Relationships between water quality, habitat quality, and macroinvertebrate assemblages in Illinois streams

The influence of specific stressors, such as nutrient enrichment and physical habitat degradation, on biotic integrity requires further attention in Midwestern streams. We sampled 53 streams throughout Illinois and examined relationships between macroinvertebrate community structure and numerous physicochemical parameters. Streams were clustered into four major groups based on taxa dissimilarity. Habitat quality and dissolved nutrients were responsible for separating the major groups in a nonmetric multidimensional scaling ordination. Furthermore, the alignment of environmental factors in the ordination suggested there was a habitat quality-nutrient concentration gradient such that streams with high-quality habitats usually had low concentrations of nutrients. Discrimination by community measures further validated the major stream groups and indicated that forested streams had generally higher biological integrity than agricultural streams, which in turn had greater integrity than urban streams. Our results demonstrate that physical habitat degradation and nutrient pollution are important and often confounded determinants of biotic integrity in Illinois streams. In addition, we suggest that management of Midwestern streams could benefit from further implementation of multivariate data exploration and stream classification techniques.     

Transport and fate of nitrate in headwater agricultural streams in Illinois

Nitrogen inputs to the Gulf of Mexico have increased during recent decades and agricultural regions in the upper Midwest, such as those in Illinois, are a major source of N to the Mississippi River. How strongly denitrification affects the transport of nitrate (NO(3)-N) in Illinois streams has not been directly assessed. We used the nutrient spiraling model to assess the role of in-stream denitrification in affecting the concentration and downstream transport of NO(3)-N in five headwater streams in agricultural areas of east-central Illinois. Denitrification in stream sediments was measured approximately monthly from April 2001 through January 2002. Denitrification rates tended to be high (up to 15 mg N m(-2) h(-1)), but the concentration of NO(3)-N in the streams was also high (>7 mg N L(-1)). Uptake velocities for NO(3)-N (uptake rate/concentration) were lower than reported for undisturbed streams, indicating that denitrification was not an efficient N sink relative to the concentration of NO(3)-N in the water column. Denitrification uptake lengths (the average distance NO(3)-N travels before being denitrified) were long and indicated that denitrification in the streambed did not affect the transport of NO(3)-N. Loss rates for NO(3)-N in the streams were <5% d(-1) except during periods of low discharge and low NO(3)-N concentration, which occurred only in late summer and early autumn. Annually, most NO(3)-N in these headwater sites appeared to be exported to downstream water bodies rather than denitrified, suggesting previous estimates of N losses through in-stream denitrification may have been overestimated.     

Assessment of chlorophyll-a as a criterion for establishing nutrient standards in the streams and rivers of Illinois

Nutrient enrichment is a frequently cited cause for biotic impairment of streams and rivers in the USA. Efforts are underway to develop nutrient standards in many states, but defensible nutrient standards require an empirical relationship between nitrogen (N) or phosphorus (P) concentrations and some criterion that relates nutrient levels to the attainment of designated uses. Algal biomass, measured as chlorophyll-a (chl-a), is a commonly proposed criterion, yet nutrient-chl-a relationships have not been well documented in Illinois at a state-wide scale. We used state-wide surveys of >100 stream and river sites to assess the applicability of chl-a as a criterion for establishing nutrient standards for Illinois. Among all sites, the median total P and total N concentrations were 0.185 and 5.6 mg L(-1), respectively, during high-discharge conditions. During low-discharge conditions, median total P concentration was 0.168 mg L(-1), with 25% of sites having a total P of > or =0.326 mg L(-1). Across the state, 90% of the sites had sestonic chl-a values of < or =35 microg L(-1), and watershed area was the best predictor of sestonic chl-a. During low discharge there was a significant correlation between sestonic chl-a and total P for those sites that had canopy cover < or =25% and total P of < or =0.2 mg L(-1). Results suggest sestonic chl-a may be an appropriate criterion for the larger rivers in Illinois but is inappropriate for small rivers and streams. Coarse substrate to support benthic chl-a occurred in <50% of the sites we examined; a study using artificial substrates did not reveal a relationship between chl-a accrual and N or P concentrations. For many streams and rivers in Illinois, nutrients may not be the limiting factor for algal biomass due to the generally high nutrient concentrations and the effects of other factors, such as substrate conditions and turbidity.     

