Summary of Research on the Effects of Sulfate and Sulfide on Wild Rice Sediment Chemistry, Growth, and Seed Production
Prof. John PastorProf. Nathan Johnson
Dept. of BiologyDept. of Civil Engineering
University of Minnesota DuluthUniversity of Minnesota Duluth
January 29, 2014
Background
Minnesota currently has a water quality standard of “10 mg/L sulfate - applicable to water used for production of wild rice during periods when the rice may be susceptible to damage by high sulfate levels.” (Minn. R. 7050.0224, subpart 2). This standard was adopted in 1973 based on a study published in 1945 by Dr. John Moyle, a botanist with the Department of Natural Resources[1]. Moyle found that wild rice populations were of low plant density and uncommon or even absent in waters with sulfate concentrations between 10 and 50 mg/L. However, questions have recently arisen as to the generality of Moyle’s research and also whether sulfate was itself toxic to wild rice or whether sulfide, to which sulfate is transfromedin some anaerobic environments, is the operative toxic agent. Some clarification of the standard’s applicability occurred in 1997, but the standard was not, at that time, proposed for revision and has not been widely enforced. Accordingly, in 2010 the MPCA initiated a multi-year effort, known as the Wild Rice Sulfate Standard Study, to begin to understand the mechanisms of transformation of sulfate in aquatic ecosystems and whether sulfate or any of its transformed products adversely affect wild rice growth. The primary hypothesis has been that elevated sulfate depresses the growth of wild rice when it is transformed into hydrogen sulfide by microbes in the wild rice’s rooting zone, and that elevated iron could mitigate the toxicity of the sulfide by forming insoluble iron sulfide compounds.
The study consisted of a three-prong approach: (1) a field study of wild rice habitats across numerous lakes and rivers, including many sites previously sampled by Moyle, to determine physical and chemical conditions correlated with the presence or absence of wild rice stands, including concentrations of sulfate in surface water and sulfide in the rooting zone; (2) outdoor container (mesocosm) experiments in whichwild rice is grown in natural sediments from a wild rice lake to determine its response to a range of sulfate concentrations in the surface water and associated sulfide in the rooting zone across the growing season. (3) controlled laboratory hydroponic experiments to determine the effect of elevated sulfate and sulfide on early stages of wild rice growth and development. These studies were completed by scientists at the Limnological Research Center at the University of Minnesota Twin Cities campus and the Departments of Civil Engineering and Biology at the University of Minnesota Duluth campus. The studies were completed and final reports were delivered to the Minnesota Pollution Control Agency in late December 2013. These reports and the data are available to the public online at ftp://files.pca.state.mn.us/pub/tmp/wildRice/.This brief report summarizes our current interpretation of these results; it should not be taken as an official statement of policy by the MPCA.
Results
Sulfide concentrations in sediments of natural waters are correlated with sulfate concentrations in overlying waters. This correlation is stronger in lakes, where anaerobic conditions in the sediment are conducive to the biological transformation of sulfate to sulfide, than in rivers, where flowing water is more likely to aerate the sediments and therefore prevent sulfate transformation to sulfide. Therefore, sulfide concentrations in sediment pore waters are high only when sulfate concentrations in the overlying water are high, but under certain circumstances overlying sulfate concentrations can be high without leading to sulfide in sediment pore waters. Wild rice is uncommon and sparse in most natural waters with sulfate concentrations greater than 10-40 mg/L, except in waters flowing over sediments that may be aerobic. These results broadly corroborate those of Moyle (1948) but go further in making a distinction between aerobic and anaerobic sediments and in quantifying concomitant rooting zone geochemistry.
In laboratory hydroponicexperiments, sulfate had no effect on wild rice seed germination and seedling growth in hydroponic solution across a broad range of concentrations from 1 to 1600 mg/L. This range is broader than the levels of sulfate concentrations in natural waters, even those impacted from mines in Minnesota. Sulfide also had no effect on seed germination. However, the growth of already-germinated seedlings was severely decreased by sulfide at low concentrations (Fig. 1). These low sulfide concentrations would be produced by as little as 1 mg/L of sulfate if 100% of the sulfate were transformed to sulfide.
Significant decline (p < 0.05)
Fig. 1 Sulfide severely reduces growth of wild rice seedlings at concentrations above 9 – 10 μM (equivalent to 1 mg/L of sulfate if 100% of the sulfate was transformed to sulfide).
During the third year of controlled experiments in outdoor mesocosms with wild rice populations growing in wild rice sediments, additions of sulfate to surface water were accompanied by increased sulfide concentrations in the upper 5 cm of sediments (Fig. 2). The concentrations of sulfide increased over the growing season and were highest in July and August when microbial activity was presumably greatest because of warmer water and sediment temperatures. High-resolution measurements of pore water chemistry revealed that sulfate concentrations declined along with a rise in sulfide concentration from the sediment surface down to 5 cm depth (Fig. 3) and greater sulfate amendments increased the peak sulfide concentration in the sediments. These depths are within the rooting zone of wild rice seedlings.The sulfide concentrations measured in the pore waters of sulfate-amended outdoor mesocosms exceed the sulfide concentration which severely reduced growth of seedlings in the hydroponic experiments.Iron concentrations were depressed in the pore fluids of sulfate amended tanks, showing that the transport and reaction of sulfur from the overlying water is consuming iron in the sediments.
