Wild rice has received considerable recent attention in the St. Louis River watershed due to concerns over the environmental effects of mining on Minnesota's Iron Range, on the northern border of the watershed. Of particular interest is the effect of higher than historical concentrations of sulfate leached into waterways from piles of waste rock and taconite tailings, and potentially from future non-ferrous mining operations.

Wild rice typically grows in shallow water - 1 to 3 feet in depth on soft organic (mucky) sediments, and prefers flowing water. Unlike many aquatic plants, wild rice is an annual, reproducing from seed each year. The seeds of wild rice require a winter dormancy of about three months, with immersion in water at freezing, or near freezing, temperatures needed to break down the waxy coating that covers the seed. Seeds germinate when water temperatures reach 45 oF (Oelke et al.). After germination, the plant develops a system of spongy white roots that grow horizontally 9-12 inches. Like other grasses, wild rice also produces tillers - underground stems that give rise to other shoots. The number of tillers depends on stem density - sparser stands of wild rice produce more tillers on individual plants. Tillers are important in that they can also produce grain, but they tend to flower later than the source plant, which affects both the length of the harvest season and total yield of grain.

Seedlings have 3-4 leaves that remain underwater, with subsequent leaves reaching to and floating on the surface (Marcum 2007). Wild rice forms characteristic floating mats of ribbon-like leaves during mid-June. At this time, the plant is sensitive to water depth - rising waters will uproot the plants and lower water levels will damage the weak hollow stems. Wind and wave action can also uproot plants, and wakes from recreational watercraft are a persistent management issue on wild rice lakes and rivers.

Aerial shoots emerge from the water in late June and early July. The emergent plants grow 3-6 feet tall, with a central stalk supporting 5-6 elongated ribbon-like leaves ½ to 1 ½ inch in width. Flowers are produced in terminal clusters, with staminate (male) flowers on the lower stalk and upright pistillate (female flowers) on the upper branches. The female flowers emerge before the males, so plants are typically cross pollinated. Wind is the primary means of pollination. The seeds are surrounded by a papery hull with a long terminal awn, both of which are removed when rice is processed as a food source.

Seeds of wild rice mature in late August. Like other grasses, an important phase of the life history is the “shattering” of the mature seed, which refers to the seeds dropping off the plant as they ripen. In natural systems, shattering occurs over a period of days to weeks, allowing multiple harvests of the wild rice beds. In cultivated rice paddies, shatter-resistant varieties are used to allow for a single mechanized harvest.

The productivity of wild rice beds varies from year to year. A rule-of-thumb provided by the Great Lakes Fisheries and Wildlife Commission is “a typical four year period will include a bumper year, two fair years and a bust”. Walker et al (2010) have attributed this pattern to the nutrient dynamics of decomposition of wild rice straw - the remains of the parent plants. Following a productive year, microbes in the bottom sediments are thought to take up much of the nitrogen as they begin the initial decomposition of plant litter, making it less available to wild rice plants. After a year or two, the final stages of decomposition occur and nitrogen again becomes available for plant growth. Understanding litter dynamics is important, in that it can lead to more informed decisions on the optimal times for seeding wild rice beds.

Environmental Stressors

The distribution of wild rice has been greatly reduced from its historic range prior to European settlement (Meeker 2000, Vennum 1988). Pillsbury and McGuire (2009) reviewed numerous studies on wild rice decline, which are variously attributed to changes in pH and the presence of heavy metals and acids from copper-zinc smelting, [reduced] sediment nitrogen, [increasing] water level changes, and competition from other plants (i.e. invasives). In their study, Pillsbury and McGuire evaluated 60 wetlands which had historically dense wild rice populations but now vary in wild rice productivity, from high, to moderate to degraded. The study included two sites in the St. Louis River Estuary - Boy Scout landing, which now has low wild rice density, and Skibo Mills, which retains a healthy population. They assessed a range of environmental factors, from water chemistry to sediment type to watershed land use, and found that the strongest declines were associated with increased residential development and increased levels of ammonia-N (NH4+-N). Residential development is a predominant environmental stressor, indirectly resulting in increased stream temperature, sedimentation, and ‘flashiness’ of streams that feed the St. Louis River Estuary. The inverse relationship with NH4+-N is less straightforward, as wild rice is often limited by nitrogen availability, but may be related to increased competition from other plants stimulated by increased nutrient levels.

