Wetland restoration: Is it worth the effort?

If forests are the lungs of our planet, then wetlands are surely the liver. Wetlands filter overland runoff through a unique suite of plants and soil microorganisms that break down harmful compounds, ensuring that waters reaching streams and coastlines are contaminant-free. Wetlands also store a massive amount of carbon in their soils, serve as a nursery ground for many fisheries, and protect shorelines from floods and storm surges.

Despite these benefits and more, humans allowed worldwide wetland degradation to continue unchecked for two hundred years. Instead of protecting existing wetlands, we gamble on restoration efforts, investing over $70 billion to bring back functional wetlands in North America alone. Until now the overall outcome remained unclear. But after investigating 621 wetland restoration projects around the world, a team of scientists led by Dr. David Moreno-Mateos and renowned ecologist Dr. Mary Power of the University of California Berkeley determined that wetland restoration is generally a losing bet. They published their results last month in PLoS Biology.

The team combed the scientific literature for studies that compare restored and newly created wetlands to adjacent undisturbed wetlands. They compiled all the data from these studies to find out how three characteristics of the restored or created wetlands recover over time: hydrology, biological community, and biogeochemistry.

Wetland hydrology includes water level, flooding patterns, and water storage. It recovers quickly because it depends on the surface features engineered by restoration teams. Before they place a single plant in the ground, restoration teams manipulate soil levels and permeability to achieve ideal patterns of wetland hydrology. The biological community, however, is much harder to recreate. Even after 100 years, biological communities in restored wetlands only recovered to 77% of their undisturbed counterparts. Moreover, plant communities recovered more slowly than animals.

Biogeochemistry, the movement and transformation of nutrients in a system, only recovered to 74% after 50 to 100 years. A healthy, functioning wetland stores large amounts of carbon and organic matter, because their flooded soils deprive decomposing microorganisms of necessary oxygen. If wetlands dry out during a disturbance, oxygen reaches decomposers, allowing them to break-down carbon compounds and larger pieces of organic matter. The decomposers eventually release carbon dioxide to the atmosphere as a byproduct of this process. Once restoration teams restore the proper wetland hydrology, decomposition shuts down and carbon begins to build up in the soil once again. However, even after twenty years, the amount of carbon and organic matter stored in wetland soils remains significantly less than levels in undisturbed wetlands.

Dr. Power's team suggests that restored wetlands do not resemble their original condition because they have shifted to alternative states. Much like building a house, successful ecosystem restoration requires that all the right pieces come together in the correct order. If a construction crew lays out the wrong foundation or neglects to build the second floor, the house will look completely different from the blueprints. The result of this botched construction project is like an alternative state. In ecosystems, the presence or absence of certain organisms determines which organisms come next. If a restoration team fails to introduce an important organism, or does so at the wrong time, a restored wetland may give rise to an alternative state that differs from the goal.

When it comes to wetland restoration, we may know which plant species to add, but the proper foundation remains unknown. Wetland plant communities and biogeochemistry depend on soil microorganisms. If the microorganism community shifts during wetland degradation, or if restoration requires imported soils, the soils may lack the microorganisms necessary for native wetland plants to grow. These unseen changes underground have cascading effects through the wetland ecosystem, producing an alternative state. Alternative states often host different organisms, function at reduced levels, and cannot confer the ecosystem benefits we depend on. Unfortunately, the only way to fix it is to start again from scratch.

Moreno-Mateos D, Power ME, Comı´n FA, Yockteng R (2012) Structural and Functional Loss in Restored Wetland Ecosystems. PLoS Biol 10(1): e1001247. doi:10.1371/journal.pbio.1001247

Photo: Vermont Department of Forests, Parks and Recreation

After 150 Years - Clarity and Consequences

Earlier this spring during a spate of unusually hot weather in the Bay Area, rays of sunlight stretched below the surface of Richardson Bay to trigger an intense algal bloom. Like all blooms, this rapid proliferation of algae was encouraged by the warm temperatures and an adequate supply of nutrients. About a month later another bloom occurred. Both events resulted in floating clumps of innocuous red algae and calls to the Baykeeper pollution incident hotline from concerned shoreline residents. My response: Don’t panic. If it doesn’t smell, it’s not a sewage spill.

