How are Endocrine-Disrupting Compounds Moving Through the Food Web in Lake Mead National Recreation Area?

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Water quality in the Lake Mead National Recreation Area (LAKE), particularly Las Vegas Bay, is affected by water coming from the Las Vegas Wash, an urban perennial stream whose water is comprised of treated wastewater and urban runoff coming from the Las Vegas metropolitan area.  Common carp collected from Las Vegas Wash and Las Vegas Bay and largemouth bass collected from Las Vegas Bay have impaired reproductive and endocrine health that have been shown to be caused by the presence of endocrine-disrupting compounds (EDCs) in water coming from the Las Vegas Wash. Understanding the movement of EDCs through the food web is critical in determining the effects of EDCs on all organisms including endangered species such as the razorback sucker and for making informed and effective management decisions on ways to control and mitigate sources of EDCs.  In addition, a new study started in 2017 looking at the effect of microplastics on the foodweb led by Austin Baldwin (ID WSC) has also been started to understand whether these plastics interrupt feeding and reproduction in the lake. 

Map of the Lake Mead National Recreation Area, Arizona-Nevada
Map of the Lake Mead National Recreation Area, Arizona-Nevada (modified from figure 1-1 in USGS Circular 1381).

A variety of contaminants including nutrients, metals, perchlorate, and organic compounds are transported to the Las Vegas Wash from various sources.  Organic compounds are some of the most commonly found contaminants in the Wash.  Some of these organic compounds, such as DDT (dichlorodiphenyltrichloroethane), PCBs (polychlorinated biphenyls), PBDEs (polybrominated diphenyl ethers), PAHs (polycyclic aromatic hydrocarbons), personal care products, and human pharmaceuticals, are suspected of being Endocrine-Disrupting Compounds (EDCs).  The sources of EDCs to water in the Las Vegas Wash include diffuse sources such as urban runoff as well as concentrated historical industrial sources located along the Wash, and the release of tertiary treated wastewater from 4 wastewater treatment plants that serve the Las Vegas Metropolitan area.  EDCs have been found in treated and untreated wastewater in many areas of the United States and many EDCs are not completely removed by even the most advanced wastewater treatment processes.

EDCs can interfere with or disrupt endocrine and reproductive systems and can cause cancerous tumors, birth defects, and developmental disorders.  In fish, EDCs are thought to be especially disruptive of the larval or developmental stages altering sexual development, behavior and fertility, and causing feminization of male fish or masculinization of female fish.  Recent studies have shown that male common carp (Cyprinus carpio) collected from Las Vegas Bay and Las Vegas Wash have shown feminization with impaired endocrine and reproductive health.  Largemouth bass (Micropterus salmoides) collected in Las Vegas Bay have also shown symptoms of impaired reproductive health.  These adverse health effects have been shown to be caused by the presence of EDCs in water and/or sediment in these areas.  Recent studies have indicated improvement in the endocrine and reproductive health of common carp collected from in Las Vegas Bay since 1995 that may have come from enhanced wastewater treatment.

Scientists preparing for fish collection at Lake Mead as part of the endocrine disruption study
Scientists preparing for fish collection at Lake Mead as part of the endocrine disruption study. Photo courtesy of Michael Rosen, USGS.

In spite of these recent improvements, the evidence that EDCs may be causing adverse health effects in fish and aquatic invertebrates in LAKE has been mounting for some time.  In a study completed in 1996, water, bottom-sediment, and tissue samples of carp taken from the Las Vegas Wash and Las Vegas Bay all showed the presence of EDCs. Tissue samples of carp taken from the Las Vegas Wash showed skewed sex hormone ratios with elevated concentrations of male hormones in female individuals and the presence of female hormones in male individuals.  Most importantly, blood plasma samples from male carp taken from the Las Vegas Wash showed extremely high levels of vitellogenin, an egg yolk precursor protein, suggesting endocrine disruption.  Samples of carp tissue collected from a reference site in Callville Bay in Boulder Basin of LAKE did not show these characteristics. The presence of EDCs in water and bottom sediment taken from the same areas of the Las Vegas Wash and Las Vegas Bay as the carp indicates that the endocrine disruption observed in carp may be due to these chemicals. However, the presence of some toxic metals may also be contributing to endocrine disruption in carp.

Microplastic sampling at Lake Mead with National Park Service
Microplastic sampling at Lake Mead with National Park Service. Photo courtesy of Michael Rosen, USGS.

