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Continental-scale increase in lake and stream phosphorus: Are oligotrophic systems disappearing in the U.S.? John L. Stoddard, John van Sickle, Alan T. Herlihy, Janice Brahney, Steve Paulsen, David V. Peck, Richard Mitchell, and Amina I Pollard Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b05950 • Publication Date (Web): 25 Feb 2016 Downloaded from http://pubs.acs.org on February 25, 2016
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Continental-scale increase in lake and stream phosphorus: Are
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oligotrophic systems disappearing in the U.S.?
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Authors: John L. Stoddard1*, John Van Sickle1,2, Alan T. Herlihy3, Janice Brahney4, Steven
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Paulsen1, David V. Peck1, Richard Mitchell5, Amina I. Pollard5
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Affiliations: *
(corresponding author): U.S. Environmental Protection Agency, 200 SW 35th Street, Corvallis,
OR, 97333; (541) 754-4441; fax (541)754-4716;
[email protected] 1
U.S. Environmental Protection Agency, 200 SW 35th Street, Corvallis, OR, 97333.
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retired.
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Oregon State University, Dept. Fish and Wildlife, Corvallis, OR 97331.
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University of British Columbia, Dept. Earth and Environmental Science, Kelowna, BC, Canada.
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U.S. Environmental Protection Agency, Office of Water, Washington, DC.
*Correspondence to:
[email protected].
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Abstract: We describe continental scale increases in lake and stream total phosphorus (TP)
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concentrations, identified through periodic probability surveys of thousands of water bodies in
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the conterminous U.S. The increases, observed over the period 2000-2014, were most notable in
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sites in relatively undisturbed catchments, and where TP was initially low (e.g., less than 10 µg
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L-1). Nationally, the percentage of stream length in the U.S. with TP ≤ 10 µg L-1 decreased from
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24.5 to 10.4 to 1.6% from 2004 to 2009 to 2014; the percentage of lakes with TP ≤ 10 µg L-1
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decreased from 24.9 to 6.7% between 2007 and 2012. Increasing TP concentrations appear to be
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ubiquitous, but their presence in undeveloped catchments suggests that they cannot be entirely
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attributed to either point or common non-point sources of TP.
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1.0 Introduction
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Phosphorus (P) is a critical nutrient that limits the productivity of many temperate freshwater
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ecosystems1, 2. Extensive research in the late 1950s to the 1970s revealed how anthropogenic
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sources of P, especially from industrial and municipal wastewater, facilitated the eutrophication
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of lakes and streams. Measures have since been implemented to control the supply of P from
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these point sources, but eutrophication attributable to non-point sources of P (e.g., agricultural,
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stormwater and wastewater runoff) continues to be widespread3. Recent sizable and
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economically critical blooms of harmful algae4 have been attributed to these non-point sources.
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Because of its critical role in aquatic ecosystems, total phosphorus (TP) has been one of the
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key variables included in the U.S. EPA’s national program of aquatic resources surveys. In this
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paper we present stream and lake data collected in periodic national surveys conducted
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cooperatively by the U.S. EPA and the States/Tribes and initiated in 2000. Results from these
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randomized, unequal probability surveys provide unbiased estimates of a range of biological,
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chemical and physical variables at a continental scale, and allow the users to determine the
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percentage of all lakes and streams that exceeds specific levels of these variables. The data
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provide a powerful source of information for examining the conditions of water resources and
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their stressors, across geopolitical, ecological and hydrologic boundaries.
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2.0 Materials and Methods
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2.1 Survey Design
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All of the national surveys described in this paper are randomized, unequally weighted
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probability surveys5 overseen by the U.S. EPA’s Office of Water
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(http://www.epa.gov/nheerl/arm/designpages/monitdesign/survey_overview.htm) with the goal
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of creating unbiased assessments of aquatic resources across the 48 conterminous states. Details
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of the stream6 and lake7 survey designs have been published elsewhere.
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The first of these surveys sampled wadeable (1st through 4th Strahler Order) streams in the
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conterminous U.S. between 2000 and 20048, 9. Additional stream and river surveys have been
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repeated on a 5 year schedule, with results from the 2008-2009 and 2013-2014 efforts available
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for the current investigation. A national lake survey was conducted in 2007 and repeated in
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201210. Each site in each survey was sampled once during a summer index period. In the second
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and third stream surveys, and the second lake survey, approximately half of the sampled sites
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were a random subsample of sites also sampled in the previous survey. Details on sample sizes in
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each survey, and the numbers of resurveyed sites, are shown in Table S1.
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The surveys use standardized sampling methods that did not change over our reported time
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period. At each stream site, a 4-L grab sample was collected from an area of flowing water at or
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near the designated geographic coordinates assigned to the site as part of the survey design 11 At
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each lake site, a 4-L integrated water sample was collected from a point on the lake that
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represented the deepest point of natural lakes or from the mid-point area of reservoirs12. All
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samples were shipped on ice as soon as possible after collection to an analytical laboratory
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(overnight courier service was used when necessary and available). Samples typically arrived at
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a laboratory within 24-48 hours of collection.
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2.2 Creation of Minimally Disturbed Dataset
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In order to quantify changes in TP in relatively undisturbed catchments, and to examine
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potential mechanisms operating in the absence of significant anthropogenic development, we
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created a minimally-disturbed subset of the resurveyed samples collected as part of each pair of
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surveys. The contributing watershed area was delineated for each sample site (lake or stream),
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and percentages of various land use categories calculated for each, using the 2006 National Land
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Cover Dataset (http://www.mrlc.gov/nlcd2006.php). In addition, the density of roads (km of
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roads per km2 of catchment) was calculated for each site’s catchment based on the 2006 TIGER
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database (https://www.census.gov/geo/maps-data/data/tiger.html). Finally, we calculated riparian
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disturbance at each stream and lake site based on data collected at the time of each survey. In
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brief, riparian disturbance was estimated from direct observations of 12 specific forms of human
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riparian disturbance, tallied in either 22 (streams) or 10 (lakes) riparian plots along the stream or
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lakeshore13, 14. We defined minimally-disturbed15 sites as those with catchments exhibiting
