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USGS2019

Resolving a paradox—high mercury deposition, but low bioaccumulation in northeastern Puerto Rico

At a “clean air” trade winds site in northeastern Puerto Rico, we found an apparent paradox: atmospheric total mercury (THg) deposition was highest of any site in the USA Mercury Deposition Network, but assimilation into the local food web was quite low. Avian blood THg concentrations (n = 31, from eight species in five foraging guilds) ranged widely from 0.2 to 32 ng g −1 (median of 4.3 ng g −1 ). Within this population, THg was significantly greater at a low-elevation site near a wetland compared to an upland montane site, even when the comparison was limited to a single species. Overall, however, THg concentrations were approximately an order of magnitude lower than comparable populations in the continental U.S. In surface soil and sediment, potential rates of demethylation were 3 to 9-fold greater than those for Hg(II)-methylation (based on six radiotracer amendment incubations), but rates of change of ambient MeHg pools showed a slight net positive Hg(II)-methylation. Thus, the resolution of the paradox is that MeHg degradation approximately keeps pace with MeHg production in this landscape. Further, any net production of MeHg is subject to frequent flushing by high rainfall on chronically wet soils. The interplay of these microbial processes and hydrology appears to shield the local food web from adverse effects of high atmospheric mercury loading. This scenario may play out in other humid tropical ecosystems as well, but it is difficult to evaluate because coordinated studies of Hg deposition, methylation, and trophic uptake have not been conducted at other tropical sites.

Puerto Rico↗

USGS2026

Per- and polyfluoroalkyl substances and pesticides in black bears (Ursus americanus) and grizzly bears (Ursus arctos horribilis) from Montana's Cabinet-Yaak ecosystem: A baseline assessment of emerging and legacy contaminants

Per- and polyfluoroalkyl substances (PFAS) and pesticides are globally distributed contaminants that persist in terrestrial food webs, yet baseline data for large omnivores in the continental U.S. remain limited. This study quantified PFAS and pesticides in whole blood from free-ranging black bears ( Ursus americanus , n = 15) and grizzly bears ( Ursus arctos horribilis , n = 9) in the Cabinet-Yaak Ecosystem of northwestern Montana. Using high-resolution mass spectrometry-based methods (GC-MS/MS and LC-MS/MS), 52 PFAS and 186 pesticides were analyzed. PFAS were detected in all bears, with total PFAS concentrations ranging from 79.5 to 317 pg/mL. PFAS profiles were dominated by long-chain perfluoroalkyl carboxylic acids, particularly PFOA, PFDA, and PFUnDA, with minimal contribution from short-chain compounds. Species was the strongest ecological predictor of PFAS concentrations, although variability across age, sex, and capture location were observed. Only two pesticides, piperonyl butoxide and permethrin, were detected at quantifiable concentrations (2070–12,600 pg/mL), and no correlations were observed between pesticide and PFAS concentrations, indicating independent exposure pathways. The predominance of long-chain PFAS suggests diffuse environmental sources and bioaccumulation within terrestrial food webs. Although measured concentrations were generally low relative to other wildlife toxicological studies, the persistence and protein-binding properties of long-chain PFAS warrant continued monitoring, particularly for long-lived omnivores. These results establish baseline contaminant concentrations for two apex omnivores in a remote ecosystem and highlight the utility of large mammals as integrative sentinels of environmental contaminant exposure for regional monitoring and wildlife conservation. Continued monitoring in the Cabinet-Yaak and across western North America will be critical for assessing temporal trends, identifying new contaminant sources, and evaluating ecological health in bear populations.

Montana↗

USGS2024

Change in growth and prey utilization for a native salmonid following invasion by an omnivorous minnow in an oligotrophic reservoir

Aquatic invasive species can affect food web structure, native fish growth, and production, depending on the traits of the invasive species and the pre-invasion conditions of the ecosystem. Thermal tolerances and behavioral traits can further influence differential exploitation of resources shared between native and invasive species. An unauthorized introduction of redside shiner ( Richardsonius balteatus ) into reservoirs in the Upper Skagit River, Washington, USA caused concern of potential competition, decreased production, and recruitment of rainbow trout ( Oncorhynchus mykiss ). We combined bioenergetics modeling and stable isotope analysis with field data to quantify consumption demand of native and invasive fishes and related consumption to the availability of key zooplankton prey. Per capita consumption on Daphnia by redside shiner was low; however, their high abundance imposed considerable demand on prey resources in Ross Lake. Although monthly consumption demand by the fish community was less than 50% of the monthly production and biomass of Daphnia in Ross Lake, the current Daphnia densities and growth of rainbow trout were considerably lower than before the invasion. These reductions correspond to lower annual consumption of Daphnia . Our study provides insight on mechanisms that influence food web impacts of an invasive omnivore in cold-water reservoirs.

