Extending vegetative cover with cover crops influenced phosphorus loss from an agricultural watershed.

Excess phosphorus (P) from agriculture is a leading cause of harmful and nuisance algal blooms in many freshwater ecosystems. Throughout much of the midwestern United States, extensive networks of subsurface tile drains remove excess water from fields and allow for productive agriculture. This enhanced drainage also facilitates the transport of P, particularly soluble reactive phosphorus (SRP), to adjacent streams and ditches, with harmful consequences. Thus, reducing SRP loss from tile-drained cropland is a major focus of regional and national efforts to curb eutrophication and algal blooms. The planting of cover crops after crop harvest is a conservation practice that has the potential to increase retention of fertilizer nutrients in watersheds by extending the growing season and limiting bare ground in the fallow season; however, the effect of cover crops on SRP loss is inconsistent at the field-scale and unknown at the watershed-scale. In this study, we conducted a large-scale manipulation of land cover in a small, agricultural watershed by planting cover crops on >60% of croppable acres for six years and examining changes in SRP loss through tile drains and at the watershed outlet. We found reduced median SRP loss from tiles with cover crops compared to those without cover crops, particularly during periods of critical export from January to June. Variation in tile discharge influenced SRP loss, but relationships were generally weaker in tiles with cover crops (i.e., decoupled) compared to tiles without cover crops. At the watershed outlet, SRP yield was highly variable over all seasons and years, which complicated efforts to detect a significant effect of changing land cover on SRP export to downstream systems. Yet, watershed-scale planting of cover crops slowed cumulative SRP losses and reduced SRP export during extreme events. Overall, this study demonstrates the potential for cover crops to alter patterns of SRP loss at both the field- and watershed-scale.

Effect of winter cover crops on soil nutrients in two row-cropped watersheds in Indiana.

The midwestern United States is a highly productive agricultural region, and extended crop-free periods in winter/spring can result in nitrogen (N) and phosphorus (P) losses to waterways that degrade downstream water quality. Planting winter cover crops can improve soil health while reducing nutrient leaching from farm fields during the fallow period. In this study, we used linear mixed effects models and multivariate statistics to determine the effect of cover crops on soil nutrients by comparing fields with cover crops (n = 9) versus those without (n = 6) in two Indiana agricultural watersheds: the Shatto Ditch Watershed, which had >60% of croppable acres in winter cover crops, and the Kirkpatrick Ditch Watershed, which had ∼20%. We found that cover crops decreased soil nitrate-N by >50% and that the magnitude of reduction was related to the amount of cover crop biomass. In contrast, cover crops had variable effects on water extractable P and Mehlich III soil test P. Finally, cover crop biomass significantly increased soil N mineralization and nitrification rates, demonstrating that cover crops have the potential to supply bioavailable N to cash crop after termination. Our study showed that widespread implementation of winter cover crops holds considerable promise for reducing nutrient loss and improving soil health. The degree to which these results are generalizable across other systems depends on factors such as climate, soil characteristics, and past and current agronomic practices.

Seasonal evaluation of biotic and abiotic factors suggests phosphorus retention in constructed floodplains in three agricultural streams.

Floodplain restoration constructed via the two-stage ditch in agricultural streams has the potential to enhance nutrient retention and prevent the eutrophication of downstream ecosystems. Identifying the role of biotic and abiotic factors influencing soluble reactive phosphorus (SRP) retention in floodplains is of interest given that changing redox conditions associated with floodplain inundation can result in a release of geochemically sorbed SRP to the water column. In three agricultural waterways (Indiana, USA), we conducted seasonal measurements of a suite of biogeochemical pools (total P, bioavailable P and Fe) and processes (SRP flux and microbial respiration) from multiple floodplain transects, along with their adjacent stream sediments, to determine the role of biotic and abiotic processes on floodplain SRP retention or release. Across floodplain soils, organic matter explained a significant amount of variation in soil respiration, and SRP flux from the water column to the floodplain soils was driven by the molar ratio of Fe: P, with values >6 indicating potential SRP sorption due to increased available sorption sites. We developed a mass balance model at a single site to relate seasonal floodplain processes with water column SRP export, above and below the study reach, using measurements in this study combined with data from the literature. Grab sample data suggest that the reach retained 26% of incoming SRP, which the mass balance model attributed to seasonal synergy between plant assimilation in spring and summer (removing P from floodplain soils) and abiotic P sorption during winter and spring inundation (adding SRP to the floodplain). Retention of SRP was higher in floodplain soils compared to stream sediments based on the modeled SRP budget. Thus, we suggest that these constructed floodplains will maximize SRP retention from the water column if they inundate regularly, have floodplain soils with Fe:P > 3-6, and that promote sustained plant life.