Evaluating agricultural best management practices in tile-drained subwatersheds of the Mackinaw River, Illinois

Best management practices (BMPs) are widely promoted in agricultural watersheds as a means of improving water quality and ameliorating altered hydrology. We used a paired watershed approach to evaluate whether focused outreach could increase BMP implementation rates and whether BMPs could induce watershed-scale (4000 ha) changes in nutrients, suspended sediment concentrations, or hydrology in an agricultural watershed in central Illinois. Land use was >90% row crop agriculture with extensive subsurface tile drainage. Outreach successfully increased BMP implementation rates for grassed waterways, stream buffers, and strip-tillage within the treatment watershed, which are designed to reduce surface runoff and soil erosion. No significant changes in nitrate-nitrogen (NO-N), total phosphorus (TP), dissolved reactive phosphorus, total suspended sediment (TSS), or hydrology were observed after implementation of these BMPs over 7 yr of monitoring. Annual NO-N export (39-299 Mg) in the two watersheds was equally exported during baseflow and stormflow. Mean annual TP export was similar between the watersheds (3.8 Mg) and was greater for TSS in the treatment (1626 ± 497 Mg) than in the reference (940 ± 327 Mg) watershed. Export of TP and TSS was primarily due to stormflow (>85%). Results suggest that the BMPs established during this study were not adequate to override nutrient export from subsurface drainage tiles. Conservation planning in tile-drained agricultural watersheds will require a combination of surface-water BMPs and conservation practices that intercept and retain subsurface agricultural runoff. Our study emphasizes the need to measure conservation outcomes and not just implementation rates of conservation practices.     

pH,dissolved oxygen,and adsorption effects on metal removal in anaerobic bioreactors

Anaerobic bioreactors were used to test the effect of the pH of influent on the removal efficiency of heavy metals from acid-rock drainage. Two studies used a near-neutral-pH, metal-laden influent to examine the heavy metal removal efficiency and hydraulic residence time requirements of the reactors. Another study used the more typical low-pH mine drainage influent. Experiments also were done to (i) test the effects of oxygen content of feed water on metal removal and (ii) the adsorptive capacity of the reactor organic substrate. Analysis of the results indicates that bacterial sulfate reduction may be a zero-order kinetic reaction relative to sulfate concentrations used in the experiments, and may be the factor that controls the metal mass removal efficiency in the anaerobic treatment systems. The sorptive capacities of the organic substrate used in the experiments had not been exhausted during the experiments as indicated by the loading rates of removal of metals exceeding the mass production rates of sulfide. Microbial sulfate reduction was less in the reactors receiving low-pH influent during experiments with short residence times. Sulfate-reducing bacteria may have been inhibited by high flows of low-pH water. Dissolved oxygen content of the feed waters had little effect on sulfate reduction and metal removal capacity.     

Phosphorus transport pathways to streams in tile-drained agricultural watersheds

Agriculture is a major nonpoint source of phosphorus (P) in the Midwest, but how surface runoff and tile drainage interact to affect temporal concentrations and fluxes of both dissolved and particulate P remains unclear. Our objective was to determine the dominant form of P in streams (dissolved or particulate) and identify the mode of transport of this P from fields to streams in tile-drained agricultural watersheds. We measured dissolved reactive P (DRP) and total P (TP) concentrations and loads in stream and tile water in the upper reaches of three watersheds in east-central Illinois (Embarras River, Lake Fork of the Kaskaskia River, and Big Ditch of the Sangamon River). For all 16 water year by watershed combinations examined, annual flow-weighted mean TP concentrations were >0.1 mg L(-1), and seven water year by watershed combinations exceeded 0.2 mg L(-1). Concentrations of DRP and particulate P (PP) increased with stream discharge; however, particulate P was the dominant form during overland runoff events, which greatly affected annual TP loads. Concentrations of DRP and PP in tiles increased with discharge, indicating tiles were a source of P to streams. Across watersheds, the greatest DRP concentrations (as high as 1.25 mg L(-1)) were associated with a precipitation event that followed widespread application of P fertilizer on frozen soils. Although eliminating this practice would reduce the potential for overland runoff of P, soil erosion and tile drainage would continue to be important transport pathways of P to streams in east-central Illinois.     

