BAY-DELTA SCIENCE CONFERENCE: Floating wetlands: A potential solution for Delta food-web support and habitat

The Delta pelagic food web no longer adequately supports native species, and developing management strategies for foodweb support and habitat for fish species of concern is a key goal as well as a substantial challenge.

It is expected that tidal marsh restoration will produce food resources to benefit species on-site, as well as export food resources to support pelagic habitat in adjacent waters and regionally.  However, deeply subsided Delta islands inhibit large-scale tidal restoration in most of the central and western Delta.  Considering the difficulty of creating tidal habitat on subsided Delta islands, floating peat mats that could provide marsh habitat is being investigated on Bouldin Island. 

Dr. Steve Deverel, president of Hydrofocus in Davis, discussed the potential of using floating peat mats for food web support and habitat at the 2021 Bay-Delta Science Conference. 

The idea was born from observations in the Montezuma wetlands when dredged material from the adjacent channel made its way underneath peat soils, causing them to lift five to 10 feet.  This prompted researchers to look at the possibilities for using floating peat mats on subsided Delta islands.

It’s pretty much accepted that the Delta food web no longer adequately supports native species, so restoring tidal wetlands to resemble somewhat the pre-development Delta can potentially restore processes that benefit the native species.  However, on subsided central and western Delta islands, the subsided land elevations prevent the creation of tidal marsh habitat that would support resident and transient species and provide a food web benefit.  So the floating peat might be a potential opportunity to provide a food web benefit and possibly habitat.

Dr. Deverel noted that floating peat maps exist worldwide; there is a large body of literature demonstrating that floating peat mats were observed in the Delta in the 19th century, as was described in the San Francisco Estuary Institute’s Sacramento-San Joaquin Delta Historical Ecology Study.  One of the anecdotes of the study was how a floating peat block supported a herd of animals near Venice Island during the flood in the 1800s.

So we can see that the peat actually does extend in the open waters and breaks off to form floating islands, and that’s probably what happened in the pre-development Delta,” said Dr. Deverel.  “Of course, we haven’t seen that for a long time, but we do know that it’s possible.”

So they reviewed the literature to discern what the characteristics of the floating peat islands are.  When they compared that to the data from Twitchell Island, the data indicated the recently accreted organic materials and sediment on Twitchell Island, which has been accreting sediment at about 3cm per year since 1997, would float.

We excavated a floating peat block which is still floating in my backyard as of this day,” Dr. Deverel said.  “You can see the amount of biomass that that accumulated. It’s very low density, highly organic, and very high for water methane concentrations on the order of 1000 micrograms per liter. That may contribute to buoyancy as well.”

So to study this idea further, they developed a pilot project with Metropolitan Water District to assess fish food potential, quantify the biomass accretion and methane, and evaluate continued flotation.

Eight Doughboy pools were built adjacent to the Mokelumne River on Bouldin Island.  Blocks of peat were excavated from Twitchell Island, moved over to Bouldin Island, and placed in the pools.  The blocks were about two feet wide, four feet long, and one foot thick.  They measured the weight of the blocks and estimated their volume.   The blocks were less dense then the water, so everything floated, without any vegetation, and without really much roots extending from the blocks. 

We developed a couple treatments – one is looking at varying residence times and the other is looking at varying vegetation density,” said Dr. Deverel.  “We can see that these blocks do float out there.”

The slides below show the growth of the peat blocks over the summer.

A large amount of biomass accumulated during the year in most pools, except for those that had the less vegetation treatment. 

We’re tasked with really trying to measure and model peat accretion and methane emissions,” said Dr. Deverel.  “We’ve done biomass harvesting,  measured methane fluxes from the blocks and the open water using collars placed in the blocks, and we have harvested biomass.  We’ve done remote sensing using a drone and we’re looking towards modeling these data as we collect more.”

The UC Davis Center for Watershed Science studied the food web benefits, analyzing the channel and pool water data for nutrients, aqueous carbon, invertebrate peat habitat utilization primarily or exclusively, zooplankton, and macro invertebrates.

He presented a slide of the zooplankton data, which shows the increase in zooplankton overtime, primarily the cladocerans and the daphnia that like slow moving environments and are filter feeders.   He pointed out that all the values are substantially higher than the channel levels which are shown in the third column.

There is a general increase over time,” said Dr. Deverel.  “It does seem to have reached a kind of static situation in which these zooplankton have adapted and created a home for themselves underneath these floating peat blocks feeding on carbon-related food stock.”

The slide shows dissolved organic carbon over time in all the pools. 

We had the spike initially and then a decrease over time, probably the result of flushing out of organic carbon that sloughed off the peat,” he said.  “But we do see concentrations on the order of two to four milligrams per liter which is consistent with the concentrations that Jacob Fleck in this paper in 2007 reported for the Twitchell Island surface water. So it does indicate that there is production of dissolved organic carbon that’s being consumed and incorporated into the microbial biomass, which is then consumed by the zooplankton and macroinvertebrates.”

The nitrate data indicates consumption by the biomass.  He noted that there are consistently higher concentrations of nitrate in the channel relative to the pool.  

Our current hypothesis is that the floating mat ecosystem foodweb and aquatic invertebrate populations will evolve as wetland plants grow, senesce and release carbon, and that there will be continued production of foodweb available carbon,” he said.

