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At least 451 records · Page 25Linked to original sources
USGS2010

Karst map of Puerto Rico

This map is a digital compilation, combining the mapping of earlier geologists. Their work, cited on the map, contains more detailed descriptions of karst areas and landforms in Puerto Rico. This map is the basis for the Puerto Rico part of a new national karst map currently being compiled by the U.S. Geological Survey. In addition, this product is a standalone, citable source of digital karst data for Puerto Rico. Nearly 25 percent of the United States is underlain by karst terrain, and a large part of that area is undergoing urban and industrial development. Accurate delineations of karstic rocks are needed at scales suitable for national, State, and local maps. The data on this map contribute to a better understanding of subsidence hazards, groundwater contamination potential, and cave resources as well as serve as a guide to topical research on karst. Because the karst data were digitized from maps having a different scale and projection from those on the base map used for this publication, some karst features may not coincide perfectly with physiographic features portrayed on the base map.

Puerto Rico↗
USGS1979

Marsh nesting by mallards

Nest-site selection by mallard (Anas platyrhynchos) hens was studied on a 52-km2, privately owned area in the Missouri Coteau of south-central North Dakota during 1974-77. Sixty-six percent of 53 nests initiated by radio-marked and unmarked hens were in wetlands in dense stands of emergent vegetation and usually within 50 m of the wetland edge. These findings and other sources of information suggest that significant numbers of mallards breeding in the Prairie Pothole Region nest in marsh habitat. Potential factors contributing to mallard use of marsh habitat for nesting purposes are discussed. Management considerations associated with marsh nesting by mallards are described and research needs are identified.

Wildlife Society Bulletin↗
USGS2021

Landscape characterization of floral resources for pollinators in the Prairie Pothole Region of the United States

Across agricultural areas of the Prairie Pothole Region (PPR), floral resources are primarily found on public grasslands, roadsides, and private grasslands used as pasture or enrolled in federal conservation programs. Little research has characterized the availability of flowers across the region or identified the primary stakeholders managing lands supporting pollinators. We explored spatial and temporal variability in flower abundance and richness across multiple grassland categories (i.e. general grassland, conservation grassland, and engineered pollinator habitat) in the PPR from 2015 to 2018 and used these data to estimate the number of flowering stems present across the region on private and public land holdings. Both flowering plant abundance and richness were greatest on engineered pollinator habitat, but this land category encompassed <0.01% of the total grassland area in the PPR. There was a steady decrease in flower abundance over the growing season across all land categories. We detected considerable variation in flower abundance and richness across grassland categories, indicating that not all natural or semi-natural covers provide similar value to pollinators. At a landscape scale, large land holdings such as privately-owned grasslands and Conservation Reserve Program lands contributed the greatest number of flowers by an order of magnitude, though these lands collectively did not support the greatest abundance of flowers per unit area. Our research depicts spatial and temporal variation in pollinator resources across the region. Further, our research will assist managers and policy makers in understanding the role of public and private lands and conservation programs in supporting pollinators.

Minnesota, North Dakota, South Dakota↗
USGS2026

Perceptions of parcel-level wildfire risk differ between homeowners and trained assessors in wildland-urban interface communities across the western United States

Wildfire risk mitigation on private property is central to reducing community wildfire vulnerability. Homeowners have control over many of the key factors that contribute to wildfire risk on their parcels, yet vulnerable conditions persist. One potential explanation is a misalignment between homeowners' and trained assessors' perceptions of parcel-level wildfire risk. Prior research has documented a “risk gap” in a single community wherein owners often underestimate their parcel-level wildfire risk; however, it is unclear whether such misalignments are widespread. This study replicates and expands previous research by examining the parcel-level wildfire risk gap in 38 wildland–urban interface (WUI) communities across the Western United States using paired data from household surveys and parcel-level wildfire risk assessments by trained assessors. We find that homeowners and assessors often perceive parcel characteristics differently. Homeowners may systematically mis-weight their importance, tending to underweight key controllable factors such as building materials and defensible space, leading to meaningful divergence from trained assessor ratings. Accordingly, we also find that homeowners generally underestimate overall parcel wildfire risk compared to trained assessors. These results suggest that even when homeowners recognize attributes on their parcels that contribute to wildfire risk, they may not fully grasp how much the attributes contribute to overall parcel risk, potentially failing to recognize the need to undertake meaningful mitigation actions. The findings from this study contribute to the broader natural hazards literature on expert-layperson risk perception gaps and offer insights for improving wildfire communication, education, and mitigation strategies in WUI communities.