Development of environmental thresholds for streams in agricultural watersheds

Global increases in consumption of chemical nutrients, application of pesticides, and water withdrawal to enhance agricultural yield have resulted in degraded water quality and reduced water availability. Efforts to safeguard or improve environmental conditions of agroecosystems have usually focused on managing on-farm activities to reduce materials loss and conserve habitat. Another management measure for improving environmental quality is adoption of environmental performance standards (also called outcome-based standards). This special collection of six papers presents the results of four years of research to devise scientifically credible approaches for setting environmental performance standards to protect water quantity and quality in Canadian agriculturally dominated watersheds. The research, conducted as part of Canada's National Agri-Environmental Standards Initiative, aimed to identify Ideal Performance Standards (the desired environmental state needed to maintain ecosystem health) and Achievable Performance Standards (the environmental conditions achievable using currently available and recommended best available processes and technologies). Overviews of the papers, gaps in knowledge, and future research directions are presented. As humans, livestock, and wildlife (both terrestrial and aquatic) experience greater pressures to share the same limited water resources, innovative research is needed that incorporates a landscape perspective, economics, farm practices, and ecology to advance the development and application of tools for protecting water resources in agricultural watersheds.     

Relationships between landscape pattern, wetland characteristics, and water quality in agricultural catchments

Water quality in streams is dependent on landscape metrics at catchment and wetland scales. A study was undertaken to evaluate the correlation between landscape metrics, namely patch density and area, shape, heterogeneity, aggregation, connectivity, land-use ratio, and water quality variables (salinity, nutrients, sediments, alkalinity, other potential pollutants and pH) in the agricultural areas of a semiarid Mediterranean region dominated by irrigated farmlands (NE Spain). The study also aims to develop wetland construction criteria in agricultural catchments. The percentage of arable land and landscape homogeneity (low value of Simpson index) are significantly correlated with salinity (r(2) = 0.72) and NO(3)-N variables (r(2) = 0.49) at catchment scale. The number of stock farms was correlated (Spearman's corr. = 0.60; p < 0.01) with TP concentration in stream water. The relative abundance of wetlands and the aggregation of its patches influence salinity variables at wetland scale (r(2) = 0.59 for Na(+) and K(+) concentrations). The number and aggregation of wetland patches are closely correlated to the landscape complexity of catchments, measured as patch density (r(2) = 0.69), patch size (r(2) = 0.53), and landscape heterogeneity (r(2) = 0.62). These results suggest that more effective results in water quality improvement would be achieved if we acted at both catchment and wetland scales, especially reducing landscape homogeneity and creating numerous wetlands scattered throughout the catchment. A set of guidelines for planners and decision makers is provided for future agricultural developments or to improve existing ones.     

Macroinvertebrate communities in agriculturally impacted southern Illinois streams: patterns with riparian vegetation, water quality, and in-stream habitat quality

Relationships between riparian land cover, in-stream habitat, water chemistry, and macroinvertebrates were examined in headwater streams draining an agricultural region of Illinois. Macroinvertebrates and organic matter were collected monthly for one year from three intensively monitored streams with a gradient of riparian forest cover (6, 22, and 31% of riparian area). Bioassessments and physical habitat analyses were also performed in these three streams and 12 other nearby headwater streams. The intensively monitored site with the least riparian forest cover had significantly greater percent silt substrates than the sites with medium and high forest cover, and significantly higher very fine organics in substrates than the medium and high forested sites. Macroinvertebrates were abundant in all streams, but communities reflected degraded conditions; noninsect groups, mostly oligochaetes and copepods, dominated density and oligochaetes and mollusks, mostly Sphaerium and Physella, dominated biomass. Of insects, dipterans, mostly Chironomidae, dominated density and dipterans and coleopterans were important contributors to biomass. Collector-gatherers dominated functional structure in all three intensively monitored sites, indicating that functional structure metrics may not be appropriate for assessing these systems. The intensively monitored site with lowest riparian forest cover had significantly greater macroinvertebrate density and biomass, but lowest insect density and biomass. Density and biomass of active collector-filterers (mostly Sphaerium) decreased with increasing riparian forest. Hilsenhoff scores from all 15 sites were significantly correlated with in-stream habitat scores, percent riparian forest, and orthophosphate concentrations, and multiple regression indicated that in-stream habitat was the primary factor influencing biotic integrity. Our results show that these "drainage ditches" harbor abundant macroinvertebrates that are typical of degraded conditions, but that they can reflect gradients of conditions in and around these streams.     

