Water quality condition monitoring - Random river and stream surveys in Minnesota
Random or probabilistic surveys simply refer to the process of randomly selecting sites to monitor. As a result, an unbiased estimate of the condition of the resource, in this case rivers and streams, can be obtained. The process is similar to that of an opinion poll where a sample of randomly chosen individuals are asked questions about their views on a particular topic and the results from this sample are used to approximate the general opinion of the population. Results from such polls are often accompanied by a margin or error. Similarly, random survey results have an associated error term (e.g., 95% confidence interval) that, given the variability observed within the sample, indicates the reliability of the estimate. The smaller the error term, the more confidence one should have that the reported estimate is close to the actual figure for the entire population.
Methods
The MPCA began using random surveys to assess rivers and streams throughout Minnesota in 1996. The focus of these surveys was, and continues to be, perennial rivers and streams within or comprising the border of the state, excluding the Mississippi River downstream of the confluence with the St. Croix River (for more information on this segment of the Mississippi (see Great River - Mississippi River). From 1996 to 2006 the organizing framework for random surveys was major river basins, with surveys typically conducted within the same summer index period for a particular basin. In addition to the random monitoring sites, a set of sites was selected to represent the range of human disturbance within each basin. Data from these ‘targeted’ sites helped to insure that an adequate dataset for developing indicators was obtained.
Beginning in 2009, a different approach to river and stream surveys will be employed. This approach will focus more on providing reliable statewide estimates of condition as randomly selected monitoring sites that span the entire state will be sampled within the same year. The previous basin framework required several years to obtain statewide estimates. Moreover, the new survey design will provide condition estimates more frequently (every 3 years), enabling it to better detect whether conditions are improving or degrading throughout the state (i.e., trend detection). Rather than individual river basins, the new survey will be able to present results separately for each of Minnesota’s three major ecoregions in addition to the statewide estimates (see map of current monitoring sites).
2009 monitoring sites
For the 2009 field season, the MPCA anticipates monitoring 150 rivers and streams randomly located throughout the state.
Map of 2009 Monitoring SitesEven though the MPCA has transitioned to a statewide assessment of river and stream condition, results from the approximately 425 sites sampled during the basin surveys can still be utilized as a baseline data set from which to compare future survey results. Since these sites were randomly selected their results can be aggregated to provide statewide estimates of condition as well as repartitioned to provide estimates for each ecoregion. To do so, however, requires an acknowledgement of the potential sources of variability associated with combining data collected across several years (e.g., wet vs. dry years); one of the primary reasons for switching to the statewide survey design.
At each monitoring site, samples are collected and measurements are recorded in an effort to characterize the biological, chemical, and physical conditions of the river or stream segment. Fish are collected using electrofishing gear, then they are identified and measured before being released back into the stream. Voucher specimens are collected for each unique species encountered. Aquatic macroinvertebrates (e.g., aquatic insects, crustaceans, snails, worms) are collected from productive habitat types present within the stream segment using a standardized dip net sampling method. Productive habitat types include riffles/rocky substrate, woody debris, undercut banks, overhanging vegetation, aquatic macrophytes, and leaf packs. Biological data are used to calculate separate fish and macroinvertebrate index of biological integrity (IBI) scores. These scores are then compared to results obtained from least-disturbed regional reference sites in order to assess the condition or ecological health of the stream.
Chemical and physical habitat data collected from each monitoring site are used to evaluate the extent and severity of any environmental stressors that may be impacting the biological communities. For instance, if biological data indicate that a stream is in poor condition, the chemical and physical measurements can be used to determine potential causes of the impairment. With this information, a strategy for improving the condition of the stream can then be formulated.
For more details on the methods and equipment used to collect data at the random monitoring sites see the following standard operating procedure (SOP) documents:
Results
Currently, the results from two of Minnesota’s ten major river basins have been analyzed and compiled in separate reports. However, it is anticipated that once IBIs have been developed for all regions of the state, a complete analysis of all major river basins will be incorporated into a single report on the condition of rivers and streams in Minnesota.