Fine Sediment Removal Influences Biogeochemical Processes in a Gravel-bottomed Stream.

The transport and processing of nutrients and organic matter in streams are important functions that influence the condition of watersheds and downstream ecosystems. In this study, we investigated the effects of streambed sediment removal on biogeochemical cycling in Fawn River, a gravel-bottomed river in Indiana, U.S.A. We measured stream metabolism as well as nitrogen (N) and phosphorus (P) retention in both restored and unrestored reaches of Fawn River to examine how sediment removal affected multiple biogeochemical functions at the reach scale. We also assessed the properties of restored and unrestored streambed sediments to elucidate potential mechanisms driving observed reach-scale differences. We found that sediment removal led to lower rates of primary productivity and ecosystem respiration in the restored reach, likely due to macrophyte removal and potentially due to changes to sediment organic matter quality. We found minimal differences in N and P retention, suggesting that these processes are controlled at larger spatial or temporal scales than were examined in this study. Denitrification enzyme activity was lower in sediments from the restored reach compared to the unrestored reach, suggesting that restoration may have decreased N removal. Our results indicate that most near-term changes in biogeochemical function following restoration could be attributed to macrophyte removal, although effects from sediment removal may emerge over longer timescales.

Initial nitrogen enrichment conditions determines variations in nitrogen substrate utilization by heterotrophic bacterial isolates.

BACKGROUND: The nitrogen (N) cycle consists of complex microbe-mediated transformations driven by a variety of factors, including diversity and concentrations of N compounds. In this study, we examined taxonomic diversity and N substrate utilization by heterotrophic bacteria isolated from streams under complex and simple N-enrichment conditions. RESULTS: Diversity estimates differed among isolates from the enrichments, but no significant composition were detected. Substrate utilization and substrate range of bacterial assemblages differed within and among enrichments types, and not simply between simple and complex N-enrichments. CONCLUSIONS: N substrate use patterns differed between isolates from some complex and simple N-enrichments while others were unexpectedly similar. Taxonomic composition of isolates did not differ among enrichments and was unrelated to N use suggesting strong functional redundancy. Ultimately, our results imply that the available N pool influences physiology and selects for bacteria with various abilities that are unrelated to their taxonomic affiliation.

Occurrence, leaching, and degradation of Cry1Ab protein from transgenic maize detritus in agricultural streams.

The insecticidal Cry1Ab protein expressed by transgenic (Bt) maize can enter adjacent water bodies via multiple pathways, but its fate in stream ecosystems is not as well studied as in terrestrial systems. In this study, we used a combination of field sampling and laboratory experiments to examine the occurrence, leaching, and degradation of soluble Cry1Ab protein derived from Bt maize in agricultural streams. We surveyed 11 agricultural streams in northwestern Indiana, USA, on 6 dates that encompassed the growing season, crop harvest, and snowmelt/spring flooding, and detected Cry1Ab protein in the water column and in flowing subsurface tile drains at concentrations of 3-60ng/L. In a series of laboratory experiments, submerged Bt maize leaves leached Cry1Ab into stream water with 1% of the protein remaining in leaves after 70d. Laboratory experiments suggested that dissolved Cry1Ab protein degraded rapidly in microcosms containing water-column microorganisms, and light did not enhance breakdown by stimulating assimilatory uptake of the protein by autotrophs. The common detection of Cry1Ab protein in streams sampled across an agricultural landscape, combined with laboratory studies showing rapid leaching and degradation, suggests that Cry1Ab may be pseudo-persistent at the watershed scale due to the multiple input pathways from the surrounding terrestrial environment.

