William G. Crumpton

Factors affecting nitrogen loss in experimental wetlands with different hydrologic loads

Abstract Constructed or restored wetlands have great potential for reducing nonpoint source contamination of surface and ground waters by agricultural chemical contaminants. The work reported here combines field and experimental studies of factors affecting nitrogen loss in the Des Plaines River Experimental Wetlands, northeastern Illinois, USA. These wetlands receive approximately 5–36 cm/week of pumped river water with significant but seasonally variable loads of nitrate and organic nitrogen. On an annual basis, the wetlands removed 78–95% of the nitrate and 54–75% of the total nitrogen received. At the low hydrologic loading rate, organic nitrogen exports approximately equalled imports. However at the higher hydrologic loading rate, the wetlands exported 22–31% more organic nitrogen than received. Seasonal variation in nitrate and organic nitrogen loads had significant effects on the effectiveness of the wetlands as sinks for total nitrogen. The wetlands were nitrogen sinks during periods of the nitrate loading and nitrogen sources during periods of low nitrate loading. Experimental studies demonstrated the effects of nitrate concentration, temperature, and location on rates of nitrate loss. Results suggest that nitrite loading rates might influence not only nitrate loss rates but also loss rate coefficients.

Effects of emergent macrophytes on dissolved oxygen dynamics in a prairie pothole wetland

Transect measurements, continuous monitoring, and synoptic surveys were used to examine patterns in light availability, temperature, and dissolved oxygen concentrations within and outside emergent vegetation zones in Goose Lake Marsh, a natural prairie pothole wetland in central Iowa. Water column light availability was less than 2% of ambient light in emergent vegetated areas due to canopy cover, small floating plants (lemnids), and plant litter. Water temperatures and dissolved oxygen concentrations were significantly lower and varied less diurnally in vegetated areas. Three habitat zones could be identified based on patterns in vegetation and dissolved oxygen: (1) a zone of dense emergent macrophytes providing significant submerged structure but with nearly or completely anoxic water, (2) a transition zone of sparse emergent macrophytes providing less structure but with more aerobic water, and (3) an open water zone with consistently acrobic water but with little submerged structure. Vegetation patterns are likely to control major aspects of wetland biogeochemistry and trophic dynamics, and wetlands should be viewed as complex mosaics of habitats with distinct structural and functional characteristics.

SPATIAL DISTRIBUTION OF HISTORICAL WETLAND CLASSES ON THE DES MOINES LOBE, IOWA

We estimated the pre-settlement density and area of different classes of palustrine wetlands on the Des Moines Lobe based on soil characteristics. Six wetland classes, ranging from temporarily flooded to permanently flooded, were identified based on soil properties that persisted after artificial drainage. Prior to drainage, wetlands covered nearly half of the Des Moines Lobe and there were differences in both the types and relative abundance of wetlands among the four geologic subdivisions of the Lobe (Bemis, Altamont, and Algona till plains and Altamont Lake). In the flat Altamont Lake zone, the most common wetlands were equally split between temporarily flooded and saturated water regimes. Among the three till plain zones, saturated wetlands were the dominant wetland type. Differences in wetland distributions among the zones probably derive from differences in initial topography and post-glacial processes such as erosion-deposition processes and stream-network formation.

Summary of Findings and Recommendations

This book responds to questions in three general areas: characterization of hypoxia; characterization of nutrient fate, transport and sources; and the scientific basis for goals and management options. In the sections below, these questions (shown in italics below) are addressed very briefly with references to those sections of this book where more detailed science on that particular question may be found.

Subsurface Drainage in Iowa and the Water Quality Benefits and Problem

It is estimated that there are approximately 3.6 million ha of land with artificial subsurface drainage in Iowa, with 2.4 million ha of that within the 3000 organized drainage districts (total land area of the state is 14.6 million ha). This drainage has made otherwise wet soils very productive. Much of this drainage was installed early last century and is reaching the end of its service life. One challenge will be the repair/replacement of these drainage systems. Because subsurface drainage “short circuits” some infiltrating water back to surface water resources, there is also a water quality challenge. Research has shown that during rainfall-runoff events, the presence of artificial subsurface drainage generally delays and reduces the volume of surface runoff. Therefore, total losses of sediment, phosphorus, ammonium-nitrogen, pesticides, and micro-organisms are decreased with subsurface drainage. However, nitrate-nitrogen leaching is increased with subsurface drainage water, and has been implicated as a major factor relative to hypoxia in the Gulf of Mexico. Research has identified several factors relative to soils, weather, and management (cropping, tillage, chemical application practices, and drainage parameters) that influence the nitrate-nitrogen leaching problem. This will be discussed along with implications for possible changes in the drainage systems and land management that may be needed to sustain production while reducing nitrate-nitrogen losses.

Transformation and loss of nitrite in agricultural stream

ABSTRACT Mass balances of nitrate nitrogen were determined for an agricultural stream receiving non-point inputs of inorganic nitrogen. Primary production and respiration of stream reaches were estimated from analyses of diurnal changes in dissolved oxygen and temperature. Substantial in-stream losses of nitrate were observed, averaging 0.66 g N m−2 day−1. The estimated nitrogen requirements to support observed rates of primary production ranged between 0.15 and 0.27 g N m−2 day−1. Laboratory investigations measuring nitrate nitrogen loss rates from stream water overlying intact sediment cores suggest that algal assimilation of inorganic nitrogen contributes to the overall nitrate decline in these systems.

