Water quality condition monitoring — Random wetlands survey in Minnesota
Random or probabilistic surveys simply refer to process of randomly selecting sites to monitor. As a result, an unbiased estimate of the condition of the resource, in this case wetlands, 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
In 2006, a comprehensive strategy for the monitoring, assessment, and mapping of Minnesota’s wetlands was developed by an interagency workgroup comprised of both state and federal agency partners (see Minnesota Comprehensive Wetland Assessment Monitoring and Mapping Strategy). One of the recommendations of this effort was a random survey to assess status and trends in wetland quantity and quality statewide. The Minnesota Department of Natural Resources is responsible for conducting the wetland quantity (i.e., acres) survey while the MPCA is responsible for conducting the wetland quality survey. Both surveys are based on 1 mi2 plots randomly selected throughout the state. Aerial photographs are taken over each plot and used to create wetland maps that can provide estimates of wetland quantity statewide. These maps also serve as the basis for selecting wetland sites for the quality survey. Since we are essentially taking a sample of wetlands from a random sample of 1 mi2 plots, the wetland quality survey design is considered a two-phase sample design.
The wetland quality survey follows a rotating ecoregion schedule to produce statewide results every three years. During at least the first three year iteration of the survey, only depressional wetlands with predominantly emergent vegetation (e.g., marshes) and open water wetlands (e.g., ponds) will be monitored. As assessment tools are developed for other types of wetlands they will become incorporated into the quality survey. Approximately 60 wetland sites are sampled each year in one of three major ecoregions (see map of sites and schedule). For the 2009 field season, the MPCA anticipates monitoring 60 wetlands randomly located within the Mixed Wood Shield ecoregion.
Map of 2009 Monitoring Sites At each monitoring site, the condition of the wetland is characterized by collecting biological, chemical, and physical data. The composition and structure of the plant community at each study site is characterized using releve sampling methodology. Releve sampling is a technique where an observer selects an area representative of the overall community to place a sampling plot (or multiple plots) where plant data can be quantified. Plots typically straddle the emergent/aquatic vegetation interface. Aquatic macroinvertebrates are also sampled from the emergent/aquatic vegetation interface using D-frame dip nets. Samples are collected by sweeping the net through the water column over a horizontal distance of approximately 1 m. Several sweeps at various locations within the wetland are collected and combined into a single sample. Two macroinvertebrate samples are collected in this manner during each site visit. Biological data are used to calculate separate plant 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 each wetland survey site.
Chemical and physical 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 wetland is in poor condition, the chemical and physical measurements can be used to determine potential causes of the impairment. Water chemistry measurements include dissolved oxygen, temperature, specific conductance, pH, nutrients, chloride, and sulfate. Water clarity is also measured using a transparency tube.
An assessment of wetland functions is also conducted at each monitoring site using the Minnesota Routine Assessment Method (MnRAM). This method uses information obtained in the office using GIS as well as observations recorded on site to assess how well the wetland is performing several functions, including: floodwater retention, maintenance of characteristic vegetation, maintenance of characteristic wildlife, maintenance of characteristic hydrologic regime, and protection of downstream water quality. The Minnesota Board of Water and Soil Resources maintains the MnRAM Web page which is an excellent resource for those interested in utilizing this functional assessment tool.
For more details on the methods and equipment used to collect data at wetland survey monitoring sites see the following standard operating procedure (SOP) documents:
Results
Sampling from the first iteration of the wetland quality survey is expected to be completed in August of 2009. Statewide results will therefore not be available until spring 2010.