Wild Rice Conservation

Because of the degradation to natural populations, there are strong interests, particularly among the Fond du Lac Band of Lake Superior Ojibway (FDL), which directly and indirectly manages the wild rice resource in the St. Louis River watershed. The dominant method for managing wild rice is by control of water levels, through water control structures, ditch maintenance and beaver dam maintenance. FDL also has a strong program for removing competing vegetation, particularly the widespread pickerelweed (Pontederia cordata), an invasive species which has extensively colonized Perch and Jaskari Lakes on the reservation (Forbes 2005). The band uses a “Cookie Cutter” - a large platform based aquatic plant harvester that severs the mat of vegetation, which is then removed from the lake. This is often followed by wild rice re- seeding.

At the landscape scale, wild rice conservation involves a complex interplay among social values, economy and ecology, all of which have underlying spatial dynamics (Drewes and Silbernagel 2012). Not only are populations and the yield of wild rice reduced from historic numbers, but the number of people participating in wild rice harvest is declining, and the average age of active harvesters is increasing. Moreover, management occurs at the level of individual lakes or individual communities - there are few active regional statewide or broader entities that look at the collective effects of local decisions. Given the current social and ecological trends, a regional perspective that allows for broader conversation and planning is essential to ensure the sustainability of this important species (Drewes and Silbernagel 2012).

Literature Cited

Drewes, A. and J. Silbernagel. 2012. Uncovering the spatial dynamics of wild rice lakes, harvesters and management across Great Lakes landscapes for shared regional conservation. Ecological Modelling 229: 97-107.

Forbes, M. E. 2005. Assessing competitive interactions between wild rice (Zizania aquatica) and pickerel week (Pontedaria cordata) in a disturbed northern Minnesota wild rice lake. M.S. Thesis, University of Minnesota Duluth, Duluth, MN 99p.

Marcum, D.B. 2007. Cultivate wild rice production in California. University of California Agriculture and Natural Resources Publication 21622. 25 p.

Meeker, J. 2000. Ecology of ‘wild’ wild rice (Zizania palustris var palustris) in the Sakagon Sloughs, a riverine wetland on Lake Superior. p 68-84 In L. S. Williamson, L. A. Dlotkowski, and A. P. McCommon-Soltis (eds) Wild rice research and management. Great Lakes Fish and Wildlife Commission publication, Odanah, Wi, USA.

Oelke, E. A., T.M. Teynor, P.R. Carter, J.A. Percich, D.M. Noetzel, P.R. Bloom, R.A. Porter, C.E. Schertz, J.J. Boedicker, and E.I. Fuller. Alternative Field Crops Manual: Wild Rice. http://corn.agronomy.wisc.edu/Crops/WildRice.aspx Accessed 6.6.2013

Orcajada, P. 2012. Harvesting Wild Rice. Dept. of Crop Science, U. of Saskatchewan. http://www.agriculture.gov.sk.ca

Vennum, T. 1988. Wild Rice and the Ojibway People. Minn Historical society Press St. Paul.

Walker, R,. J. Pastor, and B. Dewey. 2010. Litter quantity and nitrogen immobilization cause oscillations in productivity of wild rice (Zizania palustris) in Northerm Minnesota. Ecosystems 13:485-498.

Wild Rice Information Resources

GLFWIC Wild Rice Brochure

Minnesota Wild Rice Management

Wisconsin Wild Rice Harvest Data

1854 Treaty Authority

Regulatory issues

Wild rice is an important part of Minnesota’s aquatic plant communities, particularly in the north. It provides food for waterfowl and habitat for fish and wildlife. Several Native American cultures, such as the Ojibwa, consider wild rice to be an important food and a sacred element of their culture. It is also economically important to those who harvest and market wild rice. Although many environmental factors regulate its growth and distribution, evidence has pointed to sulfate (SO4 -2) concentrations greater than 10 ppm (technically 10 milligrams-per-liter, expressed as mg/L) in the water being detrimental to wild rice. Surveys across the state by Minnesota DNR ecologist John Moyle in the 1940s indicated that the rice was more prevalent in the (typically) very low sulfate waters of most of northern Minnesota. This correlation of high sulfate and less wild rice was the major scientific basis for the establishment by the Minnesota Pollution Control Agency of the 10 mg/L sulfate concentration as a regulatory standard to help protect existing wild rice “waters.” The standard could also potentially help identify degraded water bodies so the cause of the problem could be diagnosed accurately and then addressed. This action was approved by the U.S. Environmental Protection Agency as part of the requirements of the U.S. Clean Water Act.

