S. Mark Nelson

Recovery of a stream macroinvertebrate community from mine drainage disturbance

Recovery of aquatic macroinvertebrates from the effects of mine drainage was documented using a ‘weight-of-evidence’ approach which included measures of physical, chemical, and biological data. Taxa richness; number of taxa in the orders Ephemeroptera, Plecoptera, and Trichoptera; and shredder taxa richness all increased downstream of the point source after water treatment was initiated. Cluster analysis of aquatic macroinvertebrate community data along with abundance of a metals sensitive mayfly (Rhithrogena hageni) also suggested recovery from metals effects. Response to decreased metal inputs was rapid and biological measurements of impacted sites attained levels comparable to upstream reference sites in two years. Our results suggest that aquatic communities impacted by metals, in the absence of degraded habitat and with nearby colonist pools, will recover quickly if low instream concentrations of toxicants are achieved.

The influence of flow and other environmental factors on benthic invertebrates in the Sacramento River, U.S.A.

We examined how community composition of benthic invertebrates was related to current velocities and other environmental variables within the Sacramento River in California, USA. Invertebrates were collected in 1998 and 1999 from 10 sites over a gradient of 187 river kilometers. Canonical correspondence analysis revealed that current velocity was the most important variable explaining community composition. Other predicator variables that influenced community composition included periphyton biomass, altitude, and disturbance. Because of the importance of velocity in structuring benthic communities in this system, alterations of flow caused by changes in river regulation structures should be carefully considered.

INVERTEBRATE ASSEMBLAGES AND TRACE ELEMENT BIOACCUMULATION ASSOCIATED WITH CONSTRUCTED WETLANDS

Invertebrate assemblages were studied in eight monoculture wetland mesocosms constructed for wastewater treatment. Low concentrations of dissolved oxygen (D.O.) were measured in bulrush mesocosms while higher concentrations of D.O. were measured in open water mesocosms containing submerged pondweeds. Invertebrate taxa richness was positively related to D.O. concentrations that were, in turn, related to vegetation communities. Reference wetland sites contained a variety of plant species along with extensive open water areas. Invertebrate taxa richness was greater at reference sites than in any wastewater mesocosm. Invertebrate samples from the wastewater mesocosms and reference sites were analyzed for five trace elements. While the concentrations of aluminum, arsenic, mercury, and silver were below values harmful to wildlife, the concentrations of selenium reached levels of moderate concern on one occasion. Data from this study suggest that selenium bioaccumulation by invertebrates may be related to the type of vegetation community or detrital habitat type. Wetlands designed for invertebrate production for waterfowl should take into account the potential for low D.O. concentrations and trace element bioaccumulation associated with vegetation community types.

FLOOD FLOWS, LEAF BREAKDOWN, AND PLANT-AVAILABLE NITROGEN ON A DRYLAND RIVER FLOODPLAIN

We tested the hypothesis that decomposition in flood-inundated patches of riparian tree leaf litter results in higher plant-available nitrogen in underlying, nutrient-poor alluvium. We used leafpacks (n=56) containing cottonwood (Populus deltoides ssp. wislizenii) leaf litter to mimic natural accumulations of leaves in an experiment conducted on the Yampa River floodplain in semi-arid northwestern Colorado, USA. One-half of the leafpacks were set on the sandy alluvial surface, and one-half were buried 5 cm below the surface. The presence of NO3− and NH4+ presumed to result from a leafpack’s submergence during the predictable spring flood pulse was assessed using an ion-exchange resin bag (IER) placed beneath each leafpack and at control locations. Leafpacks and IERS were collected one week after flood peak (71 days total exposure) at half the stations; the remainder were collected three weeks later (93 days exposure). A multi-peaked spring flood with above-average maximum discharge inundated leafpacks for total time periods ranging from 133 to 577 hours. Litter lost from 43 to 68 percent of its initial organic matter (OM) content. Organic matter loss increased with total time inundated and total time of exposure on the floodplain. Burial retarded OM loss if the total time inundated was relatively long, and substrate texture (sand vs. silt) affected OM loss in a complex manner through interactions with total time inundated and total time of exposure. No pulse of N attributable to leaf breakdown was detected in the IERs, and leafpack litter showed no net change in the mass of nitrogen present. Patterns of leafpack and IER nitrogen levels suggested that litter removed N from floodwater and thereby reduced N availability in underlying sediment. Immobilization of floodwater-N by litter and N mineralization outside the flood period may be important components of N flux in semi-arid and arid floodplain environments.

Relationships between metals and hyporheic invertebrate community structure in a river recovering from metals contamination

Biota and chemistry from hyporheic samplers were used to describe environmental changes, after treatment of mine drainage, in the shallow hyporheic zone in the upper Arkansas River, Colorado, U.S.A. Samples were collected (1992–1997) from above and below the mine drainage source, and further downstream where a historically cleaner tributary enters the system. Hyporheic metal concentrations were high during spring runoff at the impacted site even after initiation of treatment, but declined after several years. Correspondence analysis and increased invertebrate taxa richness suggested that hyporheic macroinvertebrates were recovering at the previously impacted site. Differences in substrate between sites and high water flows in 1995 also appeared to affect hyporheic communities. High taxa richness and abundance, taxa unique from those found at the surface, and high concentrations of metals found in the shallow hyporheic zone suggests that sampling this zone is important for monitoring recovery of polluted streams.

