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At least 1,153 records · Page 64Linked to original sources
USGS2021

Trophic transfer efficiency in the Lake Superior food web: Assessing the impacts of non-native species

Ecosystem-based management relies on understanding how perturbations influence ecosystem structure and function (e.g., invasive species, exploitation, abiotic changes). However, data on unimpacted systems are scarce; therefore, we often rely on impacted systems to make inferences about ‘natural states.’ Among the Laurentian Great Lakes, Lake Superior provides a unique case study to address non-native species impacts because the food web is dominated by native species. Additionally, Lake Superior is both vertically (benthic versus pelagic) and horizontally (nearshore versus offshore) structured by depth, providing an opportunity to compare the function of these sub-food webs. We developed an updated Lake Superior EcoPath model using data from the 2005/2006 lake-wide multi-agency surveys covering multiple trophic levels. We then compared trophic transfer efficiency (TTE) to previously published EcoPath models. Finally, we compared ecosystem function of the 2005/2006 ecosystem to that with non-native linkages removed and compared native versus non-native species-specific approximations of TTE and trophic flow. Lake Superior was relatively efficient (TTE = 0.14) compared to systems reported in a global review (average TTE = 0.09), and the microbial loop was highly efficient (TTE > 0.20). Non-native species represented a very small proportion (<0.01%) of total biomass and were generally more efficient and had higher trophic flow compared to native species. Our results provide valuable insight into the importance of the microbial loop and represent a baseline estimate of non-native species impacts on Lake Superior. Finally, this work is a starting point for further model development to predict future changes in the Lake Superior ecosystem.

Lake Superior↗
USGS2021

Distribution, abundance and spatial variability of microplastic pollution on the surface of Lake Superior

In 2014, 94 paired neuston net samples (0.5 mm mesh) were collected from the surface waters of Lake Superior. These samples comprise the most comprehensive surface water survey for microplastics of any of the Great Lakes to date, and the first to employ double net trawls. Microplastic abundance estimates showed wide variability, ranging between 4000 to more than 100,000 particles/km 2 with most locations having abundances between 20,000 to 50,000 particles/km 2 . The average abundance in Lake Superior was ~30,000 particles/km 2 which was similar to previous estimates within this Laurentian Great Lake and suggests a total count of more than 2.4 billion (1.7 to 3.3 billion, 95% confidence interval) particles across the lake’s surface. Distributions of plastic particles, characterized by size fraction and type, differed between nearshore and offshore samples, and between samples collected in the eastern versus western portion of the lake. Most of the particles found were fibers (67%), and most (62%) were contained in the smallest classified size fraction (0.50–1 mm). The most common type of polymer found was polyethylene (51%), followed by polypropylene (19%). This is consistent with global plastics production and results obtained from other studies. No statistically significant difference was detected between the paired net samples, indicating that single net sampling should produce a representative estimate of microplastic particle abundance and distribution within a body of water.

Lake Superior↗
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↗
USGS2022

Bioaccumulation of perfluoroalkyl substances in a Lake Ontario food web

Per- and polyfluoroalkyl substances (PFAS) are a group of synthetic chemicals detected throughout the environment. To better understand the distribution of PFAS in an aquatic system (the Laurentian Great Lakes), stable isotope enrichment (δ13C and δ15N), fatty acid (FA) profiles, and PFAS were measured in various species from the Lake Ontario (LO) aquatic food web. Sampled organisms included top predator fish, prey fish, and benthic and pelagic macroinvertebrates. The trophic level of each species in the LO food web was determined using δ15N, and FA concentrations (range: <1–139 mg/g wet weight (ww)). The individual PFAS concentrations in the LO food web were ~1.5 to 5 times lower than previously reported. The highest PFAS concentrations were observed in deepwater sculpin (Myoxocephalus thompsonii, 150 ± 35.7 ng/g ww) suggesting a potential source of PFAS from the offshore benthic zone or sediment. The concentration of PFOS and long chain (C9-C14) perfluoroalkyl carboxylic acids (PFCAs) were significantly higher than short chain PFAS indicating the significant impact of hydrophobicity on the bioaccumulation of PFAS in organisms from the food web. However, high molecular weight PFCAs (>C8) did not exhibit increasing biomagnification factors (BMFs) and trophic magnification factors (TMFs) with log Kow, suggesting hydrophobicity does not govern the movement of PFAS from low to high trophic levels in the LO food web.

