By Kevin Rose
Why are lakes different colors?
Lakes exist in many sizes and shapes, but often the most obvious characteristic of a lake is its color (Figure 1). The differences in color or transparency between lakes can be rather striking, but even for a single lake, color changes can occur over time. The color of lake can tell you many things about the water body (e.g., including nutrient load, algal growth, and water quality) and also about the surrounding landscape.
There are three main categories of lake color: blue water lakes, green water lakes and brown water lakes. Lake color and clarity can measured using a Secchi disk or underwater light sensors such as a LiCor PAR sensor.
Blue water lakes
Blue water lakes contain low concentrations of algae and other substances, resulting in high clarity and a deep blue color. Water molecules absorb longer, visible wavelengths (e.g. red light, 600-700 nm) while shorter, blue wavelengths (< 500 nm) pass deeper into the water column. These short wavelengths scatter to create a deep blue color in clear lakes (Figure 1a, 2). Blue lakes are common in areas with fast draining soils and small lake watersheds. These lakes usually have very low algal growth, supporting few fish unless the lakes are stocked. The deep blue color of some lakes is a testament to the pristine character of the water and low human impact in the surrounding watershed. These high quality bodies of water are often the focus of local conservation efforts.
Green water lakes
Green water lakes commonly have high concentrations of chlorophyll-containing algae which can give water a green color (Figures 1b, 3). Chlorophyll can be measured with sensors such as the YSI chlorophyll probe. Green lakes are often eutrophic and typically contain more harmful algal blooms than other types of lakes. Activities such as farming or septic system failure can increase the green color of lakes through nutrient inputs which act as a fertilizer for algae. The high productivity of green lakes usually enables them to support more fish, but the poor water quality conditions can depress dissolved oxygen levels in hot summer months; these conditions can cause fish kills where oxygen drops too low for fish to survive.
Brown water lakes
Brown water lakes contain high amounts of tea-like substances, known as dissolved organic matter (Figure 1c, 4). Typically, brown lakes are surrounded by forests or wetlands. Dense forests provide dark organic material that dissolves in lake water like a teabag. This dissolved organic material stains the water brown and shades the underwater world.
Overall, brown water lakes tend to be light-limited. The algae in these lakes survive through certain adaptations that allow them to adjust to low light levels. These lakes can also sometimes be acidic and contain few fish or other organisms.
Can lakes change color?
Both natural and human activities can cause changes in lake color and clarity. The development of communities or the use of agricultural fertilizers around lakes often reduces water clarity and adds nutrients, shifting lakes from blue water to green water. Lakes can also naturally become more eutrophic and green over time. As lakes age over centuries, nutrients, sediment and plant material slowly build up. This natural process is much slower than changes caused by human impacts.
While human impacts often change lakes from blue to green, conservation and protection can improve the clarity and color of lakes. In areas where water quality has been degraded by pollution, eutrophication, or changes in land use, community action to improve water quality through enhanced laws and zoning can improve water quality and shift lakes from green water to blue water. This process is often difficult, however, as lakes tend to remain in the color state in which they currently exist.
Seasonally, lakes can change in color. In many lakes, rapid algal growth in the spring months produces a green color. However, this period is usually followed by a clear water phase (i.e., blue water lake) as zooplankton emerge and consume algae. The length of clear water phases can vary and is determined by the ecological interactions among aquatic organisms. In addition, sunlight can bleach organic matter in the same way that materials left outside for too long become bleached and faded. The bleaching typically follows day length and lakes can be most transparent (e.g. most blue in color) when the most bleaching occurs around the summer solstice.
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