According to an article by James Cloern, et al in the 2006 Pulse of the Estuary, Bay Area residents should become familiar with this sight. Algal blooms have been occurring with increasing frequency in the San Francisco Bay since the late 1990s, and the trend is likely to continue. The cause has been uncertain, however, because a host of factors promote the growth of algae. These include predators, nutrients supply, temperature, and metals. In every ecosystem one of these variables must be the limiting factor that controls algae growth and prevents bloom events. In many aquatic systems, such as the Chesapeake Bay, nutrients are the limiting factor. Given the excessive agricultural runoff in the Chesapeake Bay watershed, it is no surprise that algal blooms have been a serious problem. Despite the always reliable winter sewage spills, however, nutrient levels in the San Francisco Bay have been consistently low. So what is the variable that allows this unusual and unseasonable growth of algae? The upcoming 2009 issue of the Pulse of the Estuary will shed more light onto this question. The answer, in fact, is light.

The San Francisco Bay is becoming clearer! The concentration of suspended sediment in the Bay has been steadily decreasing since 1999, allowing sunlight to reach further below the surface of the water, stimulating algae growth and causing blooms. Incredibly, the reason for our water clarity today stems from human activities during the Gold Rush Era. In the late 1800s hydraulic gold mining sent tons of sediment, waste from the search for gold in the Sierra foothills and the Coast Range, down the Sacramento River and other Central Valley rivers. At the same time, development in the Bay Area caused the erosion of stream banks. Shoreline tidal marshes that were diked off to increase buildable and farmable land area could no longer capture this eroded sediment at the shore before reaching open water. As a result, the sediment settled on the floor of the Bay – so much sediment in fact, that the Bay became shallower. Bay Area residents are very familiar with the dredging platforms that regularly remove sediment, carving navigation channels into the floor of the Bay. In addition to dredging, natural wave patterns and burrowing wildlife can stir up sediment and re-suspend it in the water column. High concentrations of suspended sediment reduce the depth to which sunlight can penetrate the water, thus controlling algae growth and preventing most blooms.

Recent USGS data suggest that the Bay experienced a dramatic increase in clarity when this erodible supply of sediment was depleted in the late 1990s. In this year’s Pulse of the Estuary, David Schoellhammer of USGS offers an explanation as to how this may have happened. As long as the San Francisco Bay received sediment from upstream sources and held suspended sediment at capacity, erosion from the floor of the Bay was minimal. Although the Sacramento River delivered sediment, it also gently flushes the Bay and gradually pushed sediment through the Golden Gate. River banks in the Central Valley were protected during the 1900s to prevent erosion, and other sources of sediment are trapped behind dams. The remainder of the hydraulic mining supply slowly moved downstream until it reached the Bay. As a result the Sacramento River delivered clear water, which increased the erosion of sediment on the Bay floor. In 1998, a wet year during which the strong, clear flows from the Sacramento River persisted well into the summer, most of the remaining sediment supply was likely eroded pushed out of the Bay. The following year saw the suspended sediment concentration of the Bay waters decrease by 50%.

This great sweep of sediment through the Golden Gate did not unearth an ecological time capsule to the Bay’s pre-Gold Rush condition. The subsequent increase in clarity in the Bay is a major shift in water quality, which is causing a cascade of ecological and economic consequences in light of modern environmental stressors. As Bay Area residents have recently witnessed, the low concentration of suspended sediment in the Bay makes more light available to stimulate the growth of photosynthetic organisms – aquatic plants, algae, and other phytoplankton. As these organisms thrive they feed higher trophic levels, and the Bay food web becomes more robust. As Schoellhamer points out, the San Francisco Bay has crossed a threshold and become an estuary with a level of primary production that is more typical of temperate latitudes. This increased productivity has implications of its own. With a greater availability of light, nutrient inputs have a greater impact in the growth of algae. While the San Francisco Bay regularly receives nutrients from agricultural runoff or sewage spills, the low light has always prevented excessive growth of phytoplankton. Under current conditions, however, these inputs may trigger more intense bloom events and their associated problems.

The loss of sediments may also hinder coastal wetland restoration efforts. Wetland restoration usually involves opening up a previously diked area to the tides, so that suspended sediments in the water will naturally settle out along the shore, gradually building up until the land is high enough for plants to colonize. The lower the concentration of suspended sediments in the water, the longer it will take for the wetland to develop. Now with rising sea levels threatening to inundate our shorelines, the growth of new wetlands will likely be outpaced. To speed up the process, wetlands restoration projects may also utilize dredge spoils. With the loss of sediment from the Bay bottom, however, there is less of a need for dredging and a limit to sediment available for these restoration projects. Incredibly, the natural expulsion of sediments from the Bay, which caused supplies to shrink while demand has recently grown, has changed hidden Gold Rush waste into a valuable natural resource.

Learn more about Bay sediment in the 2009 Pulse of the Estuary, from the San Francisco Estuary Institute. The Pulse is the annual report for water quality in San Francisco Bay. You can find it at www.sfei.org

(Photo Credit: Michael Slater 2006)