Origin & Spread  |  Biology  |  Impacts  |  Detection  |  Prevention and Management

Introduction

Didymo (Didymosphenia geminata), a globally invasive single-celled algae (diatom), is threatening the streams and rivers of New York State. Didymo secretes massive amounts of branching stalks, creating dense mats that cover the bottoms of streams and rivers. Nicknamed “rock snot” for its gooey appearance, didymo has been confirmed at eight locations in New York State since 2007.

Origin & Spread

Historically, didymo was considered a widely distributed, yet uncommon, algal species native to the cool, running freshwaters of the northern hemisphere, including northern parts of North America, Europe, and Asia. Records collected over the last 150 years indicate that didymo widely occurred across Europe, including the UK, Norway, and northwest Russia. Didymo diatoms have been reported in the western US for over 100 years, but not as nuisance blooms.

Yet, within the last two decades, didymo blooms have been reported with increasing frequency and intensity across the globe, particularly in locations where historical blooms have not been previously recorded. In North America, didymo blooms were first documented in the 1990s in rivers on Vancouver Island, British Columbia. In the last 20 years, bloom occurrences have spread east across North America and have become increasingly common in the northeastern US, particularly in streams and rivers frequented by anglers and other aquatic recreationists. In 2011, the species could be found in 18 US states and 3 Canadian provinces.

Didymo has been confirmed in eight locations in New York State since 2007, including:

• Batten Kill and one tributary (Washington County)
• Kayderosserras Creek (Saratoga County)
• East Branch Delaware River below Pepacton Reservoir (Delaware County)
• West Branch Delaware River below Cannonsville Reservoir (Delaware County)
• West Branch Delaware River below Delhi to Cannonsville Reservoir (Delaware County)
• Mainstem Delaware River (Delaware and Sullivan Counties)
• Mouth of Little Delaware River (Delaware County)
• Esopus Creek downstream of the Shandaken Portal (Ulster County)

All of these confirmed sites are prime fishing access points where the species has been observed by numerous anglers; it is very possible that Didymo exists in other waterways where it has yet to be identified.

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Confirmed Didymo sites in New York State, 2010.

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Didymo was first detected in the southern hemisphere in New Zealand in 2004, and has continued to spread through numerous watersheds on the South Island (over 32). New Zealand’s didymo blooms have been particularly severe, with mats growing up to 8 inches thick and extending over 2.5 miles in length. Consequently, the government agency Biosecurity New Zealand has been a global leader in efforts to slow the spread and educate the public about didymo.

In 2010, the first didymo blooms in South America were confirmed in several rivers flowing through Patagonia (Chile and Argentina). It was first detected there in the Futaleufú River, a site popular with kayakers from all over the world.

Humans are largely responsible for the recent and prolific spread of didymo beyond its historical range. The microscopic diatoms can cling to fishing gear, waders, boots, and boats, and are capable of surviving at least 40 days outside of a stream as long as they remain in a damp, cool environment.

Felt-soled wading boots are major vectors of didymo as the soles absorb cells like a sponge and provide a temporary habitat for the didymo cells until an angler visits a new site. As a result, natural resource agencies as well as fishing organizations and supply stores like Trout Unlimited and L.L. Bean have promoted non-felt wading boot alternatives. Furthermore, the states of Vermont, Maryland and Alaska have outlawed the use of felt-soled wading boots.

Biology

Didymo is a freshwater diatom, a type of single-celled algae that lives attached to rocks and other substrates on the bottom of streams. Under certain conditions, didymo will secrete thick, branching stalks (composed of polysaccharides and protein) outside its cells that form dense tangled mats. The resulting didymo blooms can completely dominate streambeds, capable of lasting several months, with mats greater than 8 in thick and over 0.5 mi long. Unfortunately, the triggers of excessive stalk formation – possibly genetic and/or environmental – are unknown and the subject of current scientific research.

Thick didymo mats resemble fiber-glass insulation or wet toilet paper, inspiring its nickname, “rock snot.” It is generally light tan to brown in color (not green), with stalks sometimes forming long white strands. Clumps of didymo are not slimy, resemble wet wool, and are tough to pull apart.

In contrast to other types of freshwater algae that form nuisance blooms, didymo blooms first appeared in streams with relatively high water quality (clear, cold, low nutrients, stable flow). Often blooms occurred immediately downstream of impoundments, where flows were generally stable, regulated, and nutrient poor. More recently, as its range has expanded, didymo has been recorded in warmer, more nutrient-enriched waters, not associated with dams.