Estimating the breakdown and accumulation of emergent macrophyte litter: a mass-balance approach.

Litter accumulation within emergent macrophyte marshes may significantly influence abiotic conditions and biota but litter is rarely considered in emergent macrophyte studies. Litter is defined here as the standing and fallen dead plant material that can be collected using harvest methods in the field. Litter accumulation can be predicted by combining annual production with litter breakdown rates. Breakdown rates are typically measured using litter bag studies but these rates may also be measured using a mass-balance approach. A five year study conducted in Delta Marsh, Manitoba measured annual standing crop and harvested accumulated litter of Phragmites australis (Cav.) Trin., Typha glauca Godr., and Scolochloa festucacea (Willd.) Link. These species differ in their level of refractory material and hence the amount of litter that is expected to accumulate. Using annual estimates of standing crop and litter mass, a mass-balance model was used to estimate the litter breakdown rate for each species at three water levels. Mass-balance derived rates for Phragmites and Typha were significantly different (F1,14 = 5.07, p = 0.03) and also differed across water depths (F2,14 = 4.35, p = 0.04). For both species, predicted annual accumulation of litter tracked observed litter values. The mass-balance approach, however, was not suitable for Scolochloa because there is no litter carry over from year to year. When compared to observed litter accumulations, estimates of litter accumulation made using litter bag breakdown rates consistently overestimated annual litter accumulation. In short, breakdown rates can be estimated using a mass-balance approach and can then provide more accurate estimates of annual litter accumulation for emergent species with recalcitrant litter.

SPATIAL PATTERNS IN DISSOLVED OXYGEN AND METHANE CONCENTRATIONS IN A PRAIRIE POTHOLE WETLAND IN IOWA, USA

The presence of live and dead emergent vegetation alters the microbial metabolism of communities within the water column of prairie pothole wetlands. To demonstrate the effects, dissolved CH4, dissolved O2, plant densities, and litter densities were measured in transects from emergent vegetation to open water zones. O2 concentrations were consistently lower and CH4 concentrations consistently higher in emergent zones. Plant surface cover most likely interferes with gas flux across the air-water interface in the emergent zone, impeding O2 transport into the water column and impeding CH4 transport out of the water column. Either directly or indirectly, the presence of emergent vegetation may alter aerobic and anaerobic metabolism; since wet and dry cycles can result in emergent plant cover from nearly absent to nearly 100%, understanding these dynamics over the long-term will require studies over a range of wetland stages.

Wetland hydrologic class change from prior to European settlement to present on the Des Moines Lobe, Iowa

It has been hypothesized that wetland restoration policies have favored the restoration of the wettest classes of wetlands on the Des Moines Lobe of the prairie pothole region. To test this hypothesis we compared pre-drainage wetland distributions based on soils data and National Wetland Inventory (NWI) estimates of contemporary wetland distributions on the Des Moines Lobe. Based on the NWI data, the Des Moines Lobe today has only 3–4% of the wetland area that it had prior to the onset of drainage. On the basis of their soils, pre-drainage wetlands were predominantly temporarily flooded to saturated wetlands (84%), with only about 6% of the wetlands with water regimes classified as semi-permanently to permanently flooded. Depending on the interpretation of wetland modifiers on NWI maps, wetlands classified by the NWI as semi-permanent to permanently flooded make up more than 41% of the wetland area while wetlands with temporarily flooded to saturated water regimes account for 45–58% of the Lobe’s wetland area. The water regimes of contemporary wetlands when compared to their historic regimes suggest that many of today’s wetlands have different water regimes than they did prior to the onset of drainage. Because of the regional lowering of the groundwater table, many of today’s wetlands have drier water regimes, but some have wetter water regimes because they receive drainage tile inputs. Our results indicate that restoration has favored the wettest classes of wetlands and that temporarily to saturated wetland classes have not been restored in proportion to their relative abundance in the pre-drainage landscape.

Wetland Invertebrate Community Responses to Varying Emergent Litter in a Prairie Pothole Emergent Marsh

Plant litter produced in the interior of dense emergent stands may directly or indirectly influence invertebrate communities. Low litter may provide structure and refuge to invertebrates while high litter may displace vegetation and decrease oxygen concentration. Within an emergent stand, an edge-to-interior transect study and an interior litter treatment study were performed to investigate the impact of increasing litter densities on the invertebrate community. The interior had more litter, lemnid biomass, and hypoxia than the edge but did not differ in total invertebrate abundance. Low and moderate litter plots in the interior treatment study experienced higher lemnid biomass and greater total invertebrate abundance than the high litter plots, but the high litter plots were characterized by higher invertebrate diversity. There was a significant negative relationship between litter and invertebrate abundance in July and August. Invertebrate patterns were driven primarily by amphipod abundance and may be related to the use of lemnids as habitat. Hypoxic-tolerant and semi-aquatic taxa were associated with high litter, while several algal-feeding taxa were associated with the edge. High litter can reduce abundant invertebrates that support higher trophic levels and shift invertebrate communities. These findings underscore the importance of understanding long-term litter accumulation dynamics in wetland systems.