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Lake sediments -- Oregon, Lake sediments -- Washington (State), Paleolimnology -- Oregon, Paleolimnology -- Washington, Lakes -- Environmental aspects, Suspended sediments -- Environmental aspects, Phosphorus -- Environmental aspects


Water quality in many Northwest lakes has declined over the past century due, in part, to increased anthropogenic nutrient loading (Edmonson and Lehman, 1981). Under the Clean Water Act, resource managers such as the Washington Department of Ecology, Oregon Department of Environmental Quality, and tribes are responsible for restoring and protecting the integrity of these waters. Targets for restoration or criteria for impairment are not well defined, however, and may naturally vary by geology, hydrology, morphometry, and climate.

One way to determine whether lakes are impaired and to identify restoration targets is to assess lake reference conditions (EPA 2000). Reference conditions are the water quality conditions that would exist in the absence of anthropogenic perturbation. Reference conditions are not nutrient criteria but can be useful in establishing realistic criteria that reflect natural regional variations in water quality. The EPA suggests three general approaches for assessing reference conditions (EPA 2000):

  • Observation of reference lake or entire lake population distributions
  • Paleolimnological reconstruction of past conditions.
  • Estimation of past conditions from mass balance nutrient loading models, morphoedaphic index models, or other mathematical models.

The lake population distribution approach involves observing data from either a set of lakes that are relatively unimpacted or from a random selection of all lakes in a region. The lower percentile of each population distribution is assumed to represent reference conditions. The lower 75th percentile of the relatively unimpacted population distribution has been used as a cutoff for reference conditions while the 25th percentile has been used for the lake population distribution as a whole. The Washington Department of Ecology has used this approach to develop phosphorus criteria (Moore and Hicks 2004). This approach assumes that the percentiles in a distribution represent reference conditions.

Paleolimnological models are based on the observation that sediments continually incorporate information from the overlying water column and that sediment cores provide a history of past conditions. Sedimented diatom-based water quality reconstructions are based on two main observations (Hall and Smol 1992): (1) individual diatom species have restricted nutrient requirements and are sensitive indicators of lake trophic status, and (2) the cell walls of diatoms are abundant and well preserved in most lake sediments and can usually be identified to a specific or subspecific level. Weighted-averaging and calibration statistical models have been used to relate sediment surface diatom assemblages to contemporary water quality parameters. These speciesenvironment relationships, known as transfer functions, can then be used to infer past water quality conditions from diatom remains preserved deep in sediments. This approach has been successfully applied in several geographic regions including the northeastern United States (Dixit et al. 1999), Minnesota (Ramstack et al. 2003), and British Columbia (Reavie et al. 1995).

The morphoedaphic index (MEI) was originally developed to predict fisheries yield in lakes based on lake morphometry and alkalinity or conductivity (Ryder 1965). Vighi and Chiaudani (1985) suggested the utility of the index for inferring natural background phosphorus levels in lakes. Their major assumption was that human activities have not altered lake morphometry and ionic strength to the degree that the same activities have increased nutrient loading. While the model is simple, it has been successfully used in to infer background phosphorus concentration in Midwest lakes (Ramstack et al. 2003).

We used the MEI and the sedimented diatom total phosphorus inference models to assess lake reference conditions for Puget Lowlands and Northern Willamette Valley Ecoregion Lakes.


Prepared for: US Environmental Protection Agency and the Pacific States Marine Fisheries Commission.

A product of the Portland State University Center for Lakes and Reservoirs.

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