Alaska, Colorado, New Mexico, Utah, Wyoming↗
USGS2018

Assessing the risk of dreissenid mussel invasion in Texas based on lake physical characteristics and potential for downstream dispersal

ebra mussels ( Dreissena polymorpha ) and quagga mussels ( Dreissena bugensis ) were likely introduced from Ponto-Caspian Eurasia to the Laurentian Great Lakes inadvertently via ballast water release in the 1980s and have since spread across the US, including Texas. Their spread into the state, including reservoirs in both Brazos River and Colorado River basins, has resulted in a need to delimit suitable dreissenid habitat and dispersal potential in Texas. The objective of our research was to assess invasion risk in Texas by 1) predicting distribution of suitable habitat of zebra and quagga mussels using Maxent models; 2) refining lake-specific predictions for present zebra mussels via collection of physicochemical data; and 3) assessing the potential for downstream spread of zebra mussels by applying environmental DNA (eDNA) methods in the Leon and Lampasas Rivers downstream from the invaded Lakes Belton and Stillhouse Hollow, respectively. Maxent models did not predict the occurrence of suitable habitat for quagga mussels within Texas. However, our models accurately identified global zebra mussel habitat (AUC = 0.919), and Bioclim layers representing temperature and precipitation data both strongly influenced predictions. Predicted “hotspots” of suitable zebra mussel habitat in Texas occurred along the Red and Sabine Rivers of north and east Texas, as well as patches of suitable habitat in central Texas between the Colorado and Brazos Rivers and extending inland along the Gulf Coast. Most of the Texas panhandle, west Texas extending toward El Paso, and the Rio Grande valley were predicted to provide poor habitat suitability. Collection of physicochemical data (dissolved oxygen, pH, specific conductance, and temperature on-site as well as laboratory analysis for Ca, N, and P) from zebra mussel invaded lakes and a subset of identified high-risk lakes of North and Central Texas, did not aid predictions. Visual inspection of biplots of the first three components of a principle component analysis, which together accounted for ~80% of data variability, did not reveal separation between invaded and uninvaded lakes, and logistic regression analysis also failed to identify predictive relationships between measured variables and invasion status. Using eDNA analysis, we detected the presence of zebra mussel eDNA at 11 of 12 sites and up to at least 90.7 river km downstream from a pair of infested reservoirs. Rate of positive detection among water samples at each site ranged from 1/5 to 5/5, and within positive water samples, rate of detection among technical replicates ranged from 1/8 to 8/8, suggesting considerable heterogeneity in the zebra mussel eDNA signal in both rivers. Furthermore, no clear spatial pattern in detection rate occurred. Thus, a monitoring strategy that combines traditional sampling (e.g. settlement substrate samplers and microscopy) at sites immediately below a dam, and transitioning to more sensitive eDNA analysis at distances further from the dam may represent the most successful strategy for detection of dreissenid mussel downstream dispersal. Overall, we have demonstrated that while quagga mussels do not appear to represent an invasive threat in Texas, suitable habitat for continuing zebra mussel invasion exists within Texas, and stream and river connections may contribute to their spread. The threat of continued expansion of this poster-child for negative invasive species impacts warrants further prevention efforts, management, and research.

continental United States↗
USGS2023

Doing the same thing over and over again and getting the same result: Assessing variance in wetland invertebrate assemblages

