Bruce Vondracek

Effects of Agricultural Drainage on Aquatic Ecosystems: A Review

The extensive development of surface and subsurface drainage systems to facilitate agricultural production throughout North America has significantly altered the hydrology of landscapes compared to historical conditions. Drainage has transformed nutrient and hydrologic dynamics, structure, function, quantity, and configuration of stream and wetland ecosystems. In many agricultural regions, more than 80% of some catchment basins may be drained by surface ditches and subsurface drain pipes (tiles). Natural channels have been straightened and deepened for surface drainage ditches with significant effects on channel morphology, instream habitats for aquatic organisms, floodplain and riparian connectivity, sediment dynamics, and nutrient cycling. The connection of formerly isolated wetland basins to extensive networks of surface drainage and the construction of main channel ditches through millions of acres of formerly low-lying marsh or wet prairie, where no defined channel may have previously existed, have resulted in large-scale conversion of aquatic habitat types, from wetland mosaics to linear systems. Reduced surface storage, increased conveyance, and increased effective drainage area have altered the dynamics of and generally increased flows in larger streams and rivers. Cumulatively, these changes in hydrology, geomorphology, nutrient cycling, and sediment dynamics have had profound implications for aquatic ecosystems and biodiversity.

Multifunctional Agriculture in the United States

Abstract We evaluated possible changes to current farming practices in two Minnesota watersheds to provide insight into how farm policy might affect environmental, social, and economic outcomes. Watershed residents helped develop four scenarios to evaluate alternative future trends in agricultural management and to project potential economic and environmental outcomes. We found that environmental and economic benefits can be attained through changes in agricultural land management without increasing public costs. The magnitude of these benefits depends on the magnitude of changes to agricultural practices. Environmental benefits include improved water quality, healthier fish, increased carbon sequestration, and decreased greenhouse gas emissions, while economic benefits include social capital formation, greater farm profitability, and avoided costs. Policy transitions that emphasize functions of agriculture in addition to food production are crucial for creating change. We suggest that redirecting farm payments by using alternative incentives could lead to substantial environmental changes at little or no extra cost to the taxpayer.

Top-down control in a detritus-based food web: fish, shredders, and leaf breakdown

Abstract. We tested the hypothesis that fish decrease shredder abundance in leaf packs, thereby reducing leaf breakdown rates. Our goal was to test for the occurrence of a trophic cascade in a detritus-based food web. Willow leaves (Salix spp.) were fastened into leaf packs and placed into cages (13×13×13xa0cm) in Valley Creek, Minnesota, USA. Fish were excluded from leaf packs that were placed in cages with mesh on all sides, whereas open control cages allowed fish access to leaf packs. We collected leaf packs from two replicate cages 0, 14, 31, 55, and 112xa0days after placement in each of three riffles (n=6 per collection). Total abundance of invertebrates and shredders inhabiting leaf packs was significantly higher in exclosures than controls (P<0.01) and increased with exposure time in the stream (P<0.01). Three of the four common shredder taxa had significantly higher biomass in exclosures than controls (P<0.015). Biomass of Hesperophylax (Trichoptera) larvae was significantly higher in controls during the final collections (P<0.03), probably because these large, case-building larvae were less vulnerable to fish predation. Leaf breakdown rates differed significantly between exclosures and controls (P=0.003), but the direction of effects varied among riffles. When shredder density was analyzed separately for each riffle, we found that shredder density may explain differences in leaf breakdown rates between exclosures and controls. The differential responses of shredder taxa to predators may explain variability in fish effects on leaf breakdown. In conclusion, leaf packs did not provide invertebrates refuge from fish predation and fish reduced the densities of most shredders. Fish can indirectly affect leaf breakdown rates, but different responses to predation among taxa within the shredder guild can cause interactions that contradict trophic cascade predictions.

Impacts of Rotational Grazing and Riparian Buffers on Physicochemical and Biological Characteristicsof Southeastern Minnesota, USA, Streams