Impacts

Didymo can alter the diversity and distribution of native stream species and may have negative consequences on how stream ecosystems function.

The extensive stalks produced by didymo cells persist in invaded streams longer than the diatoms themselves and are resistant to degradation; reports from Colorado indicate thick mats of didymo stalks can persist up to 2 months after peak growth. These mats may trap stream sediments, changing the physical nature of the stream bottom and affecting the ability of native stream algae to colonize.

By covering and dominating the substrate, didymo may alter habitat and available food resources for bottom-dwelling stream invertebrates. Recent studies in New Zealand and North America have reported shifts in the invertebrate community as a result of didymo invasion. The proportion of larger sized, environmentally sensitive insect groups like mayflies, stoneflies, and caddisflies (Ephemeroptera, Plecoptera, Trichoptera; known collectively as EPT taxa) tend to decline in didymo infested waters while the proportion of smaller invertebrates, particularly midge larvae (Diptera: Chironomidae), worms (Oligochaeta, Nematoda), and crustaceans (Cladocera), increases. In most cases, the total number of invertebrates actually increases with didymo invasion, possibly as a result of the new habitat or additional food resources provided by the dense didymo stalks.

Shifts in the abundance and diversity of invertebrates as a result of didymo invasion could potentially affect the fish that feed on them. Declines in mayflies, stoneflies and caddisflies could threaten trout populations. Furthermore, didymo blooms covering large stretches of the stream bottom could alter stream habitat that is important for fish feeding and spawning. Research on the effects of didymo on native and sport fisheries is still underway. To date, research in North America and New Zealand has been inconclusive, with some studies suggesting no impact of didymo on fish growth and productivity, while others have observed native fish population decline in didymo-invaded waters.

Didymo invasions, although unsightly, do not produce an odor or threaten human health. Infestations do have significant negative impacts on all water-associated recreational activities, particularly sport-fishing. Floating didymo stalks tangle up lines, flies and lures. Additionally, didymo blooms have blocked water intake pipes and canals. Consequently, didymo remains a serious economic concern for fisheries, tourism, irrigation, and hydropower.

Detection

Didymo can exist both as a single cell without stalks or as a colony of cells with branching, mat-forming stalks. Unfortunately, invasions are not usually detected until dense didymo mats occur. Researchers who have intensively sampled stream algae by scraping rocks and filtering water from streams with no previous history of didymo blooms have identified single didymo cells when examining those samples under the microscope. Yet, this type of work is extremely time and labor-intensive as didymo cells can be quite rare. Research is now ongoing to develop DNA-based tests for detecting didymo in streams, even at very low concentrations.

Prevention and Management

Currently, there are no methods available for controlling or eradicating didymo once it has infested a water body. Research in New Zealand is underway to identify and evaluate the safety and effectiveness several didymo-killing chemicals.

Spread prevention is, therefore, the only method we have for protecting our streams and rivers from didymo invasion. Water recreationists must take great care to inspect, clean, and dry all equipment, especially waders and boots when leaving an infested stream or river.

Inspect: Look for and remove all clumps of algae and discard in designated invasive species disposal stations or upland.

Clean: Clean and disinfect any equipment that has come in contact with the water, whether you observe a didymo bloom or not as other aquatic invasive species and diseases may hitchhike on your equipment. Waders and gear should be soaked and/or scrubbed with a 2-percent solution of household bleach or a 5-percent solution of detergent and very hot tap water (> 115°F); ‘eco-friendly’ detergents are not recommended. Absorbent materials (e.g., felt soles, foam) will require prolonged soaking to kill didymo cells (> 30 minutes). Gear could also be placed in a freezer until all of the moisture is frozen solid (note: freezing may damage some gear and will only kill didymo, not necessarily invasive fish diseases).

Dry: Drying will kill didymo, but this method alone is not recommended for absorbent materials because didymo can survive in slightly moist environments for an extended period of time.

If cleaning, fully drying, or freezing is not practical, restrict equipment use to a single water body. One didymo cell transported on gear could result in an invasive didymo bloom – thus, precautionary action is essential!