Past efforts to explain variation of invertebrate assemblages in freshwater wetlands have been less productive than anticipated. To explore why efforts are disappointing, we assembled large invertebrate data sets from North Dakota prairie potholes, California rock pools, and Georgia Carolina bay wetlands that addressed spatial (among wetlands) and temporal (among seasons and years) variation. We anticipated that these large data-set sizes would enable robust conclusions to be drawn, and each place had unique environmental conditions that might contribute to greater explanatory power. We used statistical techniques that partitioned variation in invertebrate assemblages into spatial and/or temporal components, and that also yielded a measure of the amount of unexplained variation; Permutational Multivariate Analysis of Variation and Principal Coordinates Analysis assessed whole assemblage variation, and Analysis of Variance or Analysis of Covariance assessed variation in taxon richness, total abundances, and abundances of wide-spread individual taxa. Across all locations, variation explained by spatial and temporal factors, and unexplained variation were of comparable magnitudes (i.e., similar R 2 values of ~ 50%). Review of other published studies indicate that this pattern is widespread. The 50% or more unexplained variation is typically ignored by researchers, who instead focus on explained fractions. We argue that, besides addressing explained spatial and temporal variation in invertebrate assemblages (e.g., control by hydrology, resources, predation), efforts to understand what contributes to currently unexplained variation, that is unrelated to local spatial or temporal controls (e.g., broad climatic and biogeographic patterns, organism physiology and behavior), will lead to a fuller comprehension of how invertebrates in freshwater wetlands are controlled.

Wetlands↗
USGS2025

The Hawaiian Volcanoes Supersite: Open data for the benefit of science and society

The Hawaiian Volcanoes Supersite was established in 2008 with the goal of making large amounts of volcano monitoring data, especially satellite measurements, freely available at a site of international interest, scientific importance, and impactful natural hazards. The location was chosen because of the long history of volcanological research and innovation on the Island of Hawaiʻi, as well as the need for monitoring, assessing, and mitigating volcanic hazards for the local population. Ground-based data are provided by the U.S. Geological Survey Hawaiian Volcano Observatory, and several national space agencies have contributed thousands of satellite synthetic aperture radar and other data that would have otherwise required special grants or commercial purchase. Since the Hawaiian Volcanoes Supersite was initiated, the vast quantity of open space-based data has resulted in the development of new applications and methodologies, successful responses to volcanic crises, and research that has informed monitoring and hazards mitigation activities. While there remain opportunities for additional coordination among supersite users and for synergistic studies that make use of the full spectrum of available ground- and space-based data, the Hawaiian Volcanoes Supersite has achieved its goals of stimulating basic research to better understand Hawaiian volcanism and aiding in responses to hazardous geologic processes. The effort serves as a model for the benefits of open, low-latency, and comprehensive satellite data applied to disaster risk management and reduction, meeting a vision that has been laid out repeatedly in international agreements and accords.

Hawaii↗
USGS2022

Depth drives growth dynamics of dreissenid mussels in Lake Ontario

Understanding dreissenid mussel population dynamics and their impacts on lake ecosystems requires quantifying individual growth across a range of habitats. Most dreissenid mussel growth rates have been estimated in nutrient rich or nearshore environments, but mussels have continued to expand into deep, cold, low-nutrient habitats of the Great Lakes. We measured annual quagga mussel ( Dreissena rostriformis bugensis) growth at 15 m, 45 m, and 90 m in Lake Ontario using caged mussels near Oswego, New York, USA from June 2018 to May 2019. Quagga mussel growth (starting size 12 mm) was greatest at 15 m (mean shell length increase = 10.2 mm), and was lower at 45 m (5.9 mm) and 90 m (0.7 mm). Caged mussels were obtained from near the 90-m site and those reared at 15 and 45 m developed thicker shells than those that were caged at 90 m. We observed relatively high colonization by quagga and, to a lesser degree, zebra mussels ( Dreissena polymorpha ) at 15 m, very few colonizers at 45 m, and none at 90 m. Higher growth potential, but low natural mussel densities observed at 15 m and 45 m suggest factors other than growth limit dreissenid abundance at these depths. The relatively slow dreissenid growth rates observed in offshore habitats are consistent with the gradual abundance increases documented in these zones across the Great Lakes and suggest new mussels that become established in these habitats may contribute to ecosystem effects for decades.