Before taking a closer look at the potential effects of excess sulfate on wild rice, it is helpful to review some basics about wild rice and aquatic vegetation in general in terms of several important water quality parameters.

Minnesota’s sulfate standard to protect wild rice
The MPCA created this website to allow people access to “Wild Rice-Sulfate” information including its historical basis and the science behind this somewhat unique water quality standard. Since 2011, in response to the potential new sources of sulfate from proposed non-ferrous sulfide ore mines (usually just called “Copper” ) mines, MPCA has engaged in a set of field and laboratory studies to better understand the cause of the negative association between wild rice and sulfate, and determine if the 10 mg/L value is appropriate.  These studies were designed with input from a broad community of scientists and carried out largely by scientists from the University of Minnesota. Details of this process, the study designs, and preliminary data and analyses may be found here.  As you can see from the diagram below right, the processes and other water quality parameters that affect sulfur and mercury chemistry and biology in water bodies is extremely complex and dynamic. Note: 10 mg/L can also be expressed as 10 parts-per-million (ppm).

Nutrients

Nitrogen

A four year cycle between peak wild rice production years is frequently observed, recent studies of local wild rice in natural waters and in tank experiments have suggested that nitrogen availability drives this cycle. Most of the nitrogen needed by the plants is taken up from the sediments by plant roots. Decomposing plants from previous years release nitrogen, with stems and leaves decomposing faster than roots. Nitrogen also enters wetlands and shallow waters via soil and plant litter washed in naturally from the surrounding watershed and from stormwater runoff from human activities. These sources vary from year to year, throughout the year, and from watershed to watershed. Organic matter in the sediments also comes from the decomposing remains of algae and from other aquatic plants besides wild rice.

Phosphorus

Phosphorus (P) is another key nutrient that is critical to plant growth and usually is in short supply in northern Minnesota waters unless there are inputs coming from domestic wastewater (i.e., sewage from septic systems or community wastewater treatment plants) or eroded soil from lawn or farm runoff where fertilizer-P was applied. In the absence of such an “unnatural” loading of phosphorus into the water body, there is little information on how natural variations in phosphorus levels might affect annual yields of wild rice 3,4.

Nutrient impacts on Shallow Lake Ecology

Studies of many shallow lakes have shown that when aquatic vegetation (mistakenly called “aquatic weeds” by many lake enthusiasts) reaches its peak in summer, the abundance of algae freely suspended in the water (phytoplankton) declines, and the water actually becomes much clearer. What’s happening is that the aquatic plants are shading the water enough to drastically reduce sunlight available for phytoplankton photosynthesis, thus reducing growth. Also affecting water clarity, the aquatic plants and their roots combine to dampen wave action from the wind. This lets particles in the water drop to the bottom faster and reduces the amount of bottom muck or mud that is stirred up during storms.

However, excess nitrogen and phosphorus can have significant negative impacts on aquatic plant communities by promoting excess algal growth. Human-generated sources of nutrients include seepage from septic systems, lawn fertilizer runoff, erosion, and stormwater runoff. Planktonic (free-floating) algae and attached algae (the scuzzy and often-slippery green and brown slime growing on rocks, plants and other structures along the shoreline) are fertilized by these nutrients, and as their abundance increases, the clarity of the water decreases. This excess growth of algae clouds the water, and if it occurs before aquatic plants have gotten well-established, algal growth can shade the bottom where rooted plants get their start, thus reducing the area of lake bottom where plants can grow.