Photo and Map Credits

Didymo bloom observed in the Batten Kill. – VT DEC

North American distribution of didymo 21 July 2008. National Invasive Species Information Center. Modified from map created by Karl Hermann, Sarah Spaulding, and Tera Keller. For original image click here

Didymo bloom observed in Esopus Creek near Mt. Tremper, NY. – David Richardson, SUNY New Paltz

Didymo in stream on New Zealand’s South Island.  – New Zealand Fish and Game

Felt-soled wading boots – Treehugger.com

Close up of Didymo stalks – Sarah Spaulding, USGS

Didymo blooms showing dense woolly appearance – South Dakota Dept. of Environment & Natural Resources

Origin & History  |  Identification & Biology  |  Ecologic Impacts  |  Economic Impacts  | Control & Management  | New York Distribution Map |  References

Background

If a shoreline property owner in New York or the Northeast complains to you about their water chestnut problem, don’t think they are talking about Chinese takeout. The European water chestnut (Trapa natans), an invasive aquatic plant released inadvertently into waters of the Northeast in the late 1800s, is slowly but inexorably spreading throughout New York State, clogging waterways, lakes and ponds and altering aquatic habitats.

It must be pointed out that this plant species is not the same as the water chestnut which can be purchased in cans at the supermarket. The fruits of T. natans, however, are used as a source of food in Asia and have been utilized for their medicinal (and claimed) magical properties.

T. natans is native to Europe, Asia and Africa. In its native habitat, the plant is kept in check by native insect parasites. These insects are not present in North America and the plant, once released into the wild, is free to reproduce rapidly. T. natans colonizes areas of freshwater lakes and ponds and slow-moving streams and rivers where it forms dense mats of floating vegetation, causing problems for boaters and swimmers and negatively impacting aquatic ecosystem functioning.

Common names: horned water chestnut, water caltrop

Introduction History and Distribution

T. natans is native to Western Europe and Africa and northeast Asia, including eastern Russia, China, and southeast Asia to Indonesia. T. natans was first introduced to North America in the mid- to late-1870s, when it is known to have been introduced into the Cambridge botanical garden at Harvard University around 1877. It is known to have been planted in other ponds in that area, as well, and also in Concord, MA, in a pond near the Sudbury River. The plant escaped cultivation and was found growing in the Charles River by 1879. The plant was introduced into Collins Lake near Scotia, NY (in the Hudson River-Mohawk River drainage basin) around 1884, possibly as an intentional introduction for waterfowl food or possibly as a water garden escapee.

By the early part of the 1900s, water chestnut was established in the Hudson River. The first Great Lakes Basin introductions were sometime before the late-1950s when the plant was discovered growing in Keuka Lake (one of NY’s Finger Lakes). A major infestation of more than 300 acres exists throughout some 55 miles of Lake Champlain between New York and Vermont. Water chestnut can now be found throughout NY, from the Niagara Frontier through the Finger Lakes, from Lake Champlain to Long Island.

The North American distribution of water chestnut now extends throughout New England, south as far as Virginia, California, and in the Canadian Province of Quebec in a tributary of the Richelieu River. The plant has the potential to spread into the warmer regions of the U.S. as far south as Florida.

Identification and Biology

T. natans is a rooted aquatic annual herb that dies back at the end of each growing season. Re-growth is by means of seeds that germinate in the spring. Each seed produces 10 to 15 stems with submerged and floating leaves, terminating in floating rosettes. The feathery submersed leaves can be up to six inches (15 cm) long, and are alternate on the stem forming whorls around the stem. The three-quarter to one and a half inch (2 – 4 cm) glossy green floating leaves are triangular with toothed edges and form rosettes around the end of the stem. The floating leaves also have prominent veins and short, stiff hairs on their lower surface. The petioles (the stalks attaching the leaf blade to the stem; the transition between the stem and the leaf blade) of the floating leaves are two to eight feet (0.6 – 2.4 m) and contain spongy, buoyant bladders, allowing the rosettes to float on the surface of the water. Stems can reach lengths of up to 16 feet (4.9 m), although typical lengths tend to be in the six to eight foot (1.8 – 2.4 m) range. The stems are anchored to the bed of the waterbody by numerous branched roots. Single small, white flowers with four one-third inch (8.3 mm) long petals sprout in the center of the rosette.

Each rosette is capable of producing up to 20 hard, nut-like fruits. Water chestnut starts to produce fruits in July; the fruits, which ripen in about a month, each contain a single seed. The 1 to 1.5 inch (2.5 – 4 cm) wide fruits grow under water and have four very sharp spines. Water chestnut seeds generally fall almost directly beneath their parent plants and serve to propagate the parent colony. Population overwintering is accomplished through mature, greenish brown nuts sinking to the bottom where they can remain viable in the sediment for up to 12 years.  Some seeds, however, or plant parts (floating rosettes) that still contain nuts, may be moved downstream in currents. Ducks, geese and other waterfowl may also play a role in the nuts’ dispersal (the spiny nuts have been observed tangled in the feathers of Canada geese). Old nuts, black in color, will float, and are not viable. When deposited in shallow water or on the shore, water chestnut nuts can lead to injuries if stepped on. 