New York↗
USGS2025

Using hydrogeologic context and water budgets to evaluate the potential for groundwater contributions to contaminants in Lake Superior

This study presents a synthesis of the hydrogeology in the U.S. Lake Superior watershed and the contribution of groundwater to the water budget of the U.S. Lake Superior basin. Much of the shoreline of Lake Superior in Minnesota and Michigan is composed of hydrogeologic units contributing very little direct groundwater discharge to the lake. Groundwater in watersheds adjacent to the lake typically flows in short, local flow systems characterized by thin glacial sediments with active groundwater flow in fractured bedrock within the top 60–90 m below land surface. The complex groundwater system in Wisconsin’s Bayfield Peninsula has the largest groundwater reservoir near the lake, characterized by thick sand and gravel glacial deposits and underlying sandstone aquifer. Although these thick sandy glacial deposits are not in direct contact with the lake at the shoreline, groundwater discharge may still be significant via subsurface exposures beyond the shoreline or flow through the underlying sandstone aquifer. Overall, most groundwater in the watershed is contributed as indirect base flow in streams around the lakeshore and comprises about 60 percent of the upland water budget. Direct groundwater flow to the shoreline contributes 2 to 9 percent of the inflow. Identifying possible contamination sources through direct sampling of groundwater would be an inefficient way to detect problems if sources are unknown, particularly for some chemicals of concern such as PFAS, pesticides, PCBs, chloride, and nutrients. Evaluating the chemical characteristics of contamination is also important to consider in evaluating how groundwater may contribute to pollution in Lake Superior.

Lake Superior↗
USGS2023

Climate-driven mid- to late Holocene hydrologic evolution of arid wetlands documented by strontium, uranium, and oxygen isotopes from Lower Pahranagat Lake, southern Nevada, USA

Lacustrine carbonates in a 12.4-m-long core from Lower Pahranagat Lake (LPAH), southern Nevada, indicate that radiogenic isotopes of Sr and U ( 87 Sr/ 86 Sr and 234 U/ 238 U) preserve evidence of past variations in water sources and evolving hydrologic conditions. Sr and U isotope compositions in LPAH carbonates fall within the range defined by the three primary groundwater sources in Pahranagat Valley and reflect variable mixtures of those sources since the mid-Holocene. Compositions in the oldest sample (5.78 ka) closely match modern compositions of modern discharge from nearby springs, indicating that LPAH water was derived almost exclusively from the local volcanic aquifer. By ca. 5.3–5.2 ka, LPAH water compositions shifted sharply towards isotopic compositions observed in groundwater from the regional carbonate aquifer, indicating a marked increase in surface flow from high-volume springs discharging from the carbonate aquifer to the north. Sediments deposited between 3.08–1.06 ka indicate reduced contributions from the regional aquifer. A comparison of uranium- and oxygen-isotope values in LPAH carbonates suggests that wetter climate conditions favor increased supply from deeper, regional carbonate aquifers compared to drier conditions when contributions from shallower, local volcanic aquifers were more important.