Washington↗

USGS2022

In-reservoir physical processes modulate aqueous and biological methylmercury export from a seasonally anoxic reservoir

Anoxic conditions within reservoirs related to thermal stratification and oxygen depletion lead to methylmercury (MeHg) production, a key process governing the uptake of mercury in aquatic food webs. Once formed within a reservoir, the timing and magnitude of the biological uptake of MeHg and the relative importance of MeHg export in water versus biological compartments remain poorly understood. We examined the relations between the reservoir stratification state, anoxia, and the concentrations and export loads of MeHg in aqueous and biological compartments at the outflow locations of two reservoirs of the Hells Canyon Complex (Snake River, Idaho-Oregon). Results show that (1) MeHg concentrations in filter-passing water, zooplankton, suspended particles, and detritus increased in response to reservoir destratification; (2) zooplankton MeHg strongly correlated with MeHg in filter-passing water during destratification; (3) reservoir anoxia appeared to be a key control on MeHg export; and (4) biological MeHg, primarily in zooplankton, accounted for only 5% of total MeHg export from the reservoirs (the remainder being aqueous compartments). These results improve our understanding of the role of biological incorporation of MeHg and the subsequent downstream release from seasonally stratified reservoirs and demonstrate that in-reservoir physical processes strongly influence MeHg incorporation at the base of the aquatic food web.

Idaho, Oregon↗

USGS2023

Hydrodynamics structure plankton communities and interactions in a freshwater tidal estuary

Drivers of phytoplankton and zooplankton dynamics vary spatially and temporally in estuaries due to variation in hydrodynamic exchange and residence time, complicating efforts to understand controls on food web productivity. We conducted approximately monthly (2012–2019; n = 74) longitudinal sampling at 10 fixed stations along a freshwater tidal terminal channel in the San Francisco Estuary, California, characterized by seaward to landward gradients in water residence time, turbidity, nutrient concentrations, and plankton community composition. We used multivariate autoregressive state space (MARSS) models to quantify environmental (abiotic) and biotic controls on phytoplankton and mesozooplankton biomass. The importance of specific abiotic drivers (e.g., water temperature, turbidity, nutrients) and trophic interactions differed significantly among hydrodynamic exchange zones with different mean residence times. Abiotic drivers explained more variation in phytoplankton and zooplankton dynamics than a model including only trophic interactions, but individual phytoplankton–zooplankton interactions explained more variation than individual abiotic drivers. Interactions between zooplankton and phytoplankton were strongest in landward reaches with the longest residence times and the highest zooplankton biomass. Interactions between cryptophytes and both copepods and cladocerans were stronger than interactions between bacillariophytes (diatoms) and zooplankton taxa, despite contributing less biovolume in all but the most landward reaches. Our results demonstrate that trophic interactions and their relative strengths vary in a hydrodynamic context, contributing to food web heterogeneity within estuaries at spatial scales smaller than the freshwater to marine transition.

California↗

USGS2020

Spatial variability of phytoplankton in a shallow tidal freshwater system reveals complex controls on abundance and community structure

Estuaries worldwide are undergoing changes to patterns of aquatic productivity because of human activities that alter flow, impact sediment delivery and thus the light field, and contribute nutrients and contaminants like pesticides and metals. These changes can influence phytoplankton communities, which in turn can alter estuarine food webs. We used multiple approaches-including high-resolution water quality mapping, synoptic sampling, productivity and nitrogen uptake rates, Lagrangian parcel tracking, enclosure experiments and bottle incubations-over a short time period to take a “spatial snapshot” of conditions in the northern region of the San Francisco Estuary (California, USA) to examine how environmental drivers like light availability, nutrients, water residence time, and contaminants affect phytoplankton abundance and community attributes like size distribution, taxonomic structure, and nutrient uptake rates. Zones characterized by longer residence time (15–60 days) had higher chlorophyll-a concentrations (9 ± 4 µg L −1 ) and were comprised primarily of small phytoplankton cells (<5 µm, 74 ± 8%), lower ammonium concentrations (1 ± 0.8 µM), higher nitrate uptake rates, and higher rates of potential carbon productivity. Conversely, zones characterized by shorter residence time (1–14 days) had higher ammonium concentration (13 ± 5 µM) and lower chlorophyll-a concentration (5 ± 1 µg L −1 ) with diatoms making up a larger percent contribution. Longer residence time, however, did not result in the accumulation of large (>5 µm) cells considered important to pelagic food webs. Rather, longer residence time zones had a phytoplankton community comprised primarily of small cells, particularly picocyanobacteria that made up 38 ± 17% of the chlorophyll-a – nearly double the concentration seen in shorter residence time zones (22 ± 7% picocyanobacterial of chlorophyll-a). Our results suggest that water residence time in estuaries may have an effect as large or larger than that experimentally demonstrated for light, contaminants, or nutrients.