The role of dissolved organic nitrogen in a nitrate-rich agricultural stream.

Agricultural activities have heavily altered the nitrogen (N) cycle in stream ecosystems draining croplands, particularly in the midwestern United States. However, our knowledge about dissolved organic N (DON) biogeochemistry in agricultural ecosystems is limited. From January 2011 to June 2012, we investigated DON dynamics in an agricultural headwater stream in the midwestern United States. We quantified the contribution of DON to the total dissolved N (TDN) pool and examined the role of DON as a source of N for microbial metabolism. For this we measured N-acquiring enzyme activities (aminopeptidases) and whole-stream DON uptake through short-term releases of amino acids. To investigate potential coupling between the N and C cycles occurring via simultaneous uptake of these two elements during assimilation of amino acids, seven of the short-term releases were performed concurrently with acetate. We found minimal contribution of DON to the TDN pool in this stream as a result of high concentrations of nitrate. Acetate uptake suggested that C was a limiting factor for microbial metabolism in this stream. In contrast, we were not able to detect amino acid uptake during any of the 13 short-term releases we conducted, likely as a result of high availability of dissolved inorganic N. Aminopeptidase (AMP) activity did not reflect N demand. Large spatial variability in AMP was observed within and among sites, possibly as result of physicochemical characteristics of the sediments. In contrast to other human-dominated streams, DON appeared to play a minor role in microbial metabolic processes and contributed minimally to the N pool of the study stream.

Evaluation of passive samplers for the collection of dissolved organic matter in streams.

Traditional sampling methods for dissolved organic matter (DOM) in streams limit opportunities for long-term studies due to time and cost constraints. Passive DOM samplers were constructed following a design proposed previously which utilizes diethylaminoethyl (DEAE) cellulose as a sampling medium, and they were deployed throughout a temperate stream network in Indiana. Two deployments of the passive samplers were conducted, during which grab samples were frequently collected for comparison. Differences in DOM quality between sites and sampling methods were assessed using several common optical analyses. The analyses revealed significant differences in optical properties between sampling methods, with the passive samplers preferentially collecting terrestrial, humic-like DOM. We assert that the differences in DOM composition from each sampling method were caused by preferential binding of complex humic compounds to the DEAE cellulose in the passive samplers. Nonetheless, the passive samplers may provide a cost-effective, integrated sample of DOM in situations where the bulk DOM pool is composed mainly of terrestrial, humic-like compounds.

Denitrification in agriculturally impacted streams: seasonal changes in structure and function of the bacterial community.

Denitrifiers remove fixed nitrogen from aquatic environments and hydrologic conditions are one potential driver of denitrification rate and denitrifier community composition. In this study, two agriculturally impacted streams in the Sugar Creek watershed in Indiana, USA with different hydrologic regimes were examined; one stream is seasonally ephemeral because of its source (tile drainage), whereas the other stream has permanent flow. Additionally, a simulated flooding experiment was performed on the riparian benches of the ephemeral stream during a dry period. Denitrification activity was assayed using the chloramphenicol amended acetylene block method and bacterial communities were examined based on quantitative PCR and terminal restriction length polymorphisms of the nitrous oxide reductase (nosZ) and 16S rRNA genes. In the stream channel, hydrology had a substantial impact on denitrification rates, likely by significantly lowering water potential in sediments. Clear patterns in denitrification rates were observed among pre-drying, dry, and post-drying dates; however, a less clear scenario was apparent when analyzing bacterial community structure suggesting that denitrifier community structure and denitrification rate were not strongly coupled. This implies that the nature of the response to short-term hydrologic changes was physiological rather than increases in abundance of denitrifiers or changes in composition of the denitrifier community. Flooding of riparian bench soils had a short-term, transient effect on denitrification rate. Our results imply that brief flooding of riparian zones is unlikely to contribute substantially to removal of nitrate (NO3-) and that seasonal drying of stream channels has a negative impact on NO3- removal, particularly because of the time lag required for denitrification to rebound. This time lag is presumably attributable to the time required for the denitrifiers to respond physiologically rather than a change in abundance or community composition.

Pharmaceuticals suppress algal growth and microbial respiration and alter bacterial communities in stream biofilms.