More recently, a study of the gonadal condition of carp from LAKE showed that male carp from Las Vegas Bay had reduced testicular development relative to male carp from a reference site in the Overton Arm.  This difference in the reproductive health of male carp in LAKE was attributed to the greater number and concentration of EDCs detected in bed sediment from Las Vegas Bay relative to Overton Arm.  A higher incidence of testicular macrophage aggregates, a sign of abnormal testicular development, in male carp from Las Vegas Bay relative to male carp from Overton Arm further supports the hypothesis that endocrine disruption is occurring in carp in Las Vegas Bay through exposure to environmental contaminants.

Recent studies have shown that certain microbial communities may degrade some organic chemicals, but more research is needed to determine if these breakdown products are harmful, and if there is sufficient microbial activity in Las Vegas Wash to allow sufficient breakdown of the chemicals to make them harmless. 

Previous research on LAKE has identified the occurrence of EDCs in fish tissue from Lake Mead.  However, no studies have attempted to determine the pathway of these contaminants to fish including determining whether they are bioconcentrated or bioaccumulated. This study will examine the extent to which EDCs in aquatic organisms in LAKE are either bioconcentrated through direct uptake from water or bioaccumulated through the food web.  Understanding the pathways of EDCs to fish is critical to assessing impacts of EDCs to the entire aquatic ecosystem of LAKE.  If fish are accumulating EDCs directly from water or sediment the risk of adverse health effects may be relatively minor as individual species may be affected differently.  However, if fish are bioaccumulating EDCs through the food web the risk of adverse health effects is very broad as all secondary and tertiary consumers would be affected including endangered species such as the razorback sucker.  Management strategies to control the sources of EDCs to LAKE and protect and restore the ecosystem may differ depending on the mode of EDC accumulation in fish.

The main scientific questions to be answered include:

  • What are trophic relationships between organisms in LAKE? 
  • Are EDCs bioconcentrated in aquatic organisms directly from water or are they bioaccumulated through the food web? 
  • What is the total mass bioaccumulation of EDCs between various trophic levels in the food web and are concentrations of EDCs in organisms at the top of the food web, such as razorback suckers, high enough to cause reproductive or endocrine impairment? 
  • Are EDCs incorporated into quagga mussels and how do quaggas relate to other organisms in the food web? 

Samples of water, sediment, plankton, invertebrate tissue, and fish tissue have been collected and analyzed for a variety of EDCs.  Samples were collected at different depths in 3 sub-basins of LAKE; Las Vegas Bay, Boulder Basin, and Overton Arm. The Overton Arm site served as a reference site as samples of water, sediment, and fish tissue taken from a previous study showed minimal contamination in this area.  The bioavailable fraction of EDCs in the water column was measured using Semipermeable Membrane Devices (SPMD) and Polar Organic Chemical Integrative Samplers (POCIS).  Surface sediment samples were collected using a ponar sampler.  Samples of plankton were collected using plankton net.  Benthic invertebrates were collected using a D-net or hand picking.  Invertebrate tissue was composited prior to analysis with the exception of tissue of quagga mussels which was analyzed separately from other invertebrates. Samples of invertebrate organisms were identified to the lowest taxonomic resolution possible (generally family or genus) prior to compositing for analysis. In previous studies, fish near the top of the food web such as common carp and largemouth bass were sampled and analyzed for EDCs. This study focused on fish that are intermediate in the food web such as threadfin shad and bluegill.  These fish were collected in each basin using an electroshocking technique and their tissue composited for analysis.  

Samples were analyzed for a variety of potential EDCs.  Fish and invertebrate samples were analyzed for PCBs, DDT and other pesticides, PBDEs, and various antibacterial and fragrance compounds.  Sediment and water samples were analyzed for a similar set of compounds as well as a small set of chemicals related to wastewater effluent.  Prior to analysis, samples from the SPMD and POCIS were screened using the yeast estrogen screen (YES). The YES assay provides a measure of total estrogenicity of chemicals present in the sample and can be helpful in determining if chemicals in the samples are acting as estrogens or estrogen mimics and thus could be causing intersex development or other endocrine disruption effects.  Tissue samples were processed to extract chemicals of interest using various specialized methods including gel permeation chromatography and adsorption chromatography.  All water and sediment samples and tissue extracts were analyzed using gas chromatography-mass spectrometry to identify organic compounds.

Data from the sampling and chemical analyses will be evaluated to determine the food web structure of LAKE.  Contaminants detected and their relative mass will be compared to the newly established food web structures so that contaminant pathways and mass transfer rates can be determined.  From this evaluation, the potential risk of EDCs and other contaminants to razorback suckers and other fish in LAKE can be identified relative to the source of the contaminants.