California↗

USGS2024

Prey supply and predation as potential limitations to feasibility of anadromous salmonid introductions in a reservoir

Introducing anadromous fish upstream of migration barriers has frequently been proposed as a conservation strategy, but existing conditions and future changes to the ecosystems above barriers such as invasive species, climate change, and varying water operations influence the capacity to support such introductions. In the Upper Skagit River, Washington, USA, introduction of anadromous salmonids above three high-head dams was proposed; however, the proliferation of invasive redside shiner Richardsonius balteatus fundamentally altered reservoir food web interactions, presenting potential challenges for the growth and production of introduced anadromous salmonids. By combining empirical measurements of zooplankton availability and temporal patterns in thermal structure of the reservoir with bioenergetics model simulations to quantify the rearing capacity of Ross Lake, we estimated the lake could support millions of sockeye salmon fry entering in spring after accounting for temporal consumption demand by the existing planktivore community dominated by redside shiner. The initial fry estimates varied according to the expected fry-to-smolt survival rate, and whether salmonids would be thermally restricted from prey in the epilimnion. This translated to estimates of 189,000 to 285,000 smolts leaving the following spring, and 7700 to 11,700 returning adults, using mean fry-to-smolt and smolt-to-adult survival rates from a nearby sockeye salmon population. We also estimated that predation potential could pose substantial mortality for lake-rearing sockeye or Chinook salmon, although it is expected to play a lesser role in limiting survival of species that only migrate through the reservoir. These results provide a case study and framework for examining bottom-up and top-down food web processes that influence growth and survival of introduced anadromous salmonids in reservoir habitats, thus guiding the direction of future feasibility studies in Ross Lake and other regulated rivers where introduction programs are considered.

Washington↗

USGS2023

Competition between dissolved organic matter and freshwater plankton control methylmercury isotope fractionation during uptake and photochemical demethylation

Isotope fractionation related to photochemical reactions and planktonic uptake at the base of the food web is a major uncertainty in the biological application of mercury (Hg) stable isotopes. In freshwater systems, it is unclear how competitive interactions among methylmercury (MeHg), dissolved organic matter (DOM), and phytoplankton govern the magnitude of mass-dependent and mass-independent fractionation. This study investigated how DOM alters rates of planktonic MeHg uptake and photodegradation and corresponding Hg isotope fractionation in the presence of freshwater phytoplankton species, Raphidocelis subcapitata . Outdoor sunlight exposure experiments utilizing R. subcapitata were performed in the presence of different DOM samples using environmentally relevant ratios of MeHg-DOM thiol groups. The extent of Δ 199 Hg in phytoplankton incubations (2.99‰ St. Louis River HPOA, 1.88‰ Lake Erie HPOA) was lower compared to paired abiotic control experiments (4.29 and 2.86‰, respectively) after ∼30 h of irradiation, resulting from cell shading or other limiting factors reducing the extent of photodemethylation. Although the Δ 199 Hg/Δ 201 Hg ratio was uniform across experiments (∼1.4), Δ 199 Hg/δ 202 Hg slopes varied dramatically (from −0.96 to 15.4) across incubations with R. subcapitata and DOM. In addition, no evidence of Hg isotope fractionation was observed within R. subcapitata cells. This study provides a refined examination of Hg isotope fractionation markers for key processes occurring in the lower food web prior to bioaccumulation, critical for accurately accounting for the photochemical processing of Hg isotopes across a wide spectrum of freshwater systems.

ACS Earth and Space Chemistry↗

USGS2013

Gastric evacuation rate, index of fullness, and daily ration of Lake Michigan slimy sculpin (Cottus cognatus) and deepwater sculpin (Myoxocephalus thompsonii)

Accurate estimates of fish consumption are required to understand trophic interactions and facilitate ecosystem-based fishery management. Despite their importance within the food-web, no method currently exists to estimate daily consumption for Great Lakes slimy (Cottus cognatus) and deepwater sculpin (Myoxocephalus thompsonii). We conducted experiments to estimate gastric evacuation (GEVAC) and collected field data from Lake Michigan to estimate index of fullness [(g prey/g fish weight)100%) to determine daily ration for water temperatures ranging 2–5 °C, coinciding with the winter and early spring season. Exponential GEVAC rates equaled 0.0115/h for slimy sculpin and 0.0147/h for deepwater sculpin, and did not vary between 2.7 °C and 5.1 °C for either species or between prey types (Mysis relicta and fish eggs) for slimy sculpin. Index of fullness varied with fish size, and averaged 1.93% and 1.85% for slimy and deepwater sculpins, respectively. Maximum index of fullness was generally higher (except for the smallest sizes) for both species in 2009–2010 than in 1976 despite reductions in a primary prey, Diporeia spp. Predictive daily ration equations were derived as a function of fish dry weight. Estimates of daily consumption ranged from 0.2 to 0.8% of their body weight, which was within the low range of estimates from other species at comparably low water temperatures. These results provide a tool to estimate the consumptive demand of sculpins which will improve our understanding of benthic offshore food webs and aid in management and restoration of these native species in the Great Lakes.