Lake Ontario↗
USGS2022

Importance of nonindigenous harpacticoids (Crustacea: Copepoda) decrease with depth in Lake Ontario

Harpacticoid copepods can be a substantial component of the meiobenthic community in lakes and serve an ecological role as detritivores. Here we present the first species-level lake-wide quantitative assessment of the harpacticoid assemblage of Lake Ontario with emphasis on the status of nonindigenous species. Additionally, we provide COI-5P sequences of harpacticoid taxa through Barcode of Life Data System (BOLD). Harpacticoids were collected at depths from 0.1 to 184 m and from a range of substrates from August to September 2018 as part of the Cooperative Science and Monitoring Initiative (CSMI) offshore benthic survey. Twenty-six meiobenthic samples were analyzed using microscopy for community composition analysis of harpacticoids. We found thirteen indigenous and three nonindigenous species of harpacticoid, with the introduced species dominating at shallow depths. The community transitioned from nonindigenous to indigenous species dominance as depth increased. Nonindigenous species accounted for 79% of the community (by abundance) at depths <20 m, 55% from 20 to 40 m, and only 24% at depths >40 m. The nonindigenous species encountered included the first detections of Schizopera borutzkyi (Monchenko, 1967) and Heteropsyllus nunni (Coull, 1975) from Lake Ontario. S. borutzkyi was the most abundant harpacticoid species in the lake, approaching a maximum density of 50,000/m 2 and a lake-wide average density of 7,900/m 2 . Numerically important indigenous species included Bryocamptus nivalis (Willey, 1925), Canthocamptus robertcokeri (Wilson, 1958), Canthocamptus staphylinoides (Pearse, 1905), and Moraria cristata (Chappuis, 1929). The prevalence of nonindigenous harpacticoids in the meiobenthos of Lake Ontario suggests further investigations of Great Lakes meiofauna communities are warranted.

Lake Ontario↗
USGS2022

Microbial source tracking and evaluation of best management practices for restoring degraded beaches of Lake Michigan

Attempts to mitigate shoreline microbial contamination require a thorough understanding of pollutant sources, which often requires multiple years of data collection (e.g., point/nonpoint) and the interacting factors that influence water quality. Because restoration efforts can alter shoreline or beach morphology, revisiting source inputs is often necessary. Microbial source tracking (MST) using source-specific molecular markers, genomic community analyses, and physical modeling was used to identify contamination sources along three Lake Michigan beaches of the Laurentian Great Lakes with historically high fecal indicator bacteria (FIB, E. coli ) concentrations. Genetic markers for human (Bacteroides HF183) and mixed gull species ( Catellicoccus marimammalium ) fecal sources were tested from water and sediment. Gene sequencing (16S rRNA) was used to identify similarities in bacterial communities in nearshore water, river inputs, sand, sediment, and groundwater. Synoptic surveys of water exchange were conducted to determine nearshore-offshore interactions of FIB. In addition to these MST studies, best management practices to mitigate FIB, including gull deterrence, slope grading, wetland establishment, and shoreline plantings, were reviewed for their effectiveness at reducing FIB concentrations over time. Using multiple tools for MST helped identify primary and secondary sources of FIB (gulls, stormwater inputs) and the physical processes that exacerbate FIB concentrations (onshore currents, limited circulation). Management actions were successful in the short-term at reducing FIB, but scope of success was temporally limited, with FIB concentrations often rebounding. Results highlight the usefulness of MST to inform best management practices and the need for a sustained adaptive approach that adjusts for changes in the coastal system.