Physical and ecological thresholds for deposited sediments in streams in agricultural landscapes

Excessive sedimentation in streams and rivers remains a pervasive problem for the protection of aquatic habitat and the sustainability of aquatic communities. Whereas water quality criteria have been determined for suspended sediments in many jurisdictions across North America, comparably little has been done for deposited (also known as bedded) sediments. Through Canada's National Agri-Environmental Standards Initiative, assessment techniques and analytical tools were developed for estimating environmental thresholds for deposited sediments in agricultural watersheds in New Brunswick (NB) and Prince Edward Island (PEI) in the Atlantic Maritimes of Canada. Physical thresholds were developed through assessment of geomorphic metrics, which were then analyzed using y-intercept and 25th percentile approaches. For NB, there was strong agreement in physical thresholds for both analytical approaches (e.g., percent fines <2 mm were 7.5 for y-intercept and 6.9 for 25th percentile approaches). In contrast, physical thresholds for PEI differed considerably between approaches (e.g., percent fines <2 mm were 6.1 for y-intercept and 19.6 for 25th percentile approaches), likely due to a narrower range in agricultural land cover. Cross-calibration of our provisional physical thresholds for NB with ecological (i.e., benthic macroinvertebrate) assessments show that ecological thresholds, calculated as change-points in relationships between Ephemeroptera-Plecoptera-Trichoptera relative abundance or Modified Family Biotic Index and geomorphic criteria, were more liberal than physical thresholds. These results suggest that provisional thresholds developed using geomorphic criteria should demarcate change from the least disturbed condition and reduce the risk of sedimentation degrading benthic ecosystems.     

Comparative study of transport processes of nitrogen, phosphorus, and herbicides to streams in five agricultural basins, USA

Agricultural chemical transport to surface water and the linkage to other hydrological compartments, principally ground water, was investigated at five watersheds in semiarid to humid climatic settings. Chemical transport was affected by storm water runoff, soil drainage, irrigation, and how streams were linked to shallow ground water systems. Irrigation practices and timing of chemical use greatly affected nutrient and pesticide transport in the semiarid basins. Irrigation with imported water tended to increase ground water and chemical transport, whereas the use of locally pumped irrigation water may eliminate connections between streams and ground water, resulting in lower annual loads. Drainage pathways in humid environments are important because the loads may be transported in tile drains, or through varying combinations of ground water discharge, and overland flow. In most cases, overland flow contributed the greatest loads, but a significant portion of the annual load of nitrate and some pesticide degradates can be transported under base-flow conditions. The highest basin yields for nitrate were measured in a semiarid irrigated system that used imported water and in a stream dominated by tile drainage in a humid environment. Pesticide loads, as a percent of actual use (LAPU), showed the effects of climate and geohydrologic conditions. The LAPU values in the semiarid study basin in Washington were generally low because most of the load was transported in ground water discharge to the stream. When herbicides are applied during the rainy season in a semiarid setting, such as simazine in the California basin, LAPU values are similar to those in the Midwest basins.     

Whole-stream response to nitrate loading in three streams draining agricultural landscapes

Physical, chemical, hydrologic, and biologic factors affecting nitrate (NO3(-)) removal were evaluated in three agricultural streams draining orchard/dairy and row crop settings. Using 3-d "snapshots" during biotically active periods, we estimated reach-level NO3(-) sources, NO3(-) mass balance, in-stream processing (nitrification, denitrification, and NO3(-) uptake), and NO3(-) retention potential associated with surface water transport and ground water discharge. Ground water contributed 5 to 11% to stream discharge along the study reaches and 8 to 42% of gross NO3(-) input. Streambed processes potentially reduced 45 to 75% of ground water NO3(-) before discharge to surface water. In all streams, transient storage was of little importance for surface water NO3(-) retention. Estimated nitrification (1.6-4.4 mg N m(-2) h(-1)) and unamended denitrification rates (2.0-16.3 mg N m(-2) h(-1)) in sediment slurries were high relative to pristine streams. Denitrification of NO3(-) was largely independent of nitrification because both stream and ground water were sources of NO3(-). Unamended denitrification rates extrapolated to the reach-scale accounted for <5% of NO3(-) exported from the reaches minimally reducing downstream loads. Nitrate retention as a percentage of gross NO3(-) inputs was >30% in an organic-poor, autotrophic stream with the lowest denitrification potentials and highest benthic chlorophyll a, photosynthesis/respiration ratio, pH, dissolved oxygen, and diurnal NO3(-) variation. Biotic processing potentially removed 75% of ground water NO3(-) at this site, suggesting an important role for photosynthetic assimilation of ground water NO3(-) relative to subsurface denitrification as water passed directly through benthic diatom beds.     

