Water density and turnover
The weight or density of water increases as it cools – to a point. All other fluids continue to become heavier as they cool. Water is not heaviest at its freezing point of 0oC (or 32F). Instead, water is heaviest at 4°C, or about 39F. Water gets lighter between 4oC and 0oC, becoming lightest when it freezes. Thus, ice floats on top of a layer of slightly warmer water. This is important, not just so that ice floats in our drinks. Because the coldest ice floats, fish and other critters can survive in warmer water underneath the ice and the lake will not freeze solid.
Just as ice floats, warm water also floats on top of cooler water. In the summer the warmest water is again at the surface. Water also continues to become lighter the warmer it gets. So very warm water is quite buoyant compared to just slightly warm water. Water is a good thermal buffer. These different densities based on the warm temperatures can stratify and stay there – just like in cold weather a layer of ice will last on top of a layer of water. Most of the winter and most of the summer a typical lake is stratified into layers by persistent densities – just like oil and water form layers in unmixed salad dressing.
Limnologists give these layers names. The top layer is the epilimnion, and the bottom layer is the hypolimnion. The small area of rapidly changing temperature and density between these two layers is called the thermocline.
Twice a year as the lake warms in spring and cools in fall, the entire lake becomes the same temperature and mixes. That’s called turnover, and it has important lake impacts. It brings nutrients from the bottom of the lake up into the top waters, often resulting in an algae bloom and murky waters.
Click for a you-tube video that helps explain water density.
Water’s hydrological cycle
Water wondrously returns to the land from the salt ocean as fresh water in the form of precipitation. Water is present as water vapor, in streams and lakes, and in the ground in aquifers. Water flows through streams about 0.7 mph on average, and flows in aquifers in minutes per inch. How fast does water flow through lakes? The amount of flow that enters a lake depends upon the size of the watershed, the average rainfall, the land use, and the temperature. Flow through lakes is typically seasonal, with high flows as the snow melts in the spring, low flows during the summer because of high evaporation rates, and then increasing flows in fall and winter.
In our lakes, the water is estimated to take between 4 and 11 months to flow through our lakes. On average, our area gets about 49.7” of rain a year, but of course that varies. In 2011, we received 71.8″, and in 2016, only 40.6″.
A watershed is defined by any point. All the water that will flow over or through land to that point is that point’s watershed. But usually we think of the watershed for a body of water such as a lake. Activities outside the watershed generally don’t affect the lake.
All activities inside the watershed affect lake conditions. Lakes reflect the quality of the activities that we carry out around them. Lakes can tell us how we are doing on balance in the watershed. Unfortunately, some of those who pollute in the watershed may have no connection with the lakes they are affecting and may be unaware of their impacts.
Those of us fortunate enough to live around the lakes have an obligation to adjust the way we live to protect our lakes rather than to adjust the lake to protect the way we live. This can include trying to reduce impervious surfaces, limiting pesticide and fertilizer use, and picking up pet waste.
Sunlight dictates most lake processes. Winter has few hours of sun, and the light hits the lake at an oblique angle. As the ice melts in the spring, light can start to penetrate deeper into the water, aided by its movement higher in the sky and longer hours of daylight. Eventually the sun will warm the top layer enough so that spring turnover will occur.
Sunlight determines the maximum depth of the littoral zone – the depth of the lake where enough light reaches to allow plants and algae to grow. That depth is called the compensation point, generally at about 1% of incident light. If nutrients are available, sunlight can support algae across the surface of a lake. If enough sunlight hits suitable lake bottom, plants will grow. Since most lakes have adequate nutrients and soil, lake plants are a part of most lake ecosystems.
On the lake we typically measure lake clarity by the use of a secchi disk, a black and white circle lowered on a string. For comparison, the secchi disk disappears from sight at about 10% of incident light. Since light heats the water, the depth that the sunlight can penetrate will determine the thermocline. The depth of the thermocline, in turn, will determine how much of a nutrient rich lake will become devoid of oxygen.
Stratification of a lake in the summer creates two very different worlds under the water surface. The top layer or the epilimnion has light, warmth, and growth of plants. The thermocline serves as a barrier between the two layers. The hypolimnion is dark, below the depth at which light can penetrate. In nutrient rich lakes, this area quickly becomes devoid of oxygen as organic debris from the upper layer strips oxygen out of the water column, and in the dark, no plants or algae can grow to replace that oxygen.
The absence of oxygen, or anoxia, is not only bad news for fish and invertebrates. In the absence of oxygen, the water chemistry changes. This is the real devastation that comes from a eutrophic or nutrient rich lake. The changed chemistry encourages the sediment to release nutrients into the water column. When the lake mixes at turnover, those nutrients are distributed into all of the water and become available for algae to use in the top warm, bright layer.
Each year the precipitation, sunlight, and temperature characteristics can change. This in turn can mean that lake conditions will change each year. Lakes are complex and trends can be hidden by annual variability. Lake ecosystems operate on a long time scale that can frustrate our desire for quick restoration and improvements.