Note that the data published in the 2002 State of the Nation’s Ecosystems Report as well as the 2003 and 2005 Web-Only Updates have been superseded by the 2008 Report and thus should be used with caution. For the most recent data, purchase the 2008 Report from Island Press.

The Indicator

A variety of nutrients are needed for plant growth in aquatic systems: nitrogen, phosphorus, carbon, sulfur, iron, manganese, and various trace metals (e.g., copper, cobalt, molybdenum, and zinc). Silica is required by some kinds of algae (e.g., diatoms) because it is the main component of the shells that surround the cells. However, nitrogen (N) and phosphorus (P) are by far the most common nutrient elements that limit or control the amount and rate of plant growth in aquatic systems and, thus, define their trophic status and corresponding “water quality.” Of these two elements, phosphorus is widely considered to be the element that most commonly limits aquatic plant growth in fresh waters under natural conditions (i.e., minimal impacts from human activity). Total phosphorus (TP) includes all forms of phosphorus present in a water sample—dissolved and particulate, inorganic and organic; adsorbed onto suspended clays and hydrous oxides; present in planktonic organisms and in organic detritus; and phosphorus in dissolved natural organic matter. Phosphorus in macrophytes, fish, and bottom sediments generally is not included.

TP was selected for reporting because it is a comprehensive measure of the many operationally defined and chemical forms of phosphorus, most of which are directly or indirectly available for plant growth. Excess phosphorus can contribute to algal blooms, poor water clarity, and other symptoms of eutrophication.

TP levels are a measure of trophic state (Carlson 1977) and general water quality in lakes, reservoirs, and large rivers. (Large rivers typically behave as lakes; water residence times in stretches of large rivers are sufficiently long that substantial phytoplankton growth can occur in them.) The concentrations of TP that contribute to symptoms of eutrophication are poorly understood for flowing waters, but generally they are thought to be higher than the critical levels in lakes. Consequently, TP is reported separately for lakes and rivers. (The effects of phosphorus enrichment are different for lakes and rivers in tropical areas than they are for temperate zones; this discussion relates to temperate zones only.)

TP measurements are straightforward; TP in lakes should be reported as an average over the growing season (e.g., April to September), which will require several (e.g., 4–6) samples over the course of the period. Consideration was given to the appropriate number of samples each year (e.g., Knowlton et al. 1984), and complications of sampling in areas with minimal seasonal influence, such as Florida (Brown et al. 1988).

TP measurements in rivers are restricted to those large rivers with flows exceeding 1000 cubic feet per second (cfs). To ensure proper characterization of average values for each river, only sites that had at least 30 samples over the course of 2 years were included.

Information on the 1986 phosphorus recommended goal for preventing excess algae growth can be found in EPA 440/5-86- 001 (see references). Information on regional nutrient (phosphorus) criteria can be found at http://www.epa.gov/waterscience/criteria/nutrient/ecoregions/.

The Data

Data for river phosphorus are from sites operated by the U.S. Geological Survey (USGS) National Water Quality Assessment (NAWQA) and National Stream Water Quality Accounting Network (NASQAN). Data were available from 140 sites, with data collection from 1992 to 1998; 116 of these sites were either NAWQA or NAWQA and NASQAN joint sites.

NAWQA is described generally in the technical notes for the core national indicator for contaminants and for nitrate in farmland streams. While that note describes data collection from streams with relatively homogenous land cover (and often relatively low discharge volumes), the data used in this indicator are from larger rivers, with both larger discharge volumes and watersheds with generally more diverse land uses. Thus, these samples represent the integrating influences of many different land uses. The methods for processing and summarizing these data for large rivers, such as computing annual-weighted discharge concentrations, also have been described in the technical note for the Farmlands nitrate indicator.

NASQAN is a USGS program that is focused on four major river basins: the Mississippi, the Rio Grande, the Colorado, and the Columbia River. NASQAN stations are located on major tributaries in the four river basins, along the mainstem of rivers where there is a large increase in flow, and upstream and downstream from large reservoirs. The program generally measures both stream flow and a broad range of chemical constituents. An extensive quality-assurance/quality-control program enables constituents present in very low concentrations (micrograms per liter, roughly parts per billion) to be measured with definable accuracy and precision. See http://water.usgs.gov/nasqan/progdocs/index.html.

Because there was concern over the use of STORET data for this indicator (see below) with respect to the possibility that sampling locations might be strongly influenced by virtue of being located near outfalls from wastewater treatment plants, this question was also raised with respect to the NAWQA/NASQAN data. These programs collect data using procedures that ensure that the sample is representative of the entire stream cross-section. So, even if the stream at the point of collection were not well mixed, the samples would still be representative of the entire stream flow. In addition, the measure that is being reported—annual discharge- weighted average concentrations—addresses the potential concern that samples might be overly representative of summer low flows when wastewater effluent can comprise a large fraction of the flow in some rivers.

The Data Gap

In assessing the availability of data for reporting on phosphorus in lakes and rivers, we reviewed two major datasets in addition to the one reported here (NAWQA/NASQAN). These were STORET, maintained as a data repository by the Environmental Protection Agency (http://www.epa.gov/storet/), and within STORET, data from the National Water Information System (NWIS), a USGSmaintained data system (http://waterdata.usgs.gov/nwis/).

Under contract to The Heinz Center, Procter & Gamble’s Miami Valley Laboratory undertook an assessment of the quality and spatial and temporal variability of the data from these two sources. They concluded that phosphorus data were likely to be comparable in terms of reporting thresholds; that is, there were few if any problems related to the use of different reporting thresholds in different states or jurisdictions.

The second step was to determine whether either data system had sufficient numbers and geographic distribution of sampling sites. It was apparent from inspection of a map of lake phosphorus sampling sites that neither NWIS nor STORET as a whole has sufficient coverage across the country. STORET has phosphorus concentration data from a large number of river sampling sites, and this record extends into the 1980s.

However, there was significant concern among workgroup members regarding the fact that STORET data are derived from studies undertaken for many reasons and using many methods for selecting sampling sites. For example, some sampling was undertaken specifically as part of before-and-after effectiveness studies relating to phosphorus removal in publicly owned sewage treatment works (POTWs). Other studies may have been undertaken to determine the nature and extent of known phosphorus contamination problems, while others may have been located randomly as part of efforts to characterize nutrient concentrations in both “clean” and “dirty” areas.

STORET has very little information that can be used to determine the rationale for sampling-site selection. Thus any determination of the appropriate subset of STORET results to use would have to be based on a complex analysis of the proximity of sampling sites to POTWs, urban areas, and the like, which could be used to determine if the sampling was biased to inclusion or exclusion of such sites. Unfortunately, this analysis has not been done and could not be accomplished within the time and resources of this project. Therefore, given the significant potential for STORET data to be unrepresentative, we have decided that it is inappropriate to rely on it for this indicator until such studies can be completed.

References

Brown, C.D., D.E. Canfield, Jr., R.W. Bachmann, and M.V. Hoyer. 1998. Seasonal patterns of chlorophyll, nutrient concentrations and Secchi disk transparency in Florida lakes. Lake and Reserv. Manage. 14:60–76.

Carlson, R.E. 1977. A trophic state index for lakes. Limnol. Oceanogr. 22:361–369.

Knowlton, M.F., M.V. Hoyer, and J.R. Jones. 1984. Sources of variability in phosphorus and chlorophyll and their effects on use of lake survey data. Water Resour. Bull. 20:397–407. U.S. Environmental Protection Agency. 1986. Quality criteria for water—1986, U.S. Environmental Protection Agency report, EPA 440/5-86-001.