Journal of Great Lakes Research↗

USGS2011

Comparing isotope signatures of prey fish: does gut removal affect δ13C or δ15N?

Stable isotope analysis is a quick and inexpensive method to monitor the effects of food web changes on aquatic communities. Traditionally, whole specimens have been used when determining isotope composition of prey fish or age-0 recreational fishes. However, gut contents of prey fish could potentially alter isotope composition of the specimen, especially when recent foraging has taken place or when the gut contains non-assimilated material that would normally pass through fishes undigested. To assess the impacts of gut content on prey fish isotope signatures, we examined the differences in isotopic variation of five prey fish species using whole fish, whole fish with the gut contents removed, and dorsal muscle only. We found significant differences in both δ 15 N and δ 13 C between the three tissue treatments. In most cases, muscle tissue was enriched compared to whole specimens or gut-removed specimens. Moreover, differences in mean δ 15 N within a species were up to 2&permil; among treatments. This would result in a change of over half a trophic position (TP) based on a 3.4&permil; increase per trophic level. However, there were no apparent relationships between tissue isotope values in fish with increased gut fullness (more prey tissue present). We suggest that muscle tissue should be used as the standard tissue for determining isotope composition of prey fish or age-0 recreational fishes, especially when determining enrichment for mixing models, calculating TP, or constructing aquatic food webs.

Journal of Freshwater Ecology↗

USGS2021

Reduced quality and synchronous collapse of forage species disrupts trophic transfer during a prolonged marine heatwave

The Gulf of Alaska forage fish community includes a few key species that differ markedly in their timing of spawning, somatic growth and lipid storage, and in their migration behavior. This diversity in life history strategies facilitates resilience in marine food webs because it buffers predators against the naturally high variance in abundance of pelagic forage fish populations by decreasing the likelihood that all species will be scarce at the same time. During the prolonged North Pacific marine heatwave of 2014-2016, the availability and quality of at least three key forage species with different life history strategies were reduced simultaneously in the system. Capelin and sand lance occurrence in predator diets declined abruptly, and Prince William Sound herring spawning biomass declined to historically low levels. Biomass of euphausiids was also reduced, in part due to the loss of a cold-water species. Changes in age structure, growth, and energy content of capelin, sand lance, and herring were also associated with warming during the heatwave, but not all species responded in the same way. For example, spawning capelin grew faster and matured at a younger age but were shorter in length than usual, while sand lance in Prince William Sound experienced anomalously low growth rates and lipid storage in 2015-2016. Changes in forage fish populations had immediate impacts on predator populations in 2015-2016, when seabirds and marine mammals experienced shifts in distribution, mass mortality, and reproductive failures in the Gulf of Alaska. In contrast, copepod abundance increased on the shelf and in some coastal regions during the heatwave, suggesting that food availability was not a primary factor limiting forage fish populations at this time. The reduced quality and collapse of multiple forage fish populations reduced the efficiency of energy transfer through the middle trophic level of pelagic food webs, disrupting energy flow to piscivorous pelagic predators and causing abrupt and extreme reductions in their numbers and productivity.

Alaska↗

USGS2023

Selenium hazards in the Salton Sea environment—Summary of current knowledge to inform future wetland management

