San Francisco Estuary and Watershed Science

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.