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Ecological Studies at the Woodworth Study Area

Spatial and Temporal Variability of Water Quality of Wetlands in the Woodworth Study Area

Naomi E. Detenbeck*, Colleen M. Elonen, and Debra L. Taylor
U.S. Environmental Protection Agency
Mid-Continent Ecology Division
6201 Congdon Boulevard, Duluth, MN 55804

Seasonal prairie pothole wetlands play a critical role in supporting waterfowl production in the Prairie Pothole Region, particularly for early-nesting species. This wetland class also is at high risk to degradation or loss from agricultural stressors such as drainage, tillage through wetland basins, nonpoint source sediment and nutrient inputs from agricultural uplands, and pesticide overspray or spray drift. Twenty seasonal prairie pothole wetlands in the Woodworth Study Area and surrounding waterfowl production areas were selected for a collaborative study of the physical, chemical, and biological effects of summer fallow practices, best management practices (vegetated buffer strips), upland restoration through the Conservation Reserve Program (CRP), and preservation of wetlands within native prairie (NP) watersheds (1). The study period extended from the end of a prolonged drought through two abnormally wet years, so that the originally seasonally-flooded wetlands were converted to semi-permanently flooded basins.

Water quality of study wetlands was monitored on a monthly basis (May-September) during 1993 and 1994 both prior to and following implementation of summer fallow treatments. Variables measured included temperature, dissolved oxygen, specific conductance, apparent color, turbidity, total and volatile suspended solids, phosphorus (soluble reactive, dissolved, and total), nitrogen (ammonium, nitrate + nitrite, dissolved and total), organic carbon (dissolved and total), and major ions (Ca, Mg, Na, K, SO4, Cl, F, Br). Ratios of soluble reactive P to dissolved inorganic N (SRP:DIN), particulate P:N, and total P:N were calculated as indicators of relative P:N nutrient limitation for algae and grazers, respectively. Ratios of Cl: specific conductivity were calculated to determine whether changes in specific conductivity (and major ions) were due to simple evapotranspiration or to other biogeochemical processes. In addition, occasional diurnal measurements were made of pH, conductivity, temperature, dissolved oxygen, and turbidity using automated water quality recorders (Hydrolabs). Water quality variables were analyzed using multivariate analysis of variance (MANOVAs) to determine if any significant differences existed among treatment classes prior to treatment. Treatment and time effects for 1993-1994 were analyzed using a repeated measures design. Only differences between native prairie (NP) and restored upland (CRP) treatments, and differences due to interannual climatic variability will be discussed here.

No pretreatment differences among treatment classes were detected for any of the water quality variables analyzed (P> 0.05). Water quality data were highly variable among sites and over time, ranging over approximately two orders of magnitude for soluble reactive P, dissolved and total P, total suspended solids, Gran alkalinity, magnesium, sodium, chloride, and sulfate (Table 1). Only dissolved phosphorus, turbidity, surface dissolved oxygen, potassium, and ratios of chloride:specific conductivity and soluble reactive P: dissolved inorganic N showed significant differences between CRP and NP treatments (P> 0.05). Of these variables, all but surface D.O. and the Cl:conductivity ratio were greater on average for the CRP sites. N:P ratios showed evidence for nitrogen limitation in 1993, but shifted towards phosphorus limitation in 1994, following increases in water depth. Dissolved oxygen decreased to near zero as depth and duration of flooding increased and wetlands became stratified; however, diurnal oxygen profiles were strongly modified by vegetation structure within sites.

Table 1. Water quality values for twenty wetlands in the Woodworth Study Area, 1993-1994. Wetlands were classified as seasonally-flooded in previous years, but were converted to semi-permanently flooded wetlands due to above-normal precipitation. Wetlands in the following treatment classes are included: native prairie, restored upland (CRP), tilled watershed with vegetated buffer strips, and summer fallow watersheds.
Water quality variable                  Units          1993            1994

Soluble reactive phosphorus             mG P/L      0.19-3.21       <0.01-3.59
Dissolved phosphorus                    mG P/L      0.12-3.89       <0.02-3.63
Total phosphorus                        mG P/L      0.11-6.11        0.07-3.44
Particulate phosphorus                  mG P/L     <0.02-3.16       <0.02-1.25
Nitrate-N                               µG N/L       <28-118          <32-75
Total ammonia-N                         µG N/L       <22-1835         <34-678
Dissolved nitrogen                      mG N/L      1.11-9.80         0.84-4.32
Total nitrogen                          mG N/L      0.99-9.62         1.01-5.07
Particulate nitrogen                    mG N/L     <0.12-3.07        <0.30-2.09
Dissolved organic carbon                mG C/L        19-138          <3.2-52
Total organic carbon                    mG C/L        19-130            14-54
Particulate organic carbon              mG C/L      <3.1-26.6         <3.6-30.4
Apparent color                          PCU          162-133           466-411
Turbidity                               NTU         <1.2-332          <0.6-26.1
Total suspended solids                  mG/L          <1-138          <1.8-107
Volatile suspended solids               mG/L           1-155           1.8-87.5
Specific conductivity @ 25 C            µmhos/cm     132-1097          150-2710
pH                                      log [H+]     6.6-8.2           6.1-8.5
Gran alkalinity                         mG CaCO3/L    41-317             6-505
Calcium                                 mG/L           6-101            11-119
Magnesium                               mG/L        <1.4-64              3-189
Sodium                                  mG/L         0.3-7.2          <0.1-208
Potassium                               mG/L           8-45              5-43
Sulfate                                 mG/L         0.3-389            <2-1472
Chloride                                mG/L           1-14.5          0.3-13
Bromide                                 mG/L       <0.09-7.17         <0.2-0.4
Fluoride                                mG/L        0.05-0.24         <0.1-0.21

There were significant treatment x time interactions involving NP and CRP sites for soluble reactive P, total P, dissolved and total N, TP:TN and total organic C. Nutrient levels declined more over the period of the study for NP sites as compared to CRP sites, possibly related to differences in D.O. levels. In contrast, water levels increased more over time for NP sites as compared to CRP sites (P< 0.05). Water levels were constrained by the elevation of surface water outlets for three of the CRP sites, although the two sites with lowest average water depths in 1994 did not develop outlets (1).

In conclusion, our site selection process was successful in enabling us to prevent initial biases in water quality conditions among wetland treatment classes, although high variability will limit our ability to detect treatment effects on prairie pothole water quality over time. Of the water quality variables studied, phosphorus, suspended solids, and major ions were the most variable over time and space. Temporal trends (decreased dissolved oxygen, increased N:P ratios) appeared to be linked to changing water depths. Differences in water quality between NP and CRP sites were not apparent at the beginning of the study, but increased over time, possibly because of differing hydrologic responses; this accentuates the importance of long-term ecosystem studies covering more than one growing season. Differences between NP and CRP sites suggest that historical differences in land-use treatments can have significant impacts on water quality of prairie pothole wetlands. These differences need to be considered when designing whole watershed experiments.


1.     Gleason, R. A. and Euliss, N. H., Jr. (1996) Impact of agriculutral land-use on prairie
wetland ecosystems: experimental design and overview.  Proc. N.D. Acad. Sci. 50.

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