Quaternary marine and continental shales in the western United States are sources of selenium that can be loaded into the aquatic environment through mining, agricultural, and energy production processes. The mobilization of selenium from shales through agricultural irrigation has been recognized since the 1930s; however, discovery of deformities in birds and other wildlife using agricultural habitats during the 1980s spurred studies to determine the extent and effects of the contamination. Through these early studies, researchers determined that biota in the Salton Sea drainage basin was at risk from legacy selenium contamination in the Colorado River watershed. The Salton Sea and its surrounding managed and unmanaged wetlands provide vital inland habitat and trophic support for diverse assemblages of resident and migratory wildlife, and understanding regional selenium hazards for these trust species is a priority for many Federal and State agencies. The modern Salton Sea is a shallow, landlocked saline lake in Riverside and Imperial Counties (not shown) of California that is sustained by irrigation return and perennial river inflow. Changes in water transfer agreements under the 2003 Quantification Settlement Agreement (QSA) have resulted in reduced irrigation flow, declining lake levels, and the evolution of unmanaged wetlands in areas where drains and rivers no longer reach the Salton Sea. These wetlands provide additional habitat for some species of concern, but their potential to increase selenium hazards for trust species is largely unknown. From the 1980s to 2020, efforts to document selenium contamination and effects throughout the region have resulted in a considerable amount of selenium data from the Salton Sea and its surrounding drainage basin; however, no long-term (greater than 20 years), consistent sampling program has been established, and all data have been collected by different entities using a variety of protocols and analytical techniques. This lack of coordination has been previously documented in regional management plans and has led to difficulty in reliably assessing selenium hazards in the Salton Sea environment. This report provides a summary of the available disparate selenium information collected from water, sediment, and biota in the Salton Sea region since the 1980s and to identify data gaps that need to be filled to understand the potential effects of selenium on species of concern, including federally endangered desert pupfish ( Cyprinodon macularius ) and Yuma Ridgway’s Rail ( Rallus obsoletus yumanensis; formerly Yuma Clapper Rail, Rallus longirostris yumanensis ). Available data from the Salton Sea drainage basin show that water from the Colorado River has the lowest selenium concentration of all surface water sources. All other surface water flowing into the Salton Sea has elevated selenium concentrations due to evaporation and evapotranspiration that occurs in agricultural fields and associated water delivery infrastructure or leaching of selenium from irrigated farmland soils. The Salton Sea has lower selenium concentrations because of various biogeochemical processes that recycle selenium into the sediment or volatilize it to the atmosphere; however, these mechanisms are not well defined, and it is not clear if selenium cycling will change in response to possible changes in the oxidation state of the Salton Sea bottom waters as water levels decline. Agricultural drains have the highest average selenium concentrations, but few drains have been sampled since changes in irrigation practices have occurred (due to the 2003 QSA). Groundwater selenium concentrations are variable; some wells south of the Salton Sea have selenium concentrations as high as 300 micrograms per liter (µg/L), whereas selenium concentrations are below detection in other wells. Groundwater and surface-water geothermal discharge zones around the margins of the Salton Sea and in unmanaged wetlands have not been studied in detail, and published selenium measurements are not available for these surface features. Selenium concentrations in the sediment of the Salton Sea drainage basin are highest in wetland particulate organic matter and the Salton Sea lakebed, indicating that removal of selenium from the water to the sediment has been a primary mechanism for keeping selenium concentrations low in the water column. Sediment selenium concentrations in wetlands are lower than in the Salton Sea but higher than inflowing drains and rivers, indicating the lentic wetland sites also may be important sinks for selenium because of biogeochemical processes. Sediment selenium data have not been collected in agricultural drains since changes in irrigation practices occurred (due to the 2003 QSA), and it is unknown if selenium sequestration from the water column has changed in these systems. We divided biological data into broad taxonomic categories, including primary producers, invertebrates, herpetofauna, mammals, fishes, and birds to facilitate evaluation of selenium concentrations and spatiotemporal trends observed in the Salton Sea. Overall, selenium concentrations were substantially greater in algae samples compared to all vascular plant samples combined. Median selenium concentrations in several invertebrate taxa (Chironomidae, Formicidae, Corixidae, Corbiculidae and Nereididae, and Decapoda) exceeded the maximum suggested dietary threshold of 3.0–4.0 micrograms per gram (µg/g) dry weight (dw) for predators consuming invertebrates in aquatic food webs. The greatest number of samples were collected from fish, and selenium distributions among species and locations showed that the range for most samples was lower than the U.S. Environmental Protection Agency selenium criterion for aquatic life (8.5 µg/g dw whole body, 11.3 µg/g dw fillets). The median selenium concentrations for whole body fish were below the selenium criterion in most locations, except for bairdiella ( Bairdiella icistia ) from the Salton Sea and irrigation drains, a few individual tilapia spp. (family Cichlidae, including genera Tilapia, Oreochromis , and their hybrids) from the river and river outlets, and several western mosquitofish ( Gambusia affinis ) and sailfin molly ( Poecilia latipinna ) from irrigation drain outlets. For avian samples combined among years and locations, median selenium concentrations in livers from all families except waders and Ibis (family Threskiornithidae) were higher than levels expected to cause selenium toxicosis (10–20 µg/g dw), and all median egg concentrations were above or near 6.0 μg/g dw, which is a conservative threshold value for reproductive impairment. Most knowledge gaps we identified for water, sediment, and biota were interrelated, and the use of integrated approaches to address knowledge gaps can provide greater insight into the drivers behind selenium hazards. Integrated water, sediment, and biota studies could help identify cost-effective management solutions that serve multiple purposes. A comprehensive analysis of the hydrology, biogeochemistry, and food-web processes in wetlands and other habitats can inform predictive models to identify drivers of selenium bioavailability, uptake from the environment and subsequent trophic transfer, ultimately forming the basis for experimental habitat management manipulations to minimize selenium hazards to wildlife. Furthermore, a comprehensive, long-term sampling and analytical laboratory plan would enable comparison of data among different entities that are sampling at the Salton Sea. Such efforts are well suited to help fill knowledge gaps that preclude understanding of selenium hazards and future management options for biota using Salton Sea habitats, including newly formed wetlands throughout the region. All data compiled for this report are available in two U.S. Geological Survey data releases: Groover and others (2022) for water and sediment samples and De La Cruz and others (2022) for biological samples. The data releases include all publicly available data for selenium concentrations in water, sediment, and biological samples collected in and around the Salton Sea, including the Coachella and Imperial Valleys. The data releases also include previously unpublished data.