Research suggests that it is possible for two distinct and relatively stable states to exist, one in which most of the vegetative productivity is in the form of higher plants, including wild rice, and one that is dominated by algae. Dominance by algae in the form of phytoplankton productivity can cause the water to become quite green and murky – thus restricting the growth of rooted plants to only the shallowest depths nearshore. further illustrates how these impacts, if large enough, are likely to lead to very different food webs, in addition to decreased plant production – including wild rice, if historically present. In summary, excess inputs of nutrients, along with overharvesting of aquatic plants usually leads to undesirable consequences that can negatively impact many of the beneficial uses of the water body.

Bottom sediments

Oxygen, Sulfate, Sulfide, Iron – This is not simple!

Another important part of the ecology of wild rice is the character of the bottom sediments in which the plants grow. Just as water chemistry and quality varies across wetlands and with depth, some critical characteristics of the sediments vary dramatically with depth into the sediment, as well as across different parts of the wetland. The most important factor as we go down into the mucky and sometimes soupy mud of the root zone is the amount of dissolved oxygen (DO). DO is rapidly depleted by all the biological activity in the mud, making it pretty much anoxic (lacking oxygen) except near the water-mud surface. In shallow waters (less than 10 – 15 feet or 3 – 4.5 m deep) oxygen is mostly coming from the air above the water, and when it’s windy and foamy the water is aerated more quickly. Photosynthesis by aquatic plants and algae can also contribute a significant amount of dissolved oxygen to the water during daylight, but at night and on cloudy days, the DO in the water above the sediment may drop to zero because the animals and many groups of bacteria continue to consume oxygen throughout the day and night. Their combined demand for oxygen can exceed the amount diffusing downward from the water. Once you probe down into the mud, beyond the top few centimeters you’ll find no oxygen at all.

Oxygen also regulates the metabolism of many groups of naturally occurring bacteria, and low DO will kill some types of bacteria, while allowing the growth of others. The combined effect is to structure the mud into zones and layers having different amounts and types of nitrogen and phosphorus and chemical forms of iron and sulfur. In well-oxygenated streams (riffles and falls) and the surface waters of most lakes and ponds, the primary form of sulfur is sulfate (SO4-2), which is usually harmless to plants and animals even at very high levels. In fact, the drinking water standard for humans (set to control taste and laxative action, not toxicity) is 200 parts-per-million (200 mg SO4-2 /L), which is 20x higher than the wild rice standard. Preliminary results from past and recent experiments, looking at wild rice from seed germination through the annual growth cycle, have largely confirmed that it’s not the sulfate itself at current levels in the St. Louis River that is toxic to wild rice, but the transformation of sulfate to sulfide that occurs under anoxic conditions. And of course, it’s still not that straightforward.

So what happens in anoxic mud?

If you tried to grow houseplants or vegetables in this zone, the roots would die pretty quickly because they need oxygen. Conditions faced by aquatic plants are similar to those of waterlogged houseplants. But wild rice, bulrushes, reeds, cattails, sedges, irises, and some other rooted plants have special adaptations allowing them to thrive in waterlogged conditions, including hollow stems that can transport oxygen to plant roots from overlying water and emergent leaves exposed to the air. Similarly, bacteria that require oxygen to thrive cannot survive in anoxic sediments, while bacteria that can use other compounds in place of oxygen will grow well. Nitrate and nitrite, oxidized iron and manganese, and sulfate can be used by different groups of bacteria in their metabolism analogously to how plants and animals (including humans) use oxygen to breakdown organic matter (food) and extract energy and other nutrients. When oxygen is used, a final waste product is water vapor (the source of our breath steam in cold weather). When nitrate is used, a final “waste” product is nitrogen gas, which harmlessly dissolves in the water, adding a trivial amount of nitrogen relative to what diffuses in from the atmosphere (which is almost 80% nitrogen gas). But when sulfate is used, the end product is hydrogen sulfide gas (H2S), which not only stinks (think rotten eggs) but is extremely toxic to plants and animals. This appears to be the main culprit for the decline or absence of wild rice in habitats that appear suitable but have elevated sulfate levels.