Illinois, Wisconsin↗
USGS2022

Evaluation of post-stocking dispersal and mortality of juvenile lake trout Salvelinus namaycush in Lake Ontario using acoustic telemetry

Wild reproduction by stocked lake trout Salvelinus namaycush in Lake Ontario has yet to produce a self-sustaining population, requiring a reliance on stocking. Once released, age-1 juvenile lake trout are not typically surveyed until age-2, creating a gap in knowledge of fine-scale post-release behaviors. A method to track fine-scale movements and estimate mortality of juvenile lake trout could complement standard survey methods and benefit management decisions regarding stocking locations. We used acoustic telemetry to estimate post-stocking mortality and observe fine-scale spatial and temporal movements of 38 hatchery-reared, age-1 lake trout from an offshore stocking site in the eastern basin of Lake Ontario from 2017 to 2018. Cumulative post-stocking mortality was estimated at 5.3%, 10.5%, and 26.3% after one week, one month and one year, respectively. The majority of lake trout (68.4%) emigrated from the stocking location within two months and entered deep water (∼50 m) once warm-water incursions at the stocking site exceeded lake trout thermal preferences (15 °C). Lake trout made large movements (i.e., median 1.9 km, maximum 12.4 km straight-line distance) within the first hour post-release and had an average swimming speed of 1.64 km‧hr −1 over the first day. There was no statistically significant relationship between total distance traveled and time of day, although distance traveled tended to be greater during crepuscular and dark periods compared to daylight. Our results provide a conservative estimate of post-release mortality and reveal behaviors of hatchery-reared juvenile lake trout that may be helpful when selecting stocking locations beneficial to restoration program goals.

Lake Ontario↗
USGS2023

Lake Erie hypoxia spatial and temporal dynamics present challenges for assessing progress toward water quality goals

Seasonal hypolimnetic hypoxia has been documented in Lake Erie’s central basin since the 1950s. Ship-based surveys to monitor hypoxia have been conducted since the 1980s, but they occur at a relatively low frequency and focus on the deeper areas of the central basin. To better document the seasonal development of stratification and the consequent occurrence of hypoxia, we deployed eight moorings, in both nearshore-shallow areas and offshore-deep areas of the central basin, equipped with temperature and oxygen sensors at multiple depths, that recorded temperature and oxygen concentrations every 10 minutes. Results from 2017-2019 reveal that hypoxia occurs as early as July in the shallower areas west of, and around the southern perimeter of the central basin, but does not occur until August or September in the deeper central basin. Hypoxia is intermittent in the shallower perimeter areas; whereas in the deeper areas, hypoxia can persist into October, often progressing to anoxia. The intra and interannual differences in the spatial and temporal extent of hypoxia indicate that an extensive monitoring program will be necessary to more accurately assess progress toward reducing the extent of hypoxia pursuant to the lake ecosystem objectives of the 2012 Great Lakes Water Quality Agreement.

Michigan, New York, Ohio, Pennsylvania↗
USGS2023

Intra-lake trends and inter-lake comparisons of Mysis diluviana life history variables and their relationships to food limitation

The opossum shrimp, Mysis diluviana , is an important member of the offshore food webs of the Laurentian Great Lakes, but its response to ecosystem changes that have occurred over the past several decades is not well understood. We combined the data of four long-term sampling programs, adding several years of data (post and prior) to previously published analyses to offer a longer-term, cross-basin analysis of M. diluviana populations in the Great Lakes from 1997 to 2019. Densities were high in lakes Superior and Ontario (summer values 100–300/m 2 ), high and variable but declining (from 200–300/m 2 in 1997–2004 to less than 100/m 2 in 2017–2019) in Lake Michigan, low (∼20–50/m 2 since 2005) in Lake Huron, and very low in shallower eastern Lake Erie (<1/m 2 ). Biomass showed similar trends. Life history parameters (mortality, fecundity, and growth) were consistently highest in eastern Lake Erie, followed by lakes Ontario, Michigan, Huron, and Superior. Generation time was 1 year in Lake Erie and 2 years in the other lakes. Cross-basin relationships between annual M. diluviana areal densities and food indices (chlorophyll-a concentration and zooplankton biomass) were non-linear, increasing with food levels up to about 250 mysids/m 2 and about 650 mg dry wt/m 2 . Annual growth rates were also positively correlated to both food indices in the four deep lakes, but fecundity and mortality rates were not. Our results suggest food availability is a primary factor predicting M. diluviana density and biomass. Density-dependent mortality and fish predation could explain some of the inter-lake differences, but these relationships could benefit from further investigations.