He explained how the process ultimately creates fish food.  The dissolved organic matter is converted to living bacterial biomass.  Copepods and zooplankton ingest floating aggregate vegetation material and are consumed by fish.  Protozoa consume bacteria in the aggregate and are consumed by invertebrates and juvenile and larval fish.  He noted that some fish do consume the plant detritus and there is also a pathway of particulate organic matter, to algae, copepods, and other crustaceans into the fish.

The slide shows the measurements of the biomass.  “With respect to biomass, here we have measured a substantial amount of biomass and the blocks: about 1900 grams of dry weight per meter squared, and much more root biomass – almost 10,000 grams of dry weight per meter squared.”

The chart on the bottom half of the slide shows the seasonal trends for the leaf area index (LAI), which peaks in summer and decreases over time as expected.

Methane fluxes are important for considering potential greenhouse gas benefits. 

We do see this increase commensurate with temperature and then a decrease peak value here in August,” said Dr. Deverel.  “We do see much higher fluxes associated with the floating peat relative to open water in July, relative to December as we’d expect.

A key issue is the comparison with the Delta permanently flooded wetlands which are known to be weak sinks or weak sources for greenhouse gases.

These floating blocks that we measured, the 1900 grams of dry weight per meter squared, are generally consistent with the values reported by Miller and Fuji and also the peak methane fluxes were consistent with the eddy covariance in general,” said Dr. Deverel.

So, we can say from this that there is likely a benefit to creating these floating peat situations on subsided islands,” he said.  “They will be basically weak sinks or weak sources on the order of about a ton of CO2 equivalents per acre per year compared to about 10 to 20 times that of emissions of carbon dioxide equivalence from the oxidizing peat, so it will stop subsidence and create a net greenhouse gas benefit on subsiding soils.”

As for next steps, they are considering a Prop 1 grant to develop a pilot project design that would include floagint wetland plants generating a food web benefit, which can be exported over to adjacent channels and also provide fish habitat.  They are also proposing to look further into the food web benefit by proposing biogeochemical tracing isotopes and meta genomic analysis.

Our preliminary conclusions are that there is a food web benefit,” said Dr. Deverel.  “Biomass and methane production are consistent with managed wetlands.  There is likely net greenhouse gas benefit. And we predict that these things will float for some time.”

QUESTIONS & ANSWERS

QUESTION:  Are there any subsidence reversal benefits from the floating wetlands, and any material accumulating at the bottom of the pools below the root mass?

Yes to both of those,” said Dr. Deverel.  “There is material accumulating at the bottom of the pools. And if one created these, there would be a net benefit to subsidence because basically you stop that the organic soil oxidation that results in subsidence just by flooding the soil. So there would be not only a subsidence benefit, as well as, we believe, a greenhouse gas benefit.  The details are important. But in general, we see a greenhouse gas emission from Delta organic soils on average on the order of about 10 tons of CO2 equivalents per acre per year. So for a net benefit, it would be on the order of about nine or so,  considering that the average of managed Delta wetlands is on the order of amount a ton of CO2 per acre per year.

QUESTION: Since peat takes a long time to accumulate naturally, are scientists looking into peat substitutes to create more floating wetlands in the Delta?

We have looked into this a little bit,” said Dr. Deverel.  “There is some of that going on in other places where there’s an attempt to have floating vegetation that maybe serves a water quality benefit. There could be that possibility in the Delta, but I don’t think I could say right now how that would work out. We’ve done very little looking into that possibility with Delta. But certainly we know that the vegetation can float and one might be able to manage to put something together that would float as well.”

QUESTION: How do you envision building the larger scale ponds that you showed at the end? Can you establish flow through the ponds to reduce methane emissions?

We’ve been looking at this with Metropolitan Water District and trying to assess what our next step might be,” said Dr. Deverel.  “We’ve come to the preliminary conclusion that the best way to look at this may be to create a small setback levee with an area that would we would be open to the channel but not a very large area, and somehow come up with enough peat blocks to be able to fill that in. We’re thinking about how to do that as well. Certainly, more freshwater interchange might reduce methane emissions. Now the methane emissions come from the anaerobic environment that sits in the water there in peat blocks and very little of it is coming from the open water. We do have flow through these pools at various rates. So I’m not sure how much difference the water interchange might make in terms of methane emissions. Certainly the literature indicates pretty high methane emissions that are consistent with ours for freshwater tidal wetlands throughout the world.”

QUESTION: How does organic carbon accumulation from a floating peat island compare with biomass accumulation in shallow flooded wetlands, Eg Twitchell or ricefields allowed to accumulate?

For the managed wetlands, the above ground biomass that we see that was collected for our first year was within the range that was collected within the first six years and reported by Robin Miller and Roger Fuji in 2010 on the order of about 2000 grams per meter square,” said Dr. Deverel.  “I really can’t say much about the rice; I don’t know what those biomass accumulation rates are. But in general, we’re seeing above-ground biomass rates consistent with the Twitchell Island situation, but much higher root mass. I’m amazed at how far down these roots are extended and how much root mass is there.  It seems that there is a lot more biomass than what they measured at the Twitchell Island site.”

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