California↗

USGS2023

Biogeochemical and hydrologic synergy control mercury fate in an arid land river-reservoir system

Reservoirs in arid landscapes provide critical water storage and hydroelectric power but influence the transport and biogeochemical cycling of mercury (Hg). Improved management of reservoirs to mitigate the supply and uptake of bioavailable methylmercury (MeHg) in aquatic food webs will benefit from a mechanistic understanding of inorganic divalent Hg (Hg( II )) and MeHg fate within and downstream of reservoirs. Here, we quantified Hg( II ), MeHg, and other pertinent biogeochemical constituents in water (filtered and associated with particles) at high temporal resolution from 2016–2020. This was done (1) at inflow and outflow locations of three successive hydroelectric reservoirs (Snake River, Idaho, Oregon) and (2) vertically and longitudinally within the first reservoir (Brownlee Reservoir). Under spring high flow, upstream inputs of particulate Hg (Hg( II ) and MeHg) and filter-passing Hg( II ) to Brownlee Reservoir were governed by total suspended solids and dissolved organic matter, respectively. Under redox stratified conditions in summer, net MeHg formation in the meta- and hypolimnion of Brownlee reservoir yielded elevated filter-passing and particulate MeHg concentrations, the latter exceeding 500 ng g −1 on particles. Simultaneously, the organic matter content of particulates increased longitudinally in the reservoir (from 9–29%) and temporally with stratified duration. In late summer and fall, destratification mobilized MeHg from the upgradient metalimnion and the downgradient hypolimnion of Brownlee Reservoir, respectively, resulting in downstream export of elevated filter-passing MeHg and organic-rich particles enriched in MeHg (up to 43% MeHg). We document coupled biogeochemical and hydrologic processes that yield in-reservoir MeHg accumulation and MeHg export in water and particles, which impacts MeHg uptake in aquatic food webs within and downstream of reservoirs.

Environmental Science: Processes & Impacts↗

USGS2004

Selenium loading through the Blackfoot River watershed--linking sources to ecosystem