Laurentian Great Lakes↗
USGS2024

On the survival and habitat use of hatchery-reared cisco (Coregonus artedi) in Lake Erie

Cisco ( Coregonus artedi ) have been extirpated from Lake Erie in North America since the 1960s, but they once supported one of the largest Laurentian Great Lakes fisheries. Numerous potential impediments to rehabilitation have been identified, including summer habitat refugia and predation. We used acoustic telemetry to investigate the thermal habitat use and survival of hatchery-reared adult cisco in Lake Erie. Fish were experimentally released (n = 50 per site) offshore at Dunkirk, New York, in the eastern basin and Huron, Ohio, in the central basin. Cisco in both basins found suitable summer oxythermal habitat in the metalimnion, suggesting that coldwater habitat availability is likely not an impediment for reestablishment. However, track end dates or predation dates were distributed across only four months with the last detection at 155 days. Predation sensors combined with temperature values during digestion indicated different potential predators: lake trout ( Salvelinus namaycush ) for the Dunkirk group and walleye ( Sander vitreus ) for the Huron group. Additionally, digestion temperatures of two tags indicated bird predation was also important, likely underestimated and suggested substantial use of the epilimnion by tagged fish. The results highlight the need for additional studies to address stocking optimization questions in support of future reintroduction experiments and related cisco conservation efforts.

Lake Erie↗
USGS2024

Histological analysis of deepwater sculpin ovaries supports single spawning reproductive strategy

Deepwater sculpin ( Myoxocephalus thompsonii ) were considered extirpated from Lake Ontario prior to the 1990s but have since resurged and are now an abundant offshore demersal species. As deepwater sculpin reproduction is poorly described, an investigation of their gonadal development and fecundity was conducted to better understand their reproductive biology. To evaluate spawning period duration and if females spawn multiple times during their spawning period, we compared deepwater sculpin gonadosomatic index (GSI), gonadal development, and fecundity using individuals collected in fall and spring from 2018 to 2021. Our analysis revealed female GSI was greater in fall (8.1 ± 6.2 %) than spring (4.4 ± 4.3 %). Absolute fecundity averaged 763 ± 246 oocytes and relative fecundity averaged 19 ± 6 oocytes per gram of fish. Histological analysis revealed the presence of only one batch of developing oocytes in the ovary (n = 60), indicating group-synchronous ovarian organization. Our findings suggest deepwater sculpin spawn once annually but have a protracted spawning season indicated by prolonged elevated GSI values. Therefore, protracted spawning in deepwater sculpin likely results in an extended period of larval emergence rather than the majority occurring in late spring as previously suggested. A longer timeframe for deepwater sculpin larval emergence may increase reproductive success and contribute to their population’s recovery.