Fig.2 Porewater sulfide concentrations in 0-5 cm of sulfate amended mesocosms. Note log scale. Sulfide is clearly related to overlying water sulfate amendment and increases over the course of the summer.
Fig.3 Depth profiles in August 2013 for pore water sulfate, sulfide, and iron in mesocosms amended with nominally (a) 10 mg/L, (b) 50 mg/L and (c) 300 mg/L sulfate in the overlying water. 0 depth represents the sediment-water interface. Sulfide is clearly higher and pore water iron is depleted in porewaters of amended mesocosms.
Wild rice seedling survival in these mesocosms declined with increased sulfate and hence sulfide concentrations in the sediments (Fig. 4). This decreased survival of seedlings is consistent with the results of the hydroponic experiments (Fig. 1) and the increased sulfide concentrations within the rooting zone in mesocosms amended with sulfate (Fig. 3).
Plants grown in the control tanks had white or light tan roots, but plants in the tanks amended with sulfate had blackened roots (Fig. 5). Visual estimates of the proportion of blackened roots increased progressively from approximately 50% in the tanks with 50 mg · L-1 SO4 to 100% in tanks with 300 mg/l sulfate. Examination of these roots byscanning X-ray showed that this blackened deposit is almost entirely iron sulfide which precipitated on the surface of the roots.
Fig. 4 Decline in wild rice seedling survival with increased sulfate concentrations.
Fig. 5 Blackened roots from plants grown in tanks with 300 mg SO4 · L-1 (left) compared to tan roots from plants grown in control tanks (right).
We hypothesize that these iron sulfide deposits may have inhibited late season uptake of nutrients which the wild rice plant uses to fill out seeds because the proportion of viable seeds and the mean seed weight declined with sulfate ammendments (Fig. 6).Therefore plants grown with enhanced sulfate concentrations produced fewer viable seeds and those seeds which were viable had less food reserves to support germination. These effects of sulfide on seed production resulted in lower germination rates the following year. The decline in germination rate with increased sulfide concentrations in the sediments does not appear to be a direct effect of sulfide on the germinating seeds, since we found no effect of sulfide on seed germination in the hydroponic experiments. Rather reduced germination in the outdoor mesocosms appears to be due to an indirect effect of sulfide on seed production by the parent plant.
Fig. 6Declines in seed production associated with increased sulfate concentrations in the water columns.
Summary
Wild rice populations are uncommon and sparse in lakes with concentrations of sulfate in overlying waters greater than 10-30 mg/L. Sulfide concentrations in sediments of these lakes also increased with overlying water sulfate concentrations. In controlled hydroponic experiments, enhanced sulfate concentrations had no effect on wild rice seed germination and seedling growth, but sulfide concentrations severely inhibited seedling growth at very low concentrations. In controlled experiments in mesocosms with wild rice growing in wild rice sediments, sulfate amendments increased sediment sulfide concentrations within the seedling rooting zone to levels above those observed to be toxic to seedlings seen in the hydroponic experiments. Seedling survival also declined in these mesocosms with increased sulfate amendments. Iron sulfide precipitates on roots of mature plants are associated with reduced production of viable seeds and reduced average seed weight. This may have indirectly decreased seed germination the following year.
Future Research
We are currently consulting with scientists at the MPCA to synthesize the results of these studies into a final report which will be completed by MPCA at the end of February 2014. In that final report, MPCA will answer two questions: 1. Should the current standard of 10 mg sulfate/L for wild rice waters in Minnesota be changed? 2. Should the standard concentration be increased or decreased? The final report will not specify a new standard. If a change to the existingstandard is recommended and a new standard developed, it would be adopted into Minnesota Rules via the administrative rulemaking process and subject to U.S. EPA approval before the changes could be implemented. In this case, several more years of research will likely be needed to specify what the new standard should be.
We have also received a $200,000 award from Minnesota Sea Grant to continue this research along with our colleague Prof. James Cotner of the Dept. of Ecology, Evolution, and Behavior on the Twin Cities campus. This award will be supplemented by a matching award from the Natural Resource Management Division of the Fond du Lac Band of Lake Superior Ojibway. This new research will further quantify the geochemical interactions between iron and sulfide in the sediments, examine whether the iron sulfide precipitates on roots impede nutrient uptake, and determine whether rates of these processes are modified during long term population cycles of wild rice. New field sites will be established at the University of Minnesota’s Itasca Biological Station. The mesocosm experiment at the University of Minnesota Duluth’s Field Studies Research Station will also be expanded.
1
[1]Moyle, J. B. 1945. Some chemical factors influencing the distribution of aquatic plants in Minnesota. American Midland Naturalist 34: 402-420.