Pharmaceutical and personal care products are ubiquitous in surface waters but their effects on aquatic biofilms and associated ecosystem properties are not well understood. We measured in situ responses of stream biofilms to six common pharmaceutical compounds (caffeine, cimetidine, ciprofloxacin, diphenhydramine, metformin, ranitidine, and a mixture of each) by deploying pharmaceutical-diffusing substrates in streams in Indiana, Maryland, and New York. Results were consistent across seasons and geographic locations. On average, algal biomass was suppressed by 22%, 4%, 22%, and 18% relative to controls by caffeine, ciprofloxacin, diphenhydramine, and the mixed treatment, respectively. Biofilm respiration was significantly suppressed by caffeine (53%), cimetidine (51%), ciprofloxacin (91%), diphenhydramine (63%), and the mixed treatment (40%). In autumn in New York, photosynthesis was also significantly suppressed by diphenhydramine (99%) and the mixed treatment (88%). Pyrosequencing of 16S rRNA genes was used to examine the effects of caffeine and diphenhydramine on biofilm bacterial community composition at the three sites. Relative to the controls, diphenhydramine exposure significantly altered bacterial community composition and resulted in significant relative increases in Pseudomonas sp. and decreases in Flavobacterium sp. in all three streams. These ubiquitous pharmaceuticals, alone or in combination, influenced stream biofilms, which could have consequences for higher trophic levels and important ecosystem processes.

Responses of stream macroinvertebrates to Bt maize leaf detritus.

In the midwestern United States, maize detritus enters streams draining agricultural land. Genetically modified Bt maize is commonly planted along streams and can possibly affect benthic macroinvertebrates, specifically members of the order Trichoptera, which are closely related to target species of some Bt toxins and are important detritivores in streams. The significance of inputs of Bt maize to aquatic systems has only recently been recognized, and assessments of potential nontarget impacts on aquatic organisms are lacking. We conducted laboratory feeding trials and found that the leaf-shredding trichopteran, Lepidostoma liba, grew significantly slower when fed Bt maize compared to non-Bt maize, while other invertebrate taxa that we examined showed no negative effects. We also used field studies to assess the influence of Bt maize detritus on benthic macroinvertebrate abundance, diversity, biomass, and functional structure in situ in 12 streams adjacent to Bt maize or non-Bt maize fields. We found no significant differences in total abundance or biomass between Bt and non-Bt streams, and trichopterans comprised only a small percentage of invertebrate biomass at all sites (0-15%). Shannon diversity did not differ among Bt and non-Bt streams and was always low (H' range = 0.9-1.9). Highly tolerant taxa, such as oligochaetes and chironomids, were dominant in both Bt and non-Bt streams, and macroinvertebrate community composition was relatively constant across seasons. We used litterbags to examine macroinvertebrate colonization of Bt and non-Bt maize detritus and found no significant differences among litter or stream types. Our in situ findings did not support our laboratory results; this is likely because the streams we studied in this region are highly degraded and subject to multiple, persistent anthropogenic stressors (e.g., channelization, altered flow, nutrient and pesticide inputs). Invertebrate communities in these streams are a product of these degraded conditions, and thus the impact of a single stressor, such as Bt toxins, may not be readily discernable. Our results add to growing evidence that Bt toxins can have sublethal effects on nontarget aquatic taxa, but this evidence should be considered in the context of other anthropogenic impacts and alternative methods of pest control influencing streams draining agricultural regions.

Occurrence of maize detritus and a transgenic insecticidal protein (Cry1Ab) within the stream network of an agricultural landscape.