Journal of Great Lakes Research↗
USGS2024

Larval coregonine density and diet composition along beaches of northern Lake Michigan

Recent oligotrophication in Lake Michigan has contributed to reduced biomass of spring zooplankton and a shift in the zooplankton assemblage toward more calanoid copepods. These changes have likely altered prey availability for first feeding native fish species that hatch in early spring, including coregonines. While spring zooplankton density and community composition are routinely monitored in offshore areas of Lake Michigan, descriptions of zooplankton in nearshore areas such as shallow beach environments and associated diet characteristics among larval coregonine species are limited. In this study, we describe the nearshore (<1 m depth, beach environment) zooplankton community in northeastern Lake Michigan during early spring 2015-2019 and compare the diets and diet selectivity of larval Lake Whitefish ( Coregonus clupeaformis ) and Cisco ( C. artedi ). Zooplankton density and composition varied among years, but calanoid copepods and copepod nauplii consistently dominated the zooplankton assemblage. Cisco and Lake Whitefish larvae were captured regularly, with Elk Rapids containing the highest proportion of Ciscoes each year. Calanoid and cyclopoid copepods were common in diets and estimated to have a positive selection. Although previous research has indicated high consumption of cyclopoid copepods by larval coregonines in Lake Michigan, our results provide new evidence that larval Lake Whitefish and Cisco of northeastern Lake Michigan will also consume and positively select for calanoid copepods when they are abundant. As such, should calanoid zooplankton continue to dominate the copepod community in Lake Michigan, larval coregonines appear capable of exploiting this abundant prey to improve their likelihood of survival to later life stages.

Michigan↗
USGS2025

Lake Superior fish community and fisheries, 2001–2022: An era of stability

Lake Superior is the least anthropogenically impacted of the Laurentian Great Lakes ecosystems, yet dramatic changes to the fish community are evident. Previous published works chronicled those changes and the efforts to rehabilitate the fish community through the year 2000. Here, we review through the year 2022, where post-rehabilitation stability was driven by lean lake trout ( Salvelinus namaycush namaycush) as the most abundant piscivore in nearshore waters, siscowet lake trout ( Salvelinus namaycush siscowet) as the most abundant piscivore in offshore waters, and a healthy, intact assemblage of native prey species, which created ecological redundancies and helped stabilize the food web. Stocking of non-native salmonines was reduced 74%, and populations of Chinook salmon ( Oncorhynchus tshawytscha) and coho salmon ( Oncorhynchus kisutch) were maintained through natural reproduction. Despite reduced stocking, yield from recreational fisheries was stable. Likewise, developments in population modeling led to evaluations and refinement of management strategies that helped create stability for lake trout, lake whitefish ( Coregonus clupeaformis ), and cisco ( Coregonus artedi ) fisheries. With lake trout rehabilitation achieved, focus shifted toward rehabilitation of native brook trout ( Salvelinus fontinalis ), lake sturgeon ( Acipenser fulvescens ), and walleye ( Sander vitreus ). Despite continued control efforts, sea lamprey ( Petromyzon marinus ) abundance increased considerably, and estimates of fish killed by lampreys averaged 2.65 million kg annually. Environmental changes have benefited sea lampreys and fostered thermal habitats more suitable to non-native organisms, posing new challenges for managers and researchers. Nevertheless, the post-rehabilitation stability in the contemporary fish community will help provide resilience to future perturbations in the ecosystem.

Lake Superior↗
USGS2026

Competition between invasive round goby and native slimy sculpin in a coldwater laboratory setting

Slimy sculpin ( Cottus cognatus ) abundance has substantially declined in many regions of the Laurentian Great Lakes coincident with the arrival and range expansions of invasive round goby ( Neogobius melanostomus ). Previous studies have shown that round goby can outcompete native mottled sculpin ( C. bairdii ) for shelter and negatively affect native spoonhead sculpin ( C. ricei ) growth, but direct behavioral interactions of slimy sculpin and round goby have not been observed. We conducted a laboratory experiment in which a pair of slimy sculpin were placed in an aquarium that contained an artificial spawning shelter and one round goby (treatment, n = 10) or no other fish (control, n = 10) for at least 30 days at temperatures reflective of their potential offshore overlap. We measured slimy sculpin egg deposition, aggressive interactions, artificial shelter occupancy, growth, and survival. Slimy sculpin were less likely to occupy shelters and experienced increased intraspecific competition when a round goby was present. In contrast, round goby rarely chased slimy sculpin, slimy sculpin were more likely to chase round goby, and slimy sculpin growth, egg deposition, and survival were not significantly different between groups. Differences in density between treatments and the relative size of fish could confound our observations. However, the dominant slimy sculpin occupied the shelter at similar rates between experimental groups if the round goby was smaller. Our results indicate that larger round goby overlapping with slimy sculpin could affect slimy sculpin shelter and space use, but the Great Lakes environment may create more stress for slimy sculpin than we observed.

