Geology topics

Carl W. Isaacson

Publications and source records attributed to Carl W. Isaacson.

4 recordsLinked to original sources

USGS2025

Pyrethroid insecticide pollution of wetlands reduces amphipod density

Freshwater amphipods play a key role as forage for breeding and migrating waterfowl in wetlands throughout the Prairie Pothole Region (PPR) of North America. Amphipod populations declined in recent decades, but there is a limited understanding of mechanisms for their decline and their uneven distribution across the landscape. Row crop agriculture is abundant in the PPR, but the sensitivity of amphipods and wetland ecosystems to agrochemical pollution has rarely been studied. We investigated relationships among amphipod abundances (specifically, Gammarus lacustris and Hyalella azteca ), land uses, water quality, and pyrethroid insecticide contamination of wetland sediments. Our study design targeted a large gradient of amphipod abundances and accounted for water quality, hydrology, and habitat metrics that commonly influence amphipods. We found a significant, negative relationship between pyrethroid concentrations and the abundance of the two amphipod species. Pyrethroids were detected at relatively low concentrations (<2.5 ng/g sediment) in 44% of study wetlands and occurred most frequently in intensively cropped watersheds with low vegetative filter strip coverage. Interestingly, wetlands on state and federal wildlife reserves had regular occurrence of pyrethroids, demonstrating the pervasive transport of these compounds and the intensity of agriculture in the PPR. The pyrethroids are likely entering these wetlands through overland transport during rain events or aerial spray drift, and our results show that forest patches and vegetative filter strips may reduce pyrethroid exposure to both wetlands and amphipods.

Ecotoxicology↗

USGS2024

Low rate of population establishment of a freshwater invertebrate (Gammarus lacustris) in experimental conservation translocations

Conservation translocations may be a useful tool for the restoration of declining freshwater invertebrates, but they are poorly represented in the literature. We conducted a before-after/control-impact (BACI) experiment to test the efficacy of conservation translocation for re-establishing abundant populations of the amphipod Gammarus lacustris , a declining species and wildlife food resource in depressional wetlands in the upper Midwest of the United States of America. Each study site ( n = 19) contained at least one treatment wetland receiving translocated G. lacustris from a local donor and one control wetland. We selected study wetlands based on a suite of wetland characteristics and randomly assigned recipient versus control treatment. Gammarus lacustris was detected post-translocation at only 2 of 22 recipient wetlands (1 of 19 sites). Overall, there was a statistical increase in G. lacustris density in recipient wetlands compared to controls; however, the results were of minimal biological significance due to being driven by a single site with low G. lacustris densities. Accordingly, our results suggest that future conservation translocations of amphipods might be successful if limited to recently restored wetlands or informed by a more complex habitat suitability model to differentiate dispersal limitations from habitat limitations. To develop such a model would involve identifying the fewest, most influential physical and biological factors (e.g. wetland size/structure, fish, aquatic vegetation, and water chemistry) from the numerous inter-related factors that correlate with the abundance of naturally occurring G. lacustris ; candidate wetlands to receive amphipods would be those for which the model predicts abundant G. lacustris but in which they do not presently occur.

Minnesota↗

USGS2024

Fish invasion of prairie pothole wetlands reduces amphipod abundance, a key vertebrate forage

Fishes have spread into previously fishless wetlands, likely affecting other species. In the Prairie Pothole Region of North America, the invasion of fish into wetlands is facilitated by interactions of altered land use, climate, and hydrology. We aimed to understand the effects of fishes on amphipods, which are macroinvertebrates that vertebrates rely on as forage. We hypothesized the presence and abundance of fish, particularly benthivores, would have detrimental effects on amphipod abundance. Our study design targeted a large gradient of amphipod abundances among wetlands, including very high abundances of two amphipod species: Gammarus lacustris and Hyalella azteca . We found that fishless basins had twice as many amphipods as those with fish, on average. Gammarus lacustris were not detected in the presence of Black Bullhead Ameiurus melas . The abundance of both amphipod species had negative associations with the most common fishes, Fathead Minnow Pimephales promelas and Brook Stickleback Culaea inconstans . A multivariate community analysis showed the benthivore-fish functional feeding guild was negatively associated with the amphipod community, as hypothesized. However, our study design captured several wetlands with anomalies of high abundances of both fish and amphipods, obscuring their relationships. Our results aid resource managers by confirming several fish guilds and species are associated with lower abundances of amphipods. These findings can inform resource managers who make decisions about managing for fish and wildlife; for example, they may choose to manage existing fish populations or protect existing wetlands with high amphipod densities from new fish invasions.

Wetlands↗

USGS2022

Microbially induced anaerobic oxidation of magnetite to maghemite in a hydrocarbon-contaminated aquifer

Iron mineral transformations occurring in hydrocarbon-contaminated sites are linked to the biodegradation of the hydrocarbons. At a hydrocarbon-contaminated site near Bemidji, Minnesota, USA, measurements of magnetic susceptibility (MS) are useful for monitoring the natural attenuation of hydrocarbons related to iron cycling. However, a transient MS, previously observed at the site, remains poorly understood and the iron mineral phases acting as reactants and products associated with this MS perturbation remain largely unknown. To address these unknowns, we acquired mineral magnetism measurements, including hysteresis loops, backfield curves, and isothermal remanent magnetizations on sediment core samples retrieved from the site and magnetite-filled mineral packets installed within the aquifer. Our data show that the core samples and magnetite packs display decreasing magnetization with time and that this loss in magnetization is accompanied by increasing bulk coercivity consistent with decreased average grain size and/or partial oxidation. Low-temperature magnetometry on all samples displayed behavior consistent with magnetite, but samples within the plume also show evidence of maghemitization. This interpretation is supported by the occurrence of shrinkage cracks on the surface of the grains imaged via scanning electron microscopy. Magnetite transformation to maghemite typically occurs under oxic conditions, here, we propose that maghemitization occurs within the anoxic portions of the plume via microbially mediated anaerobic oxidation. Mineral dissolution also occurs within the plume. Microorganisms capable of such anaerobic oxidation have been identified within other areas at the Bemidji site, but additional microbiological studies are needed to link specific anaerobic iron oxidizers with this loss of magnetization.

Minnesota↗