EFFECTS OF RIVER FLOW REGIME ON COTTONWOOD LEAF LITTER DYNAMICS IN SEMI-ARID NORTHWESTERN COLORADO

Abstract We compared production and breakdown of Fremont cottonwood (Populus deltoides wislizenii) leaf litter at matched floodplain sites on the regulated Green River and unregulated Yampa River in semi-arid northwestern Colorado. Litter production under trees was similar at sites in 1999 (250 g/m2, oven-dry) but lower in 2000 (215 and 130 g/m2), a drought year that also featured an outbreak of defoliating beetles at the Yampa River site. Our production values were similar to the few others reported for riparian forests within semi-arid or arid areas. Leaf litter in portions of the floodplain not inundated during the spring flood lost organic matter at the same rate as leaves placed in upland sites in 1998 and 2000: 35 to 50% of organic matter during an approximately 160-day spring and summer period. Inundated litter lost 55 to 90% of its organic matter during the same period. Organic matter loss from inundated leaves increased with duration of inundation and with deposition of fine sediment. Pooled across locations, leafpack data suggested that nitrogen concentration (mg N/kg organic matter) increased until about 65% of the initial organic matter was lost. This increase likely reflected the buildup of microbial decomposer populations. The role of insects and other macroinvertebrates in litter breakdown apparently was minor at both sites. Large spatial and temporal variation in litter dynamics in aridland floodplain settings is ensured by microtopographic variation in the alluvial surface coupled with year-to-year variation associated with most natural flood regimes. Factors reducing flood flow frequency or magnitude will reduce overall breakdown rates on the floodplain towards those found in drier upland environments.

Effects of Cottonwood Leaf Beetle Chrysomela Scripta (Coleoptera: Chrysomelidae) on Survival and Growth of Fremont Cottonwood (Populus Fremontii) in Northwest Colorado

Abstract Greenhouse and field studies were used to assess the effects of a cottonwood leaf beetle (Chrysomela scripta) outbreak on growth and survivorship of Fremont cottonwood (Populus fremontii) along a segment of the Yampa River in northwestern Colorado. Exposure of greenhouse-reared seedlings to either 1 or 3 larvae reduced survivorship and the shoot and root masses of surviving seedlings. In the field sapling survivorship in unmanipulated, naturally established patches of saplings was inversely related to the intensity of beetle damage noted the previous year in both 1998–1999 and 1999–2000. Floodplain saplings treated with the insecticide esfenvalerate during two growing seasons added more height and radial growth than untreated saplings. Field observations in riparian stands along the Yampa River indicated that beetles preferentially oviposited and fed on cottonwood seedlings over saplings and mature trees during late summer. Taken together, the greenhouse and field studies suggest herbivory by C. scripta is a major determinant of Fremont cottonwood distribution and abundance at the Yampa River site. Repeated surveys detected few or no beetles in stands along the neighboring but regulated Green River. We hypothesize that flow regulation, by reducing P. fremontii seedling abundance, precludes C. scripta from reaching outbreak status along the Green River.

The Western Viceroy butterfly (Nymphalidae: Limenitis archippus obsoleta): an indicator for riparian restoration in the arid southwestern United States?

Abstract Life history characteristics of the Western Viceroy (Limenitis archippus obsoleta), an obligate riparian nymphalid butterfly in the desert southwestern United States, are described and related to Colorado River riparian restoration efforts. Riverine disturbance regimes and associated fluvial and hydrological dynamics may provide resources critical to this butterfly. Puddling by adult butterflies may require flood-cleared surfaces and an obligate riparian plant, Gooddings willow, was a larval host plant. This butterfly needs a variety of resources that are only found in close proximity in naturally functioning riparian ecosystems. Habitat heterogeneity required for colony persistence depends largely upon the natural dynamic character of flowing water systems. Because of the links between this butterfly and riparian structure and function it may be a useful indicator for monitoring riparian ecosystem restoration in the area.

Integrated Assessment of Metals Contamination in a Lotic System Using Water Chemistry, Transplanted Bryophytes, and Macroinvertebrates

ABSTRACT Complementary lines of evidence were used to understand distribution and effects of metal contaminants above and below Ridgway Reservoir on the Uncompahgre River in Colorado. Water chemistry identified sites that were contaminated by Zn, while analysis of bryophyte tissue identified sites which had statistically significant differences in metal uptakes, including metals which were not detected in water samples (i.e., Hg and Se). Macroinvertebrate metric values also detected environmental degradation at some sites and generally supported results of water and tissue analyses. In general, water quality improved longitudinally from upstream to downstream. The two stations above Ridgway Reservoir had relatively inferior water quality and higher metal concentrations compared to the station below the reservoir. The exception was in Se concentrations which were enriched in bryophyte tissue at the downstream station.

Selection of the Mayfly Rithrogena hageni as an Indicator of Metal Pollution in the Upper Arkansas River

ABSTRACT Macroinvertebrates from the upper Arkansas River were studied to identify taxa sensitive to heavy metal (zinc) impacts. Field collections and introduced substrates indicated that Rithrogena hageni was intolerant of conditions caused by mine drainage into the Arkansas River. Numbers of R. hageni at the impacted site increased significantly after treatment of mine drainage water was initiated. Data collected from this study suggest that the use of colonized substrates may be useful in studying metal impacts on macroinvertebrate communities.