Lake Michigan, Lake Superior↗
USGS2026

Describing the seasonal abundance and growth rates of larval fishes across productivity gradients in Lake Huron in 2017

Several of the Laurentian Great Lakes, including Lake Huron, have undergone oligotrophication in the past decades and prey fish biomass has concomitantly declined. One potential mechanism to explain declines in prey fish is slower growth and lower survival at the larval stage. To determine whether reduced productivity affects the growth of larval fish, we sampled larval fishes and their environment approximately monthly from May through August 2017 at nine nearshore to offshore transects across Lake Huron that included North Channel, Georgian Bay, and the main basin. North Channel transects had the highest chlorophyll a concentrations and zooplankton densities. Rainbow smelt ( Osmerus mordax ), burbot ( Lota lota ), bloater ( Coregonus hoyi ), and shiners ( Notropis spp.) were the most abundant larval fish taxa, peaking in June and July. We aged rainbow smelt and bloater using otoliths, and estimates of growth rate revealed rainbow smelt always grew faster. For both species, we explained variation in total length by comparing 16 candidate linear mixed-effects models, with age, chlorophyll a , zooplankton, water temperature, larval fish density, and interactions with age as predictor variables. For rainbow smelt, the full model was best; zooplankton had the greatest effect, but it was negative and opposite from our hypothesis. For bloater, four candidate models were most parsimonious; water temperature had the greatest effect, and it was positive as predicted from our hypothesis. To more effectively evaluate whether zooplankton can limit larval fish growth and survival, we recommend that future designs conduct more frequently sampling within a year even at the expense of fewer transects.

Lake Huron↗
USGS2026

A post-restoration evaluation of fish spawning on Coreyon Reef in Saginaw Bay, Lake Huron

Habitat diversity is important to fisheries for promoting resiliency to interannual environmental variation. Historical land use change resulting in increased sedimentation has impaired fish rock reef habitats in Saginaw Bay, Lake Huron. Pursuant to the recovery of Saginaw Bay’s walleye population, and to benefit other species, came the decision to restore the offshore habitat Coreyon Reef, a 0.8 ha rock reef complex. Following a pre-restoration assessment in 2014–2016 and construction in 2019, we conducted a comprehensive assessment from fall 2020 to spring 2022 to document the restored habitat’s viability for fish spawning. This assessment featured two objectives: (1) to evaluate reproductive usage by lake whitefish and walleye, and (2) to evaluate their corresponding egg incubation environment. We sampled directly on Coreyon Reef and at nearby unrestored control sites for comparison, with gears standardized to the pre-restoration assessment. We confirmed spawning of lake whitefish and walleye on the restored reef less than 2 years post-construction. Relative to the pre-restoration assessment, peak egg deposition increased substantially for walleye and marginally for lake whitefish. However, concurrent reproductive utilization at nearby unrestored sites was consistent or even greater than directly on the restored site. Dissolved oxygen concentration was sufficient for embryo development, but invasive round goby were abundant and consumed eggs. The fish assemblage at our sampling sites was consistent before and after restoration. We predict eggs deposited on the restored site have improved survival relative to unrestored sites, as the new reef structure may protect incubating embryos from predation and sedimentation.