Determination of dissolved oxygen limitation in aerobic biofilm reactors

The concentration of dissolved oxygen (DO) strongly influences the performance of aerobic biofilm reactors because organic oxidation is limited by the availability of oxygen. However, it is not necessary to maintain a high DO level in the reactors in order to overcome this limitation. Excessive aeration wastes energy. Therefore, the determination of the onset of DO limitation against organic substrate removal in aerobic biofilm reactors is important for their effective operation. This study is aimed at developing an expression to determine the onset of DO limitation and hence to control the aeration system. The expression developed is as follows: , where S_b and C_b are the bulk concentrations of organic substrate and DO, respectively; D_ws and D_wc are the diffusion coefficients of organic substrate and oxygen in the reactors respectively; and R_b is an overall ratio of oxygen consumption to organic substrate removal in the reactors. The latter is the key parameter in the equation, and is determined by the characteristics of the substrate, biofilm, and reactor. In order to measure the value of R_b, the authors have developed a micro-biofilm reactor. The value of R_b was determined to be 0.13 (mg O₂ mg⁻¹ COD_cr) for glucose removal with this reactor. The equation has, subsequently, been verified with data from batch and continuous experiments.     

Sorption and fractionation of dissolved organic matter and associated phosphorus in agricultural soil

Mobility of dissolved organic matter (DOM) strongly affects the export of nitrogen (N) and phosphorus (P) from soils to surface waters. To study the sorption and mobility of dissolved organic C and P (DOC, DOP) in soil, the pH-dependent sorption of DOM to samples from Ap, EB, and Bt horizons from a Danish agricultural Humic Hapludult was investigated and a kinetic model applicable in field-scale models tested. Sorption experiments of 1 to 72 h duration were conducted at two pH levels (pH 5.0 and 7.0) and six initial DOC concentrations (0-4.7 mmol L(-1)). Most sorption/desorption occurred during the first few hours. Dissolved organic carbon and DOP sorption decreased strongly with increased pH and desorption dominated at pH 7, especially for DOC. Due to fractionation during DOM sorption/desorption at DOC concentrations up to 2 mmol L(-1), the solution fraction of DOM was enriched in P indicating preferred leaching of DOP. The kinetics of sorption was expressed as a function of how far the solution DOC or DOP concentrations deviate from "equilibrium." The model was able to simulate the kinetics of DOC and DOP sorption/desorption at all concentrations investigated and at both pH levels making it useful for incorporation in field-scale models for quantifying DOC and DOP dynamics.     

Herbicides and herbicide degradation products in Upper Midwest agricultural streams during August base-flow conditions

Herbicide concentrations in streams of the U.S. Midwest have been shown to decrease through the growing season due to a variety of chemical and physical factors. The occurrence of herbicide degradation products at the end of the growing season is not well known. This study was conducted to document the occurrence of commonly used herbicides and their degradation products in Illinois, Iowa, and Minnesota streams during base-flow conditions in August 1997. Atrazine, the most frequently detected herbicide (94%), was present at relatively low concentrations (median 0.17 microg L(-1)). Metolachlor was detected in 59% and cyanazine in 37% of the samples. Seven of nine compounds detected in more than 50% of the samples were degradation products. The total concentration of the degradation products (median of 4.4 microg L(-1)) was significantly greater than the total concentration of parent compounds (median of 0.26 microg L(-1)). Atrazine compounds were present less frequently and in significantly smaller concentrations in streams draining watersheds with soils developed on less permeable tills than in watersheds with soils developed on more permeable loess. The detection and concentration of triazine compounds was negatively correlated with antecedent rainfall (April-July). In contrast, acetanalide compounds were positively correlated with antecedant rainfall in late spring and early summer that may transport the acetanalide degradates into ground water and subsequently into nearby streams. The distribution of atrazine degradation products suggests regional differences in atrazine degradation processes.     