The upper Blackfoot River watershed in southeast Idaho receives drainage from 11 of 16 phosphate mines that have extracted ore from the Phosphoria Formation, three of which are presently active. Toxic effects from selenium (Se), including death of livestock and deformity in aquatic birds, were documented locally in areas where phosphatic shales are exposed ( Piper et al., 2000 ; Presser et al., Chapter 11). Current drainage conditions are leading to Se bioaccumulation at concentrations that pose a risk to fish in the Blackfoot River and its tributaries (Hamilton et al., Chapter 18). A gaging station on the Blackfoot River was re-activated in April 2001 to assess hydrologic conditions and concentration, load, and speciation for Se discharges on a watershed scale. The gaging-station data are considered to represent regional drainage conditions in the upper Blackfoot River water- shed because of its location near the outlet of the watershed and directly upstream of the Blackfoot Reservoir. Watershed discharges for 2001 and 2002 were below minimum hydrologic conditions for the gage as documented by the historical record. Drought emergencies were declared in the area in both 2001 and 2002. Unmonitored diversions for irrigation that routinely take place during the snowmelt season also affected conditions downstream. Annual cycles in Se concentration, load, and selenate (Se 6+ ) reached maxima in the spring during the period of maximum flow at the gaging station. Thirty-seven to 44% of annual flow occurred dur- ing the three-month high-flow season (April through June) in 2001 and 56% of annual flow occurred during that time period in 2002. Extrapolation from historical hydrographs for average and wet years and a limited data set of regional Se concentrations for 2001 and 2002 indicated potential for a 3.6- to 7.4-fold increase in Se loading because of increased seasonal flows in the Blackfoot River watershed. Supplementation data indicate that: (a) the difference between total Se and dissolved Se, as a measure of the contribution of particulate Se, was < 10% except at the peak of con- centration when total Se was 18% more than dissolved Se; (b) selenite (Se 4+ ) represented less than 10% of the dissolved species during all months of 2001; and (c) dissolved Se was approximately a 50:50 mixture of selenate and organic selenide (operationally defined Se 2- ) during summer 2001 (June through August). Ecological risk based on regional Se drainage occurred during both the high- and low-flow seasons. Seventy to 83% of the Se load occurred during the high-flow season. During early May of both years, dissolved-Se concentrations exceeded the criterion for the protection of aquatic life and the ecological threshold of 5 gL 1 Se at which sub- stantive risk occurs. During the majority of the three-month high-flow season, dissolved- Se concentrations exceeded the 2 gL 1 Se concern level for aquatic biota. The Se concentration in suspended material during high flow in 2002 was within the range of marginal risk to aquatic life (2-4 gg 1 Se, dry weight). Selenate was the major species during peak flows, with both selenate and organic selenide being major species during relatively low-flow periods in summer. A change in speciation to reduced Se may indicate elevated biotic productivity during summer months and could result in enhanced Se uptake in food webs. In addition to the magnitude of regional Se release in the Blackfoot River watershed, Se concentrations in individual source drains and waste-rock seeps, and those predicted by experimental column leaching of proposed mining overburden materials, also indicate that drainage options that currently meet existing demands for phosphate mining cause eco- logical risk thresholds to be exceeded. At times, the drinking-water Se standard (50 g L 1 Se) and the criterion for hazardous Se waste (1000 L -1 Se) ( US Department of the Interior, 1998 ; US Environmental Protection Agency, 1987 ) are also exceeded. For water-years 2001 and 2002, seasonal increased input of water in the mining area resulted in increased Se transport, suggesting a mechanism of contamination that involves a significant Se reservoir. Hence, recognition and monitoring of Se loading to the envi- ronment on a mass balance basis (i.e. inputs, fluxes and storage within environmental media, and outputs) are essential to evaluating how to control Se concentrations within environmentally protective ranges ( Presser and Piper, 1998 ). In areas where release of Se to aquatic systems is anticipated as a product of future expansion of phosphate mining, continuous monitoring of flow and development of seasonal Se loading patterns would help to model watersheds in terms of sources, flow periods, and environmental-Se con- centrations that most influence bioavailability. These data, in turn, could be linked to Se- bioaccumulation models specific to food webs and vulnerable species of the impacted areas to accurately project ecological effects. Gaging at this site on the Blackfoot River is planned to continue in order to establish a long-term (>10 year) record of hydrologic conditions.

Idaho↗

USGS2025

Effects of flow on pesticides in water and zooplankton in the northern Sacramento-San Joaquin Delta

Zooplankton are a key food source for juvenile fishes in estuaries worldwide, including California’s Sacramento–San Joaquin Delta (hereafter Delta); both zooplankton quality and quantity are critical to ecosystem health. Zooplankton may be affected by pesticides in water and the food web, and the Delta is known to contain complex pesticide mixtures. In this study, we evaluated pesticide concentrations in water and zooplankton in the northern Delta during (1) the summer–fall of 2017, 2018, and 2019, which included periods of augmented pulse flows from agriculture tailwater, and (2) across a full seasonal cycle from May 2019 to March 2020. We quantified changes in pesticide concentration in response to environmental factors. We found that zooplankton showed more frequent detections of hydrophobic pesticides compared to more frequent detections of hydrophilic compounds in water. Pesticide concentrations were influenced by flow, pesticide application, and season, but the effects of these environmental factors differed by habitat (Sacramento River or Yolo Bypass Toe Drain). Pesticides in water responded similarly to environmental factors in the Sacramento River and Yolo Bypass, whereas pesticides in zooplankton responded differently. In water, we found more detections and higher concentrations at higher flows in the Yolo Bypass and Sacramento River, but responses to pesticide application varied by habitat. Alternatively, pesticide concentrations in zooplankton increased in the Yolo Bypass with increasing flow (correlated with flow pulses) and changed seasonally; whereas, pesticide concentrations in zooplankton in the Sacramento River decreased at higher flows, and decreased with or did not respond to higher pesticide application in the watershed. Our study suggests that augmented flows—particularly those using agricultural tailwater—may have unintended negative ecological effects that could partially offset benefits to the food web and fishes in the northern Delta, underscoring the complex interplay among factors that drive increased pesticide exposure.