The upper boundary of the St. Louis River watershed, of course, is defined by the Mesabi Iron Range, which supplied iron ore to Midwestern steel mills for over 50 years. But the taconite tailings resulting from open pit mining have been a long-term source of elevated sulfate levels (Lindgren et al 2006). This increase in sulfate levels in the St. Louis River and other water bodies over the past 40 or more years from iron mining activities has the potential to negatively impact wild rice by enhancing the amount of sulfate that reaches bottom water and sediment, where naturally occurring anaerobic bacteria convert it into toxic H2S. You can read summaries of the lab experiments and field studies that MPCA and UMD scientists have been conducting for the past few years here . These recent studies are currently being evaluated by MPCA in the context of the validity of the current 10 mg/L sulfate standard.

Other factors that affect the amount of H2S in the water and root zone

The biogeochemistry of the root zone is even more complicated than described above because if there is enough dissolved iron in the root zone mud, the iron can react and bind some of the H2S to form iron sulfide. Iron sulfide is very insoluble in water, forming patches of black mud that might act to delay or decrease the toxicity of the sulfide. Finally, there can be additional reactions between phosphate and iron in the mud that further modify the net amount of sulfide available to impact aquatic plants.

Climate change and wild rice

Our climate has been changing, and current understanding of its likely impacts in northern Minnesota suggests that there will be an increase in severe storms and droughts. This would likely add additional stress to wild rice stands because the plant is very sensitive to water depth, i.e., rising waters can uproot the plants and lower water levels can damage the weak hollow stems. Wind and wave action can also uproot plants, adding to other stresses such as the damage that can result from the wakes created by recreational watercraft, which are a persistent management issue on wild rice lakes and rivers.

Invasive species and wild rice

Invasive and exotic species of aquatic plants are thought to benefit from disturbances to natural habitats. Mechanically harvesting aquatic “weeds” or dosing aquatic plants with herbicides creates open areas, which can increase the likelihood of an invasive plant species gaining a toehold that later leads to its spread. Research suggests that a healthy and diverse native assemblage of aquatic plants has a better chance of resisting the invasion of an exotic plant or animal. It may not completely exclude invaders but they are much less likely to completely dominate the near shore zone. In general, “weedy” species (think dandelions on lawns, fields, and vacant lots) are excellent colonizers and well adapted to exploiting disturbed and open bottom sediments, particularly when nutrients are abundant. This, along with their importance as habitat for fish and wildlife, is an important reason that lake managers and educators emphasize the importance of maintaining a healthy aquatic plant community in your lake.

Additional information on the ecology of wild rice

• 1854 Authority
• Fond du Lac Band of Lake Superior Chippewa
• Great Lakes Indian Fish & Wildlife Commission
• Save Our Rice Alliance
• Minnesota DNR
• Minnesota PCA

References

1. Walker, R. D., J. Pastor, and B. Dewey. 2010. Litter quantity and nitrogen immobilization cause oscillations in productivity of wild rice (Zizania palustris L.) in northern Minnesota. Ecosystems 13: 485-498.
2. Walker, R. D., J. Pastor, and B. Dewey. 2006. Effects of wild rice (Zizania palustris L.) straw on biomass and seed production in northern Minnesota. Canadian Journal of Botany 84: 1019-1024.
3. Sims, L., J. Pastor, T. Lee, and B. Dewey. 2014In press. Nitrogen, phosphorus, and light effects on growth and allocation of biomass and nutrient in wild rice. Oecologia. Pastor, J. and R. D. Walker. 2006. Delays in nutrient cycling and plant population oscillations. Oikos 112: 698-705.
4. MPCA Wild Rice Literature Holdings (Last Update: 12/31/2013)
5. Moss, B., J. Madgwick and G. Phillips. 1996. A guide to the restoration of nutrient enriched shallow lakes. Environment Agency, Broads Authority & European Life Programme. Norwich, England. Natural History Book Services, Totnes, Devon, UK. 180 p.
6. >Moss, B. 2003. Cinderella goes to the ball: A story of shallow lakes. Lakeline (a publication of the North American Lake Management Society, http://www.nalms.org) 23 (1): 11-16 (Spring 2003).
7. Lindgren J., Schuldt, N., Borkholder, B., Howes, T., Levar, A., Olson, C., Tillma, J. and Vogt, D. (2006) A Study of the St. Louis River. Minneosta Department of Natural Resources Division of Fisheries Report, Duluth, MN, 48 p. plus figures, tables, and appendicies.