Floating Wetlands Project

by Rob Zisette

Floating treatment wetland systems (floating wetlands) are engineered, vegetated rafts made of natural or inert materials that mimic floating bog mats. Root systems of the planted vegetation extend deep into the water and become covered with a biofilm comprised of a variety of microbes (algae, bacteria, fungi, and protozoans). The biofilm grows rapidly, taking nutrients and other pollutants from the water, improving water quality by reducing the nutrient supply for phytoplankton growth, and hence reducing algae blooms. The biofilm also readily adsorbs toxic pollutants dissolved or suspended in the water. Ultimately, the biofilm sloughs and becomes incorporated into lake sediments. Some nutrients (approximately 20%) are also incorporated into the planted vegetation during active growth that become incorporated into the wetland media. Floating wetlands also reduce high water temperatures from shade. The degree of water quality improvement depends primarily on the floating wetland area in relation to the water body area, but also depends on nutrient concentrations, water currents, temperature, and dissolved oxygen.

floating ecosystems

Floating wetlands also provide valuable habitat for fish and wildlife, particularly in urban water bodies lacking natural shoreline vegetation. Small fish seek refuge from predators and feed extensively on insects and other invertebrates that populate the underlying biofilm. Floating wetlands can be designed for waterfowl access using ramps, or for waterfowl exclusion using perimeter netting to protect planted vegetation and/or using plant species that are not a preferred waterfowl food source. Recent innovative designs include submerged platforms to create shallow water habitat above the plant matrix for juvenile salmonid rearing and optimum habitat for native bulrush species and other emergent vegetation historically used by Native Americans.

Floating wetlands offer several advantages over shoreline wetlands. Plant survival is higher because there is no lack of water since they are not affected water level fluctuation or drought. Plants are also less disturbed by foot traffic and vandalism since they can only be accessed by boat. It is much less expensive to expand wetland habitat using floating wetlands than constructing shoreline wetlands. Floating wetlands are also used as breakwaters to reduce shoreline erosion or bulkhead undermining, including some in the Gulf of Mexico that have survived hurricane winds and waves.

Floating islands are not a new technology, but they have not been used very often in the Pacific Northwest. Those installed in Washington State’s public lakes have been to decrease water temperature and nutrients, but not the occurrence of toxic algal blooms. Floating islands have been used in other areas throughout the world, most often in stormwater detention ponds, sewage treatment lagoons, and urban canals for nutrient removal to inhibit algal blooms. Multiple studies have shown that floating wetlands have been shown to be effective in reducing high nutrient concentrations in stormwater detention ponds (e.g., Headley and Tanner, 2011; White and Cousins, 2013; Wang and Sample, 2014). Limited water quality effectiveness data are available for floating wetland treatment systems in natural lakes or other water bodies. Even lakes with nuisance algal blooms typically have nutrient concentrations substantially lower than in sewage-contaminated canals or stormwater detention ponds. Although inconclusive, three years of monitoring data collected from Hicklin Lake after the installation of two small floating wetlands indicated that the wetlands were too small to significantly reduce nutrient or algae concentrations (King County 2018).

The Green Lake floating wetlands would be constructed to benefit water quality, fish, and native waterfowl, and to educate the public on their functional value. We have teamed with Biomatrix Water for this project because their Floating Ecosystems are designed with the highest engineering quality for buoyancy and longevity and made with sustainable materials. Biomatrix Water has improved their floating wetland design since their Floating Ecosystems were installed in Hicklin Lake in 2012. Information about Floating Ecosystem design features are shown in Figure 3 and the following links:

We recognize that Green Lake floating wetlands will not be large enough to eliminate a need for future alum treatments. However, some water quality benefit would be expected, and we would include the Biomatrix Water’s underwater media columns to maximize surface area for biofilm growth and nutrient uptake. Our site selection and design will primarily focus on providing native waterfowl habitat with consideration of the high potential for education and vandalism in this highly used lake. One waterfowl species of particular interest is the pied-billed grebe. This small, widespread, opportunistic feeder, and floating nest builder is frequently observed at Green Lake and popular among local naturalists (https://www.inaturalist.org/observations/21003420). Their unique characteristics are described by: FOGL (http://friendsofgreenlake.org/pied-billed-grebes-at-green-lake/), Seattle Audubon Society (http://birdweb.org/birdweb/site/seattle_-_green_lake/3), and the Cornell Lab of Ornithology (https://www.allaboutbirds.org/guide/Pied-billed_Grebe/overview).

floating wetlands map

Three sites of interest have been initially selected for further consideration(see Figure 4):

  1. Duck Island located in the western portion of the lake where floating wetlands would be anchored just off shore of the island to provide protected, shallow water nesting habitat that is lacking along the island’s shore, in addition to water quality benefits.
  2. Densmore drain outfall located adjacent to the northwest shore of the lake where the islands would be anchored off shore of the outfall to filter pollutants discharged from the lake’s largest source of stormwater runoff, in addition to waterfowl benefits.
  3. Northeast cove located north of the East Beach and the public dock where floating wetlands would be anchored just off shore of the segment of lake shoreline that accumulates the most toxic algae scum and invasive Eurasian watermilfoil fragments, effectively capturing the drifting algae and plant fragments to reduce shoreline accumulations in addition to water quality and waterfowl benefits.

We anticipate the floating wetlands would have a linear, angled shape at each of these sites, similar to that shown in the adjacent drawing of connected Floating Ecosystem sections.

The sections would be shipped to Green Lake and then in one day planted with bare-root native plants from a local source by trained volunteers, placed in shallow water for connection at the corners by stainless steel cotter pins, and towed by a canoe to the site for anchoring with heavy-duty buoy lines and anchors.

In addition to public involvement in the design and planting activities, FOGL would provide weeding and other maintenance of the floating wetlands through our association with Green Lake Stewards and other interested volunteers.

The project would include signs for educating Green Lake visitors about the Floating Wetland Project. A large sign could be designed to show how floating wetlands benefit water quality, fish, and waterfowl. The large sign could be installed next to the alum treatment sign located on the amphitheater next to the Small Craft Center. In addition, each floating wetland would have a small sign identifying the purpose of the floating wetland and discouraging boaters from disturbing the habitat.

Materials are estimated to cost approximately $35 per square foot, including Floating Ecosystem sections, underwater media columns, shipping, plants, and the anchoring system. This cost is based on a discount offered to FOGL by Biomatrix Water. At this unit cost, two floating wetlands with dimensions of 20 meters(66 feet) long by 2.3 meters (7.5 feet) wide would cost approximately $35,000. An additional $5,000 would be needed for education for a total project cost of $40,000, assuming SPR provides permitting assistance.