California↗

USGS2026

Assessing the status and drivers of phytoplankton blooms along the riverine to freshwater-tidal continuum in a modified delta

Shifts in phytoplankton communities, from beneficial taxa that fuel food webs to harmful taxa that degrade water quality, are often linked to anthropogenic modifications to estuaries. We analyzed monitoring data from 2000 to 2023 in the heavily modified southern, landward portion of the Sacramento-San Joaquin Delta (California, USA) to determine the status and drivers of phytoplankton abundance and community composition along the riverine to freshwater tidal transition. Diatoms and green algae dominated the phytoplankton community, especially when chlorophyll a (chl- a ) concentrations exceeded 16 µg L − 1 . Common riverine diatoms (e.g., Cyclotella and Aulacoseira ) dominated upstream riverine stations, while taxa associated with tidally influenced conditions (e.g., colonial cyanobacteria, cryptophytes) were more prevalent in western, freshwater tidal stations. The potentially toxigenic cyanobacterium, Microcystis , was present each summer, but was not associated with elevated chl- a or cyanotoxins (microcystins were detected in 12% of samples with a maximum concentration of 3.9 µg L − 1 ). A Bayesian ordinal regression showed that mean prior week water speeds below 0.18 m s − 1 and water temperatures above 22 ˚C were associated with the presence of Microcystis. Random forest modeling to assess factors associated with phytoplankton blooms (chl- a > 16 µg L − 1 ) found that mean prior week turbidity, inflows from the San Joaquin River, region, and mean prior week water temperature were important drivers of blooms. These modeling results emphasize the importance of hydrodynamics in controlling phytoplankton dynamics in lowland rivers and tidal deltas, offering insights for how anthropogenic impacts on flow regimes may affect the base of estuarine food webs.

California↗

USGS2012

Oceanographic gradients and seabird prey community dynamics in glacial fjords

Glacial fjord habitats are undergoing rapid change as a result of contemporary global warming, yet little is known about how glaciers influence marine ecosystems. These ecosystems provide important feeding, breeding and rearing grounds for a wide variety of marine organisms, including seabirds of management concern. To characterize ocean conditions and marine food webs near tidewater glaciers, we conducted monthly surveys of oceanographic variables, plankton, fish and seabirds in Kenai Fjords, Alaska, from June to August of 2007 and 2008. We also measured tidal current velocities near glacial features. We found high sediment load from glacial river runoff played a major role in structuring the fjord marine ecosystem. Submerged moraines (sills) isolated cool, fresh, stratified and silt-laden inner fjord habitats from oceanic influence. Near tidewater glaciers, surface layers of turbid glacial runoff limited availability of light to phytoplankton, but macrozooplankton were abundant in surface waters, perhaps due to the absence of a photic cue for diel migration. Fish and zooplankton community structure varied along an increasing temperature gradient throughout the summer. Acoustic measurements indicated that low density patches of fish and zooplankton were available in the surface waters near glacial river outflows. This is the foraging habitat occupied most by Kittlitz's murrelet (Brachyramphus brevirostris), a rare seabird that appears to be specialized for life in glacially influenced environments. Kittlitz's murrelets were associated with floating glacial ice, and they were more likely to occur near glaciers, in deeper water, and in areas with high acoustic backscatter. Kittlitz's murrelet at-sea distribution was limited to areas influenced by turbid glacial outflows, and where prey was concentrated near the surface in waters with low light penetration. Tidewater glaciers impart unique hydrographic characteristics that influence marine plankton and fish communities, and this has cascading effects on marine food webs in these ecosystems.

Fisheries Oceanography↗

USGS2020

Introduction: Ecological subsidies as a framework for understanding contaminant fate, exposure, and effects at the land-water interface

Ecologists have long recognized that ecological subsidies (the flow of organic matter, nutrients, and organisms between ecosystems) can strongly affect ecosystem processes and community structure in the recipient ecosystem. Animal movements, organic matter flows, and food web dynamics between linked aquatic and terrestrial systems can also influence contaminant fate, exposure, and effects at the land-water interface. Here and in this book, we develop a broad framework that highlights two important ways that ecological subsidies and contaminants interact. Ecological subsidies from the donor system can drive exposure to recipient systems, and contaminant exposures in the donor system can control subsidies and contaminant fluxes to the recipient systems. In the case of prey movement between ecosystems, subsidies drive exposure when contaminants present in aquatic environments bioaccumulate in the tissues of prey organisms at levels that are relatively non-toxic to the prey themselves. Conversely, exposure in the aquatic system can limit subsidies when pollutants are relatively toxic to prey organisms themselves and the magnitude of the subsidy (i.e., biomass of aquatic insects emerging to the terrestrial environment) is reduced. These effects of contaminants on subsidies are shaped by other global stressors that are ubiquitous in aquatic-riparian ecosystems (e.g., climate and land use change, species extinction and invasion, and eutrophication). As our understanding of these ecological and toxicological processes advances, there are increasing opportunities to make landscape-scale predictions of contaminant and animal fluxes and to integrate this knowledge of aquatic-riparian linkages into managing contaminant risks. Through these efforts to integrate the fields of ecology and ecotoxicology on this subject, we expect to gain greater insight on the ecological effects of contaminants on linked ecosystems as well as the ways in which food web dynamics and ecosystem processes can themselves govern the fate, transport, and exposure to contaminants in the environment.

Book chapter↗