Nevada↗
USGS2005

Population trends of Mariana Crow Corvus kubaryi on Rota, Commonwealth of the Northern Mariana Islands

Endemic to the islands of Guam and Rota in the Mariana Islands, Mariana Crow Corvus kubaryi is the only corvid in Micronesia. Currently, it survives on Guam only because of translocation of individuals from Rota (1999-2003). Island-wide surveys in 1982 and 1995 on Rota yielded population estimates of 1,348 and 592 respectively, indicating a 56% decrease in only 13 years. A sharp decline in the only viable Mariana Crow population has serious implications for conservation efforts on Rota and for efforts to re-establish the Guam population. However, the validity of the apparent decline has been debated among scientists and government management agencies. We augmented the 1982 and 1995 island-wide VCP surveys with (1) an additional island-wide survey conducted in 1998, and (2) roadside surveys conducted during 1991-1993 and again during 1999-2002. We also outline historical changes in Rota's limestone forest based on aerial photographs and historical information. Data from all surveys indicate a significant decline in the Mariana Crow population. Declines occurred especially along the north-central coast and in the area east of the airport known as As Dudo in the 1990s, but the data indicate an island-wide decline over the entire span of the surveys. introduced predators, human persecution, and habitat loss and degradation by anthropogenic and natural causes have all contributed to the decline. Long-term preservation of this species will require effective brown treesnake Boiga irregularis control, habitat protection, continued monitoring and research, and increased public education and awareness of Rota's rare and endangered species. ?? BirdLife International 2005.

Bird Conservation International↗
USGS2012

Seismicity in the twenty years preceding the Loma Prieta California Earthquake

Persistent seismicity occurred at a low rate during at least the twenty years before the Loma Prieta earthquake along the 60 km-long rupture zone. The depth distribution of this seismicity forms a broad “U”-shape that delineates the previously locked rupture zone. Relocations of seismicity during the ten years before the earthquake relative to the Loma Prieta aftershocks show that this “U”-shaped distribution can be partitioned lengthwise into activity on two adjacent subparallel structures: a vertical fault beneath the San Andreas fault trace and an eastward-dipping blind fault beneath the Sargent fault trace. The 11–18 km deep, southwest-dipping part of the Loma Prieta rupture was not active during at least the preceding ten years. The slip geometry of intersecting fault structures in this zone could contribute to both the preparation process and the complexity of the rupture.

California↗
USGS2025

Estimates of global surface water dynamics harnessing near real-time land cover observations and open science geospatial capabilities

Spatio-temporal changes to our world’s surface water resources are escalating. Translating how these changes impact communities and ecosystems requires time-varying data of Global Surface Water Extents (GSWE). Traditionally, GSWE mapping has been limited to static estimates, with recent efforts focusing on annual averages, frequency and occurrence of long-term variations. Building upon these foundational capabilities, we harnessed remotely sensed Sentinel-2 based near real-time Dynamic World (DW) land cover products to produce the first-of-its-kind 10 m resolution GSWE dataset representing 2015–2023. Our dataset estimated 2.5 million km 2 of permanent waters and 8 million km 2 of seasonal waters worldwide. Comparing our Sentinel-2 based data to contemporary Landsat-based GSWE, we observed that our data mapped less water within the >50% probability of occurrence range, suggesting a lower presence of open permanent water especially in high latitudes, deviating from what we previously learnt from Landsat data. Statistical analysis compared to well-established observational products and widely used GSWE datasets across some of the world’s most ecologically significant regions, including Pantanal in South America and Haor in South Asia, supports the overall physical realism of our data in predicting global open surface water dynamics. Our key contribution is a prototype Open Science operational framework that extracts routinely available DW products, runs geospatial analytics, and creates actionable water information for educators, researchers, and stakeholders at any scale of practical interest. We present examples of this operational capability through instant mapping of flood in Spain and drought in Lake Urmia, Central Asia, frequent monitoring of river extent changes at the Ganges–Brahmaputra confluence, and above all, interoperability with other existing GSWE applications.

Environmental Research Letters↗
USGS2025

Three decades of World Recreational Fishing Conferences: What have we learned about the dynamics of recreational fisheries?