We assessed the relationship between riparian management and stream quality along five southeastern Minnesota streams in 1995 and 1996. Specifically, we examined the effect of rotationally and continuously grazed pastures and different types of riparian buffer strips on water chemistry, physical habitat, benthic macroinvertebrates, and fish as indicators of stream quality. We collected data at 17 sites under different combinations of grazing and riparian management, using a longitudinal design on three streams and a paired watershed design on two others. Continuous and rotational grazing were compared along one longitudinal study stream and at the paired watershed. Riparian buffer management, fenced trees (wood buffer), fenced grass, and unfenced rotationally grazed areas were the focus along the two remaining longitudinal streams. Principal components analysis (PCA) of water chemistry, physical habitat, and biotic data indicated a local management effect. The ordinations separated continuous grazing from sites with rotational grazing and sites with wood buffers from those with grass buffers or rotationally grazed areas. Fecal coliform and turbidity were consistently higher at continuously grazed than rotationally grazed sites. Percent fines in the streambed were significantly higher at sites with wood buffers than grass and rotationally grazed areas, and canopy cover was similar at sites with wood and grass buffers. Benthic macroinvertebrate metrics were significant but were not consistent across grazing and riparian buffer management types. Fish density and abundance were related to riparian buffer type, rather than grazing practices. Our study has potentially important implications for stream restoration programs in the midwestern United States. Our comparisons suggest further consideration and study of a combination of grass and wood riparian buffer strips as midwestern stream management options, rather than universally installing wood buffers in every instance. RID= ID= The Unit is jointly sponsored by the US Geological Survey, Biological Resources Division; the Minnesota Department of Natural Resources; the University of Minnesota; and the Wildlife Management Institute.

Factors influencing wood mobilization in streams

[1]xa0Natural pieces of wood provide a variety of ecosystem functions in streams including habitat, organic matter retention, increased hyporheic exchange and transient storage, and enhanced hydraulic and geomorphic heterogeneity. Wood mobilization is a critical process in determining the residence time of wood. We documented the characteristics and locations of 865 natural wood pieces (>0.05 m in diameter for a portion >1 m in length) in nine streams along the north shore of Lake Superior in Minnesota. We determined the locations of the pieces again after an overbank stormflow event to determine the factors that influenced mobilization of stationary wood pieces in natural streams. Seven of 11 potential predictor variables were identified with multiple logistic regression as significant to mobilization: burial, effective depth, ratio of piece length to effective stream width (length ratio), bracing, rootwad presence, downstream force ratio, and draft ratio. The final model (P < 0.001, r2 = 0.39) indicated that wood mobilization under natural conditions is a complex function of both mechanical factors (burial, length ratio, bracing, rootwad presence, draft ratio) and hydraulic factors (effective depth, downstream force ratio). If stable pieces are a goal for stream management then features such as partial burial, low effective depth, high length relative to channel width, bracing against other objects (e.g., stream banks, trees, rocks, or larger wood pieces), and rootwads are desirable. Using the model equation from this study, stewards of natural resources can better manage in-stream wood for the benefit of stream ecosystems.

Mixed-source reintroductions lead to outbreeding depression in second-generation descendents of a native North American fish.

Reintroductions are commonly employed to preserve intraspecific biodiversity in fragmented landscapes. However, reintroduced populations are frequently smaller and more geographically isolated than native populations. Mixing genetically, divergent sources are often proposed to attenuate potentially low genetic diversity in reintroduced populations that may result from small effective population sizes. However, a possible negative tradeoff for mixing sources is outbreeding depression in hybrid offspring. We examined the consequences of mixed‐source reintroductions on several fitness surrogates at nine slimy sculpin (Cottus cognatus) reintroduction sites in south‐east Minnesota. We inferred the relative fitness of each crosstype in the reintroduced populations by comparing their growth rate, length, weight, body condition and persistence in reintroduced populations. Pure strain descendents from a single source population persisted in a greater proportion than expected in the reintroduced populations, whereas all other crosstypes occurred in a lesser proportion. Length, weight and growth rate were lower for second‐generation intra‐population hybrid descendents than for pure strain and first‐generation hybrids. In the predominant pure strain, young‐of the‐year size was significantly greater than any other crosstype. Our results suggested that differences in fitness surrogates among crosstypes were consistent with disrupted co‐adapted gene complexes associated with beneficial adaptations in these reintroduced populations. Future reintroductions may be improved by evaluating the potential for local adaptation in source populations or by avoiding the use of mixed sources by default when information on local adaptations or other genetic characteristics is lacking.

Comparison of Catch Rate, Length Distribution, and Precision of Six Gears Used to Sample Reservoir Shad Populations

Abstract Hydroacoustics, trawling, gillnetting, electrofishing, shoreline seining, and cove rotenoning were used concurrently in August 1991 at Lake Texoma, Texas–Oklahoma, to compare sampling efficiency for gizzard shad Dorosoma cepedianum and threadfin shad D. petenense. A simple random-sampling design was used at nearshore and offshore stations in each of three 400–2,000-ha sites in the reservoir. Most gears provided similar evidence of spatial patterns of shad abundance among sites, but length distributions and sampling precision varied among methods. At offshore transects, catch-per-unit-effort (CPUE) data for hydroacoustics, trawling, and gillnetting were positively correlated (r = 0.45–0.80; P < 0.05) when data from all sites were combined. Gears differed in proportions of small (age-0) shad versus larger (age-1 and older) fish. Gears that collected mainly age-0 shad were trawls, surface-set gill nets, and seine, whereas catches of age-1 and older shad were greater with bottom-set gill nets, electr...

