Agenda - Public Works Committee - 09/16/2014

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Grass Lake has a catchment area of 178 acres. This is a small -sized watershed relative to the size of the lake (5:1 watershed: lake area ratio). With such a small catchment area, Grass Lake has a very small water load - meaning that Table 1. Land use composition water drainage into Grass Lake, either by ground or Land use Grass Lake catchment NCHF typical land land use percentage use percentage surface water, is minimal. One of the main concerns in regards to Developed 57 2-9 water load is the amount of nutrients present within the load, which could affect water quality and 5 22 50 Cultivated (Ag) clarity. The nutrient content of any water load Pasture & Open 5 11-25 depends upon land use within the catchment area. Forest 12 6-25 Highly developed or cultivated land often Water & Wetland 21 14-30 contributes higher amounts of nutrients (fertilizers, erosion and organic debris such as leaves and grass clippings) than forest, pasture or wetland areas, which capture nutrients and sediment before they enter lakes. Grass Lake has the potential for an elevated nutrient content water load since its catchment area is highly developed - 50% higher than typical lakes located in the NCHF ecoregion (Table 1). The catchment area is balanced out by a below average percentage of cultivated and pastureland and an average percentage of forest, water and wetland (Figure 2). Lake mixing and stratification Lake size, depth and the shape of the basin, affect whether a lake stratifies (forms distinct temperature layers) and how it mixes, which have a significant influence on water quality. Deep lakes that stratify during the summer months fully mix, or turn over, twice per year; typically in spring and fall. Shallow lakes (maximum depths of 6 meters or less), in contrast, typically do not stratify and mix continuously. Lakes with moderate depths may stratify intermittently during calm periods, but mix during heavy winds and during spring and fall. Mixing events allow nutrient -rich lake sediments to be re -suspended, which, under high temperature, can introduce phosphorus into the water where it may encourage the growth of algae. As a result, lakes that continuously mix are at more risk of developing algal blooms than deeper lakes that stratify. Lakes that strongly stratify often have little or no oxygen near the lake bottom. Low oxygen can allow phosphorus to be released from the lake sediments, which is another way nutrients are introduced to the water and can stimulate the growth of algae after the fall turn over. To determine if a lake stratifies or not, water temperature and dissolved oxygen are measured throughout the water column (surface to bottom) at selected intervals (e.g. every meter) several times during the open -water season. These measurements, called "profiles", will reveal specific patterns if the lake stratifies and will also show how oxygen changes with depth. The small size and shallow conditions of Grass Lake led to frequent mixing during 2012 and 2013 — typical for a lake of this size and depth. Lakes that mix frequently are referred to as polymictic. Both 2012 and 2013 temperature and dissolved oxygen profiles for Grass Lake show consistent uniformity in readings from surface to lake bottom during the monitoring season, with no stratification taking place in either year. A DO concentration of 5 milligrams per liter (mg/L) or more is required to maintain a healthy game fish population. The Department of Natural Resources has not conducted a fish survey on Grass Lake, so fish population and species diversity information is not available at this time, but DO concentrations in Grass Lake are generally high enough to support a fish population. Water quality Nutrients, typically phosphorus and nitrogen, are the primary drivers of algal productivity in lakes. In general, high nutrient levels increase the likelihood that nuisance algal blooms will grow and that lakes will not support aquatic recreational uses; however, there are other factors at play that also must be considered. For this reason, it's important to collect information on suspended solids, temperature, dissolved oxygen, and a number of other parameters. All May - September water chemistry data for Grass Lake gathered in 2012 and 2013 were averaged (referred to as "summer Grass Lake (02-0113) • August 2014 Minnesota Pollution Control Agency 2