Recreational fishing extends beyond catching fish, connecting individuals with nature, generating environmental stewards and contributing significantly to both local, regional and national economies. Assessing potential impacts on ecosystems and social-ecological systems requires dedicated multi- and interdisciplinary research and integrative management and policy approaches. Managing recreational fisheries effectively also necessitates transdisciplinary thinking, yet this poses continued challenges, as researchers and managers balance specialized expertise with innovative, boundary-crossing perspectives in light of limited funding in a rapidly changing social-ecological environment. The World Recreational Fishing Conference (WRFC) serves as the leading global scientific forum for addressing these challenges, fostering interdisciplinary exchange among scientists, managers, policy makers and stakeholders. The papers in this special issue represent the output of the last WRFC in Melbourne, Australia in 2023. By reviewing the content of the papers published in the present special issue, we illustrate the critical role of collaborative spaces, such as the WRFC, in bridging different expertise and approaches and fostering innovation, thereby securing adaptive management and conservation of recreational fisheries in response to global changes. Evaluating the development of the WRFC over three decades highlights how this conference series represents a leading think tank serving the entire network of recreational fisheries professionals globally.

Fisheries Management and Ecology↗
USGS1992

Geologic studies in Alaska by the U.S. Geological Survey, 1990

This collection of papers continues the annual series of U.S. Geological Survey (USGS) reports on geologic investigations in Alaska. From 1975 through 1988, the series was published as USGS circulars. The first of these appeared under the title "The United States Geological Survey in Alaska: Accomplishments during 1975," and the series continued to the last annual circular entitled "Geologic studies in Alaska by the U.S. Geological Survey during 1987," which reflects a title change made in 1986. This 1990 volume continues the bulletin format started in 1988. As in 1989, this volume separates shorter contributions as Geologic Notes from more extensive Articles. This 1990 volume of 18 Articles and 4 Geologic Notes represents the broad range of USGS research activities carried out in Alaska over the past few years. These studies include topics on mineral and other resources such as gold (Y eend), platinum-group elements (Cathrall and Antweiler), coal (Roberts, Stricker, and Affolter), and petroleum (Howell, Bird, and others). Many other investigations provide background geochemical (Kilburn, Box, and others) and geologic data needed for resource evaluation as well as for determining the general geologic framework of Alaska, as in stratigraphic, sedimentologic, and paleontologic and radiometric age studies (Blodgett, Clough, and others; Box and Elder; Dickinson and Skipp; Marincovich and Moriya; McLean and Stanley; Stanley, Flores, and Wiley; Roeske, Pavlis, and others); geophysics (Sampson, Labson, and Long); structure and tectonic evaluations (Bradley and Kosky; Clendenen, Sliter, and Byrne; Karl; Csejtey; Howell, Johnsson, and others); and geomorphic and late Quaternary studies (Carter and Hillhouse; Galloway, Huebner, and others; McGimsey, Richter, and others; Nelson and Carter). These studies span nearly the entire State from the North Slope and Brooks Range to interior, southwestern, and south-central Alaska (fig. 1). Two bibliographies (White) at the end of the volume list (1) reports about Alaska in USGS publications released in 1990 and (2) reports about Alaska by USGS authors in publications outside the USGS in 1990. A bibliography and index of papers in past USGS circulars that are devoted to geologic research and accomplishments in Alaska (1975 to 1986) is published as USGS Open-File Report 87-420.

Alaska↗
USGS2007

Climate Variability and Change

In 2007, the U.S. Geological Survey (USGS) developed a science strategy outlining the major natural science issues facing the Nation in the next decade. The science strategy consists of six science directions of critical importance, focusing on areas where natural science can make a substantial contribution to the well-being of the Nation and the world. This fact sheet focuses on climate variability and change and how USGS research can strengthen the Nation with information needed to meet the challenges of the 21st century.

