 79% relative to the status quo but reduced GHG emissions by 43,000 t CO₂-e yr^{− 1} (57% reduction). When mixtures of rice and wetlands were introduced, farm profits decreased 16%, and the GHG emission reduction was 33,000 t CO₂-e yr^{− 1} (44% reduction). When rice was cultivated on 38% of the island, profit increased 12% and emissions were 22,000 t CO₂-e yr^{− 1} lower than baseline emissions (30% reduction). Conversion to a mosaic of wetlands and crops including rice could substantially reduce overall GHG emissions of cultivated lands in the Delta without greatly affecting profitability.

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.