Ecological Impacts

Water chestnut has become a significant environmental nuisance throughout much of its range, particularly in the Hudson, Connecticut and Potomac Rivers, and in Lake Champlain. The plant can form nearly impenetrable floating mats of vegetation. These mats create a hazard for boaters and other water recreators. The density of the mats can severely limit light penetration into the water and reduce or eliminate the growth of native aquatic plants beneath the canopy. The reduced plant growth combined with the decomposition of the water chestnut plants which die back each year can result in reduced levels of dissolved oxygen in the water, impact other aquatic organisms, and potentially lead to fish kills. The rapid and abundant growth of water chestnut can also out-compete both submerged and emergent native aquatic vegetation.

Water chestnut has little nutritional or habitat value to fish or waterfowl and can have a significant impact on the use of an infested area by native species.

T. natans likely impacts non-native and invasive plant and animal species in the same manner it impacts natives. Some of the potentially impacted invasive plant species might include: Eurasian watermilfoil (Myriophyllum spicatum), curly pondweed (Potamogeton crispus), and Eurasian or brittle water-nymph (Najas minor). It is not yet known in a match up of T. natans or and hydrilla (Hydrilla verticillata, which invader would outcompete which. Because of its invasiveness and severity of its impacts, T. natans has been listed in federal regulations prohibiting interstate sale/transportation of noxious plants.

Economic Impacts

Economic impacts result from T. natan’s impenetrable mats of vegetation which can impede swimming, boating, commercial navigation, fishing, and waterfowl hunting. Untreated populations of such an aquatic invasive species also can result in losses to shoreline property values and, as a result, to local government property tax revenues. As mentioned earlier, the sharp, spiny nuts can result in puncture injuries to swimmers and recreators walking along the shore of infested areas and can injure the feet of livestock and horses, as well.

One example of the cost of managing T. natans in a waterbody is the experience of the States of New York and Vermont on Lake Champlain. From 1982 through 2011, $9,600,000 has been spent on Trapa control in the lake with funding from a number of sources including: the two states; the U.S. Army Corps of Engineers; the U.S. Fish and Wildlife Service; the U.S. Department of Agriculture; Ducks Unlimited; the Lake Champlain Basin Program; and The Nature Conservancy. A combination of hand pulling and mechanical harvesting has been used on the lake since the early-1980s. Significant reductions of T. natans populations resulted from this prolonged annual control effort, however, every time that funds were reduced, rapid grow back of the species and extension of its range in the lake was observed.

Control

Mechanical and Chemical Control

It is much easier and less expensive to control newly introduced populations of T. natans. Early detection of introductions and a rapid control response are key to preventing high-impact infestations. Because T. natans is an annual plant, effective control can be achieved if seed formation is prevented. Small populations can be controlled by hand pulling working from canoes or kayaks.

Large infestations usually require the use of mechanical harvesters or the application of aquatic herbicides. Regardless of treatment type, it should ideally take place before the fruit has ripened and dropped to the bottom forming a long-term seed bank. Because of the potential of unintentional spread of floating plant parts offsite, mechanical harvesting should be undertaken only by trained and certified equipment operators. Since water chestnut overwinters entirely by seeds that may remain viable in the sediment for up to 12 years, repeated annual control is critical to deplete the seed bank. Treatment generally is needed for five to twelve years to ensure complete eradication and can be very expensive (see Economic Impact, above).

Potential negative impacts to non-target species and public perceptions regarding the use of chemicals in recreational waters have limited chemical control of T. natans except as a treatment of last resort and usually only in still or sluggishly flowing waters. The herbicide 2,4-D has been tested and shown to be non-adverse on non-target species. 2,4-D has been used widely in the U.S. Another herbicide that is effective on T. natans is glyphosate. Application of aquatic herbicides requires both a licensed pesticide applicator and a permit from your state environmental regulatory agency.

Biological Control

The unfortunate fact is that for large infestations of water chestnut (i.e. those too large to be controlled by hand-pulling) over the long-term mechanical and chemical control measures have proven to be impractical to provide an economically sustainable control of water chestnut. Scientists have now turned to the potential of biocontrol agents to serve as a long-term solution to water chestnut infestations.