Fact Sheet↗
USGS2002

Human-health pharmaceutical compounds in Lake Mead, Nevada and Arizona, and Las Vegas Wash, Nevada, October 2000-August 2001

The U.S. Geological Survey and the National Park Service conducted a reconnaissance study to investigate the occurrence of selected human-health pharmaceutical compounds in water samples collected from Lake Mead on the Colorado River and Las Vegas Wash, a waterway used to transport treated wastewater from the Las Vegas metropolitan area to Lake Mead. Current research indicates many of these compounds can bioaccumulate and may adversely affect aquatic organisms by disrupting physiological processes, impairing reproductive functions, increasing cancer rates, contributing to the development of antibiotic-resistant strains of bacteria, and acting in undesirable ways when mixed with other substances. These compounds may be present in effluent because a high percentage of prescription and non-prescription drugs used for human-health purposes are excreted from the body as a mixture of parent compounds and degraded metabolite compounds; also, they can be released to the environment when unused products are discarded by way of toilets, sinks, and trash in landfills. Thirteen of 33 targeted compounds were detected in at least one water sample collected between October 2000 and August 2001. All concentrations were less than or equal to 0.20 micrograms per liter. The most frequently detected compounds in samples from Las Vegas Wash were caffeine, carbamazepine (used to treat epilepsy), cotinine (a metabolite of nicotine), and dehydronifedipine (a metabolite of the antianginal Procardia). Less frequently detected compounds in samples collected from Las Vegas Wash were antibiotics (clarithromycin, erythromycin, sulfamethoxazole, and trimethoprim), acetaminophen (an analgesic and anti-inflammatory), cimetidine (used to treat ulcers), codeine (a narcotic and analgesic), diltiazem (an antihypertensive), and 1,7-dimethylxanthine (a metabolite of caffeine). Fewer compounds were detected in samples collected from Lake Mead than from Las Vegas Wash. Caffeine was detected in all samples collected from Lake Mead. Other compounds detected in samples collected from Lake Mead were acetaminophen, carbamazepine, cotinine, 1,7-dimethylxanthine, and sulfamethoxazole.

Nevada, Arizona↗
USGS1997

Plan of study to determine if the isotopic ratios [delta]15 N and [delta]18 O can reveal the sources of nitrate discharged by the Mississippi River into the Gulf of Mexico

Nitrate and other nutrients discharged from the Mississippi River basin are suspected of causing a zone of depleted dissolved oxygen (hypoxic zone) in the Gulf of Mexico each summer. The hypoxic zone may have an adverse effect on aquatic life and commercial fisheries. Commercial fertilizers are the dominant source of nitrogen input to the Mississippi basin. Other nitrogen sources include animal waste, fixation of atmospheric nitrogen by legumes, precipitation, domestic and industrial effluent, and the soil. The inputs of nitrogen from most of these sources to the Mississippi basin can be estimated and the outputs in surface water can be measured. However, nitrogen from each source is affected differently by physical, chemical, and biological processes that control nitrogen cycling in terrestrial and aquatic systems. Hence, the relative contributions from the various sources of nitrogen to nitrate load in the Mississippi River are unknown because the different sources may not contribute proportionally to their inputs in the basin. It may be possible to determine the relative contributions of the major sources of nitrate in river water using the stable isotopic ratios d15N and d18O of the nitrate ion. A few researchers have used the d15N and/or d18O isotope ratios to determine sources of nitrate in ground water, headwater catchments, and small rivers, but little is known about the isotopic composition of nitrate in larger rivers. The objective of this study is to measure the isotopic composition of nitrate and suspended organic matter in the Mississippi River and its major tributaries, in discharge to the Gulf of Mexico, and in streamflow from smaller watersheds that have distinct sources of nitrogen (row crops, animal wastes, and urban effluents) or are minimally impacted by man (undeveloped). Samples from seven sites on the Mississippi River and its tributaries and from 17 sites in smaller watersheds within the Mississippi River basin will be analyzed for d15N and d18O of dissolved nitrate. Suspended sediment collected from these sites will also be analyzed to determine the d15N, d13C, and d34S of the suspended organic material. Six samples will be collected at each site during the winter, spring, and summer of 1996-97. Results from these samples will be used to identify seasonal and flow-related variability in d15N and d18O ratios from each site and may also help determine the principal sources of the nitrate entering the Gulf of Mexico.

Open-File Report↗