Michigan↗
USGS2014

Quantity and quality of groundwater discharge in a hypersaline lake environment

Geophysical and geochemical surveys were conducted to understand groundwater discharge to Great Salt Lake (GSL) and assess the potential significance of groundwater discharge as a source of selenium (Se). Continuous resistivity profiling (CRP) focusing below the sediment/water interface and fiber-optic distributed temperature sensing (FO-DTS) surveys were conducted along the south shore of GSL. FO-DTS surveys identified persistent cold-water temperature anomalies at 10 separate locations. Seepage measurements were conducted at 17 sites (mean seepage rate = 0.8 cm/day). High resistivity anomalies identified by the CRP survey were likely a mirabilite (Na 2 SO 4 ·10H 2 O) salt layer acting as a semi-confining layer for the shallow groundwater below the south shore of the lake. Positive seepage rates measured along the near-shore areas of GSL indicate that a ∼1-m thick oolitic sand overlying the mirabilite layer is likely acting as a shallow, unconfined aquifer. Using the average seepage rate of 0.8 cm/day over an area of 1.6 km 2 , an annual Se mass loading to GSL of 23.5 kg was estimated. Determination of R/Ra values (calculated 3 He/ 4 He ratio over the present-day atmospheric 3 He/ 4 He ratio) <1 and tritium activities of 1.2–2.0 tritium units in groundwater within and below the mirabilite layer indicates a convergence of regional and local groundwater flow paths discharging into GSL. Groundwater within and below the mirabilite layer obtains its high sulfate salinity from the dissolution of mirabilite. The δ 34 S and δ 18 O isotopic values in samples of dissolved sulfate from the shallow groundwater below the mirabilite are almost identical to the isotopic signature of the mirabilite core material. The saturation index calculated for groundwater samples using PHREEQC indicates the water is at equilibrium with mirabilite. Water samples collected from GSL immediately off shore contained Se concentrations that were 3–4 times higher than other sampling sites >25 km offshore from the study site and may be originating from less saline groundwater seeps mixing with the more saline water from GSL. Additional evidence for mixing with near shore seeps is found in the δD and δ 18 O isotopic values and Br:Cl ratios. Geochemical modeling for a water sample collected in the vicinity of the study area indicates that under chemically reducing conditions, arsenic- (As) bearing minerals could dissolve while Se-bearing minerals will likely precipitate out of solution, possibly explaining why the shallow groundwater below and within the mirabilite salt layer contains low concentrations of Se (0.9–2.3 μg/L).

Utah↗
USGS2017

Environmental conditions and prey-switching by a seabird predator impact juvenile salmon survival

Due to spatio-temporal variability of lower trophic-level productivity along the California Current Ecosystem (CCE), predators must be capable of switching prey or foraging areas in response to changes in environmental conditions and available forage. The Gulf of the Farallones in central California represents a biodiversity hotspot and contains the largest common murre ( Uria aalge ) colonies along the CCE. During spring, one of the West Coast's most important Chinook salmon ( Oncorhynchus tshawytscha ) populations out-migrates into the Gulf of the Farallones. We quantify the effect of predation on juvenile Chinook salmon associated with ecosystem-level variability by integrating long-term time series of environmental conditions (upwelling, river discharge), forage species abundance within central CCE, and population size, at-sea distribution, and diet of the common murre. Our results demonstrate common murres typically forage in the vicinity of their offshore breeding sites, but in years in which their primary prey, pelagic young-of-year rockfish ( Sebastes spp.), are less available they forage for adult northern anchovies ( Engraulis mordax ) nearshore . Incidentally, while foraging inshore, common murre consumption of out-migrating juvenile Chinook salmon, which are collocated with northern anchovy, increases and population survival of the salmon is significantly reduced. Results support earlier findings that show timing and strength of upwelling , and the resultant forage fish assemblage, is related to Chinook salmon recruitment variability in the CCE, but we extend those results by demonstrating the significance of top-down impacts associated with these bottom-up dynamics. Our results demonstrate the complexity of ecosystem interactions and impacts between higher trophic-level predators and their prey, complexities necessary to quantify in order to parameterize ecosystem models and evaluate likely outcomes of ecosystem management options.

California↗