Phytoestrogens and mycotoxins in Iowa streams: an examination of underinvestigated compounds in agricultural basins

This study provides the first broad-scale investigation on the spatial and temporal occurrence of phytoestrogens and mycotoxins in streams in the United States. Fifteen stream sites across Iowa were sampled five times throughout the 2008 growing season to capture a range of climatic and crop-growth conditions. Basin size upstream from sampling sites ranged from 7 km2 to > 836,000 km2. Atrazine (herbicide) also was measured in all samples as a frame-of-reference agriculturally derived contaminant. Target compounds were frequently detected in stream samples: atrazine (100%), formononetin (80%), equol (45%), deoxynivalenol (43%), daidzein (32%), biochanin A (23%), zearalenone (13%), and genistein (11%). The nearly ubiquitous detection of formononetin (isoflavone) suggests a widespread agricultural source, as one would expect with the intense row crop and livestock production present across Iowa. Conversely, the less spatially widespread detections of deoxynivalenol (mycotoxin) suggest a more variable source due to the required combination of proper host and proper temperature and moisture conditions necessary to promote Fusarium spp. infections. Although atrazine concentrations commonly exceeded 100 ng L(-1) (42/75 measurements), only deoxynivalenol (6/56 measurements) had concentrations that occasionally exceeded this level. Temporal patterns in concentrations varied substantially between atrazine, formononetin, and deoxynivalenol, as one would expect for contaminants with different source inputs and processes of formation and degradation. The greatest phytoestrogen and mycotoxin concentrations were observed during spring snowmelt conditions. Phytoestrogens and mycotoxins were detected at all sampling sites regardless of basin size. The ecotoxicological effects from long-term, low-level exposures to phytoestrogens and mycotoxins or complex chemicals mixtures including these compounds that commonly rake place in surface water are poorly understood and have yet to be systematically investigated in environmental studies.     

Influence of geomorphological variability in channel characteristics on sediment denitrification in agricultural streams

Within fluvial systems, the spatial variability of geomorphological characteristics of stream channels and associated streambed properties can affect many biogeochemical processes. In agricultural streams of the midwestern USA, it is not known how geomorphological variability affects sediment denitrification rates, a potentially important loss mechanism for N. Sediment denitrification was measured at channelized and meandering headwater reaches in east-central Illinois, a region dominated by intensive agriculture and high NO(3)-N stream export, between June 2003 and February 2005 using the chloramphenicol-amended acetylene inhibition procedure. Sediment denitrification rates were greatest in separation zones, ranging from 0.6 to 76.4 mg N m(-2) h(-1), compared with riffles, point bars, pools, and a run ranging from 0 to 36.5 mg N m(-2) h(-1). Differences in benthic organic matter (r = 0.70) and the percentage of fine-grained sediments (r = 0.93) in the streambeds controlled much of the spatial variations in sediment denitrification among the geomorphological features. Although two meandering study reaches removed 390 and 99% more NO(3)-N by sediment denitrification than adjacent channelized reaches, NO(3)-N loss rates from all reaches were between 0.1 and 15.7% d(-1), except in late summer. Regardless of geomorphological characteristics, streams in east-central Illinois were not able to process the high NO(3)-N loads, making sediment denitrification in this region a limited sink for N.     

Relationships among plant available phosphorus, fertilizer sales, and water quality in northwestern Ohio

Soluble reactive phosphorus (SRP) in northwestern Ohio river water has declined over the past 20 yr in response to decreased applications of fertilizer P. Our objective was to evaluate changes in soluble P (Bray-1 P) levels in the soil over time as influenced by fertilizer P management, cultivation practice, soil properties, and landscape factors. Because soil is the intermediary between added P and SRP measured in river water, we examined the relationship between fertilizer P, soluble soil P, and SRP. Using historical soil survey sample sites as a baseline for original soluble P concentrations (Por), we resampled Ap horizons to establish current levels of soluble P (Pcu). The Por baseline extended from 1953-1982 and Pcu from 1996-1998. Thirty percent of the Pcu values and 17% of the Por values were > or = 40 mg kg(-1). Log-transformed means for Pcu were significantly higher than for Por. The principal determining factors for Por were physiography, soil texture, and soil series. Current P is affected by present tillage practice and drainage class. Change in soluble P in the soil is not as responsive to fertilizer P sales as is SRP in river water. This suggests that as fertilizer P sales decline, a declining percentage of P added as fertilizer is annually dissolved and transported into the drainage system. Soluble P in soil is governed by a combination of fertilizer and tillage management, soil properties, and landscape factors interacting over time.