Relationship of Riparian Buffer Type to Water Temperature in the Driftless Area Ecoregion of Minnesota

Abstract We used the U.S. Fish and Wildlife Services Stream Network Temperature Model to examine the role of riparian buffer type in mediating summer water temperatures for the reintroduction of brook trout Salvelinus fontinalis into Wells Creek, a tributary to the Mississippi River located in southeastern Minnesota. Stream temperatures measured from 23 July to 3 September 1997 were used to calibrate the model, evaluate existing temperatures, generate simulations for different shade conditions and channel morphologies, and generate simulations for “average” and “warm” summers (we define a warm summer as one that is 2.8°C above the 30-year mean). The simulations indicated that successional buffers (grasses and forbs) provided as much shade as wooded buffers in streams with a width less than 2.5 m. With a low width:depth ratio, the successional buffer vegetation mediated mean temperature as well as the wooded buffer when discharge was held constant. At a discharge characteristic of our study reach, the mea...

Insects as indicators of land use in three ecoregions in the Prairie Pothole Region

We sampled populations of insects in the prairie pothole region of North Dakota, USA, to determine whether relationships existed between community- or taxon-level indicators and 11 land-use types. Our goal was to determine if agricultural impacts were reflected in measurable differences for insect indicators: abundance, taxa richness, and diversity. Insects were sampled with light traps at 126 wetland basins in three ecoregions. Sampling was conducted three times each year during the spring and early summer of 1995 and 1996. Sites were selected based on the proportion of cropland to grassland, hayland, and Conservation Reserve Program land surrounding wetland basins at 50 and 400 m radii. Other land-use types included in our analyses were woodland, roadways, and five wetland types: permanent, semipermanent, seasonal, temporary, and riverine. In both years, taxa richness, abundance, and diversity were greater for the second (June) and third (July) sampling periods than for the first period (May), and indicators were greater in the Drift Plain and Red River Valley ecoregions than in the Missouri Coteau ecoregion. Our analyses indicated several significant associations between insect indicators and land-use types; however, r2 values were generally low. Much more of the variance in insect measures was explained by temperature, seasonal, and ecoregion effects. Several associations were significant within individual ecoregions (i.e., abundance of aquatic insects. Caenidae, Scarabaeidae, and Lepidoptera and number of Ephemeroptera families). However, no indicators were found in common for all three ecoregions. Several significant associations with land use were identified across all sites (i.e., all ecoregions combined). A small number of the significant relationships found across all sites were related to agricultural land use, and several indicated a negative relationship with grasslands. However, we observed several positive relationships between our insect indicators and riverine wetlands across sites and in the Red River Valley ecoregion for both years and spatial scales (i.e., the abundance of Caenidae, Scarabaeidae, Ceratopogonidae, Hydropsychidae, and Hydroptilidae).

Grazed Riparian Management and Stream Channel Response in Southeastern Minnesota (USA) Streams

The U.S. Department of Agriculture-Natural Resources Conservation Service has recommended domestic cattle grazing exclusion from riparian corridors for decades. This recommendation was based on a belief that domestic cattle grazing would typically destroy stream bank vegetation and in-channel habitat. Continuous grazing (CG) has caused adverse environmental damage, but along cohesive-sediment stream banks of disturbed catchments in southeastern Minnesota, short-duration grazing (SDG), a rotational grazing system, may offer a better riparian management practice than CG. Over 30 physical and biological metrics were gathered at 26 sites to evaluate differences between SDG, CG, and nongrazed sites (NG). Ordinations produced with nonmetric multidimensional scaling (NMS) indicated a gradient with a benthic macroinvertebrate index of biotic integrity (IBI) and riparian site management; low IBI scores associated with CG sites and higher IBI scores associated with NG sites. Nongrazed sites were associated with reduced soil compaction and higher bank stability, as measured by the Pfankuch stability index; whereas CG sites were associated with increased soil compaction and lower bank stability, SDG sites were intermediate. Bedrock geology influenced NMS results: sites with carbonate derived cobble were associated with more stable channels and higher IBI scores. Though current riparian grazing practices in southeastern Minnesota present pollution problems, short duration grazing could reduce sediment pollution if managed in an environmentally sustainable fashion that considers stream channel response.