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Eutrophication is the state with high nutrient concentration, which can cause plant growth including algae blooms. Nutrients drain into the Baltic Sea from agricultural lands and drainage systems, from weak or insufficient wastewater treatment, from contaminated industrial waste water, and even from the rain water, if it contains dust, unburnt oil from the roads and parking areas or other pollutants washed from hard coverings or accumulated in the air. High nutrient level into the natural waters affects the human economic activity. An inexpensive way to reduce nutrient load to water objects from agricultural lands is formation of constructed wetlands or restoration of natural wetlands. The main tasks of this study were to describe the processes in constructed wetlands, the conditions of construction and to analyze the capacity of constructed wetlands to reduce nutrients and suspended particles inflow to open waters. Another task was to determine nutrient retention efficiency in the Mežacīruļi constructed wetland in Latvia by the descriptive methods of mathematical statistics and Wilcoxon T test method and to compare the obtained results with ones from Bergaholm constructed wetland in Sweden. In the end of this article, the evaluation of efficient theoretical design parameters for constructed wetlands in Latvia is made. The research results indicate that the Mežacīruļi constructed wetland does not provide the sufficient nutrient retention. To improve the nutrient retention efficiency, it would be necessary to develop it, e.g., to increase the amount of vegetation, such as planting Typha spp. or to make a small constructed wetland extension to a depth of 0.3 m for Carex spp. to stabilize the sediments. [ABSTRACT FROM AUTHOR]