A number of potential biological control agents were found in field surveys in the native European and Asian ranges of water chestnut. The most promising biocontrol species appeared to be the leaf beetle Galerucella birmanica. Unfortunately, field observations in China suggested that G. birmanica may also attack native water shield (Brasenia schreberi) in addition to Trapa natans. This host non-specificity could be problematic to the use of the beetle for biocontrol in North America.

Laboratory and field tests initially indicated that out of 19 different plant species in 13 different families, G. birmanica laid eggs and completed development only on species of Trapa and B. schreberi. Adult G. birmanica in the field and lab indicated that the beetles showed a strong preference for T. natans. This preference continued even after the water chestnut was completely defoliated; adults resisted migrating to nearby water shield. While this is very promising news, additional studies on host specificity with additional North American aquatic plants are on-going. [Ding, et. al., 2006]

New York Distribution Map

This map shows confirmed observations (green points) submitted to the NYS Invasive Species Database. Absence of data does not necessarily mean absence of the species at that site, but that it has not been reported there. For more information, please visit iMapInvasives.

References

Blossey B, Schroeder D, Hight S, Malecki R. 1994. Host specificity and environmental impact of two leaf beetles (Galerucella calmariensis and G. pusilla) for biological control of water chestnut (Trapa natans). Weed Science 42:134-140.

Deck J, Nosko P. 2002. Population establishment, dispersal, and impact of Galerucella pusilla and G. calmariensis, introduced to control water chestnut in central Ontario. Biological Control 23: 228-236.

Ding J, Blossey B, Du Y,  Zheng F. 2006. Galerucella birmanica (Coleoptera: Chrysomelidae), a promising potential biological control agent of water chestnut, Trapa natans. Biological Control. Vol.36, Issue 1, Pages 80–90

Fernald ML. 1950. Gray’s Manual of Botany. 8th ed. American Book Company, N.Y.

Gleason HA. 1957. The New Britton and Brown Illustrated Flora of the Northeastern U.S. and Adjacent Canada. New York Botanical Gardens, N.Y.

Methe BA, Soracco RJ, Madsen JD, Boylen CW. 1993. Seed production and growth of water chestnut as influenced by cutting. J. Aquat. Plant Manage. 31: 154-157.

Mills EL, Leach JH, Carlton JT, Secor CL. 1993. Exotic species in the Great Lakes: A history of biotic crises and anthropogenic introductions. Journal of Great Lakes Research 19: 1-54.

Mullin BH. 1998. The biology and management of water chestnut (Trapa natans L.). Weed Technology 12:397-401.

Pemberton RW. 2002. Water Chestnut. In: Van Driesche R., et al. 2002. Biological Control of Invasive Plants in the Eastern United States. USDA Forest Service Publication FHTET-2002-04.

Rawinski T. 1982. The ecology and management of water chestnut (Trapa natans L.) in central New York. M.S. thesis, Cornell University.

Vermont Invasive Exotic Plant Fact Sheet Series: Water Chestnut. Vermont Agency of Natural Resources and The Nature Conservancy, Vermont Chapter. June, 1998.

Hunt T, Marangelo P. 2012. 2011 Water Chestnut Management Program: Lake Champlain and Inland Vermont Waters, Final Report. Vermont Department of Environmental Conservation. May 2012.

Photo Credits

Patch of floating water chestnut (Trapa natans) leaves. John M. Randall, The Nature Conservancy, www.forestryimages.org

NYS Distribution of water chestnut as of January 2014. © 2014 imapinvasives.org/nyimi  The Nature Conservancy. Accessed January 2014

North American distribution of water chestnut as of September 2014. Plants Database. USDA Natural Resources Conservation Service

Drawing of floating and submerged leaves and fruit (nut). Connecticut River Coordinator’s Office, US Fish & Wildlife Service

Hand holding water chestnut rosette. Alfred Cofrancesco, U.S. Army Corps of Engineers, www.forestryimages.org

Water chestnut infestation on Lake Champlain. Alfred Cofrancesco, U.S. Army Corps of Engineers, www.forestryimages.org

Riverine infestation of water chestnut. Leslie J. Mehrhoff, University of Connecticut, www.forestryimages.org

Chart of Lake George, NY, water chestnut annual control costs, 1982 – 2011. Data from Hunt T, Marangelo P. 2012.

Water chestnut harvesting machine. US Fish & Wildlife Service, Silvio Conte National Fish and Wildlife Refuge