Widespread planting of maize throughout the agricultural Midwest may result in detritus entering adjacent stream ecosystems, and 63% of the 2009 US maize crop was genetically modified to express insecticidal Cry proteins derived from Bacillus thuringiensis. Six months after harvest, we conducted a synoptic survey of 217 stream sites in Indiana to determine the extent of maize detritus and presence of Cry1Ab protein in the stream network. We found that 86% of stream sites contained maize leaves, cobs, husks, and/or stalks in the active stream channel. We also detected Cry1Ab protein in stream-channel maize at 13% of sites and in the water column at 23% of sites. We found that 82% of stream sites were adjacent to maize fields, and Geographical Information Systems analyses indicated that 100% of sites containing Cry1Ab-positive detritus in the active stream channel had maize planted within 500 m during the previous crop year. Maize detritus likely enters streams throughout the Corn Belt; using US Department of Agriculture land cover data, we estimate that 91% of the 256,446 km of streams/rivers in Iowa, Illinois, and Indiana are located within 500 m of a maize field. Maize detritus is common in low-gradient stream channels in northwestern Indiana, and Cry1Ab proteins persist in maize leaves and can be measured in the water column even 6 mo after harvest. Hence, maize detritus, and associated Cry1Ab proteins, are widely distributed and persistent in the headwater streams of a Corn Belt landscape.

Effect of road salt application on seasonal chloride concentrations and toxicity in south-central Indiana streams.

Contemporary information on road salt runoff is needed for management of water resources in regions experiencing urbanization and increased road density. We investigated seasonal Cl(-) concentrations among five streams in south-central Indiana that drained watersheds varying in degree of urbanization and ranging in size from 9.3 to 27 km(2). We also conducted acute toxicity tests with Daphnia pulex to assess the potential effects of the observed Cl(-) concentrations on aquatic life. Periods of elevated Cl(-) concentrations were observed during the winters of 2007-08 and 2008-09 at all sites except the reference site. The highest Cl(-) concentration observed during the study was 2100 mg L(-1) and occurred at the most urbanized site. The Cl(-) concentration at the reference site never exceeded 22 mg L(-1). The application of road salt caused large increases in stream Cl(-) concentrations, but the elevated Cl(-) levels did not appear to be a significant threat to aquatic life based on our toxicity testing. Only the most urbanized site showed evidence of salt retention within the watershed, whereas the other sites exported the road salt relatively quickly after its application, suggesting storm drains and impervious surfaces minimized interaction between soils and salt-laden runoff. During winter at these sites, the response in stream Cl(-) concentrations appeared to be controlled by the timing and intensity of road salt application, the magnitude of precipitation, and the occurrence of air temperatures that caused snowmelt and generated runoff.

Nitrogen mass balance of a tile-drained agricultural watershed in East-Central Illinois.

Simple nitrogen (N) input/output balance calculations in agricultural systems are used to evaluate performance of nutrient management; however, they generally rely on extensive assumptions that do not consider leaching, denitrification, or annual depletion of soil N. We constructed a relatively complete N mass balance for the Big Ditch watershed, an extensively tile-drained agricultural watershed in east-central Illinois. We conducted direct measurements of a wide range of N pools and fluxes for a 2-yr period, including soil N mineralization, soybean N(2) fixation, tile and river N loads, and ground water and in-stream denitrification. Fertilizer N inputs were from a survey of the watershed and yield data from county estimates that were combined with estimated protein contents to obtain grain N. By using maize fertilizer recovery and soybean N(2) fixation to estimate total grain N derived from soil, we calculated the explicit change in soil N storage each year. Overall, fertilizer N and soybean N(2) fixation dominated inputs, and total grain export dominated outputs. Precipitation during 2001 was below average (78 cm), whereas precipitation in 2002 exceeded the 30-yr average of 97 cm; monthly rainfall was above average in April, May, and June of 2002, which flooded fields and produced large tile and riverine N loads. In 2001, watershed inputs were greater than outputs, suggesting that carryover of N to the subsequent year may occur. In 2002, total inputs were less than outputs due to large leaching losses and likely substantial field denitrification. The explicit change in soil storage (67 kg N ha(-1)) offsets this balance shortfall. Although 2002 was climatically unusual, with current production trends of greater maize grain yields with less fertilizer N, soil N depletion is likely to occur in maize/soybean rotations, especially in years with above-average precipitation or extremely wet spring periods.

Rapid decomposition of maize detritus in agricultural headwater streams.

Headwater streams draining agricultural landscapes receive maize leaves (Zea mays L.) via wind and surface runoff, yet the contribution of maize detritus to organic-matter processing in agricultural streams is largely unknown. We quantified decomposition and microbial respiration rates on conventional (non-Bt) and genetically engineered (Bt) maize in three low-order agricultural streams in northwestern Indiana, USA. We also examined how substrate quality and in-stream nutrient concentrations influenced microbial respiration on maize by comparing respiration on maize and red maple leaves (Acer rubrum) in three nutrient-rich agricultural streams and three low-nutrient forested streams. We found significantly higher rates of microbial respiration on maize vs. red maple leaves and higher rates in agricultural vs. forested streams. Thus both the elevated nutrient status of agricultural streams and the lability of maize detritus (e.g., low carbon-to-nitrogen ratio and low lignin content) result in a rapid incorporation of maize leaves into the aquatic microbial food web. We found that Bt maize had a faster decomposition rate than non-Bt maize, while microbial respiration rates did not differ between Bt and non-Bt maize. Decomposition rates were not negatively affected by genetic engineering, perhaps because the Bt toxin does not adversely affect the aquatic microbial assemblage involved in maize decomposition. Additionally, shredding caddisflies, which are known to have suppressed growth rates when fed Bt maize, were depauperate in these agricultural streams, and likely did not play a major role in maize decomposition. Overall, the conversion of native vegetation to row-crop agriculture appears to have altered the quantity, quality, and predictability of allochthonous carbon inputs to headwater streams, with unexplored effects on stream ecosystem structure and function.

Relationships between benthic sediments and water column phosphorus in Illinois streams.

Sediments can be important in regulating stream water P concentrations, and this has implications for establishing nutrient standards that have not been fully investigated. We evaluated abiotic and biotic processes to better understand the role of sediments in determining stream water dissolved P concentrations. Sediment and stream water samples were collected during low discharge from 105 streams across Illinois and analyzed for equilibrium P concentration at zero release or retention (EPC(0)), P sorption characteristics, stream water P concentration, and sediment particle size. In addition, four east-central Illinois streams were repeatedly sampled to examine temporal patterns in sediment P retention and biotic processing of P. Median dissolved reactive P (DRP) and total P concentrations across the state were 0.081 and 0.168 mg L(-1), respectively. Sediment EPC(0) concentrations were related to stream water DRP concentrations (r(s) = 0.75). Sediment silt+clay (and co-correlated organic matter) was related to sorbed P (r(s) = -0.49) and the reactive sediment pool of P (r(s) = 0.76). However, for most sites this pool was small given the coarse textures present (median silt+clay was 5.7%). Repeated sampling at the four intensive sites showed little variation in EPC(0) values or alkaline phosphatase activity, suggesting overall stream conditions regulated the biotic processing. Biotic retention of P was 32% of short-term P removal. We conclude that sediments in Illinois streams are a reflection of and partially affected by stream water P concentrations through both abiotic and biotic processes. Sediments seem unlikely to alter annual stream P loads, but may affect concentrations at low discharge.

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.

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.

Diversity of fungi, bacteria, and actinomycetes on leaves decomposing in a stream.

Although fungi, bacteria, and specific bacterial taxa, such as the actinomycetes, have been studied extensively in various habitats, few studies have examined them simultaneously, especially on decomposing leaves in streams. In this study, sugar maple and white oak leaves were incubated in a stream in northeastern Ohio for 181 days during which samples were collected at regular intervals. Following DNA extraction, PCR-denaturing gradient gel electrophoresis (DGGE) was performed using fungus-, bacterium-, and actinomycete-specific primers. In addition, fungal and bacterial biomass was estimated. Fungal biomass differed on different days but not between leaves of the two species and was always greater than bacterial biomass. There were significant differences in bacterial biomass through time and between leaf types on some days. Generally, on the basis of DGGE, few differences in community structure were found for different leaf types. However, the ribotype richness of fungi was significantly greater than those of the bacteria and actinomycetes, which were similar to each other. Ribotype richness decreased toward the end of the study for each group except bacteria. Lack of differences between the two leaf types suggests that the microorganisms colonizing the leaf biofilm were primarily generalists that could exploit the resources of the leaves of either species equally well. Thus, we conclude that factors, such as the ecological role of the taxa (generalists versus specialists), stage of decay, and time of exposure, appeared to be more important determinants of microbial community structure than leaf quality.