Lake turnover is a natural process where the entire water column, from surface to bottom, mixes. This phenomenon is common in many lakes, particularly in temperate regions. It is a fundamental physical process driven by changes in water temperature and density, influencing the distribution of elements within the lake.
The Mechanics of Lake Turnover
Water density changes with temperature, reaching its maximum at approximately 4 degrees Celsius (39.2 degrees Fahrenheit). This property governs lake stratification and mixing. During summer, temperate lakes develop distinct layers due to solar heating. The warmest, least dense water forms the uppermost layer, the epilimnion, while cooler, denser water settles at the bottom, forming the hypolimnion. A transitional zone, the metalimnion or thermocline, separates these layers, exhibiting a rapid temperature decrease with depth.
As autumn progresses, surface waters cool and increase in density. This denser surface water sinks, displacing warmer, less dense water below. This continuous sinking and displacement, aided by wind, gradually erodes summer stratification. The entire water column eventually reaches a uniform temperature, typically around 4 degrees Celsius. At this point, the water throughout the lake has a similar density, allowing for complete vertical circulation from surface to lakebed. This autumnal mixing event effectively breaks down the summer’s thermal layering.
In winter, if the lake freezes, inverse stratification can occur. The coldest water, near 0 degrees Celsius, forms ice and floats on top, while slightly warmer, denser water (closer to 4 degrees Celsius) remains at the bottom. When spring arrives and the ice melts, the surface water warms to 4 degrees Celsius. This warming surface water becomes denser and sinks, initiating another period of complete vertical mixing. This spring turnover ensures the uniform distribution of water properties throughout the lake after the winter period.
Seasonal Patterns of Turnover
In temperate regions, lake turnover typically follows distinct seasonal patterns, with two main mixing periods: spring and fall. Fall turnover begins as surface water temperatures drop below summer highs, eventually reaching approximately 4 degrees Celsius, the temperature of maximum water density. This density change causes surface water to sink, initiating mixing across the entire water column. The consistent density throughout the lake allows wind to effectively drive this complete circulation.
Spring turnover occurs after winter ice melts and surface waters warm from near 0 degrees Celsius to 4 degrees Celsius. As surface water approaches this temperature of maximum density, it sinks, displacing cooler, less dense water from deeper parts. This leads to thorough mixing of the water column, distributing dissolved gases and nutrients. Lakes in tropical or subtropical regions often exhibit different mixing patterns or may not turn over at all. Their water temperatures remain relatively stable, preventing the significant density differences required for full vertical circulation. Some tropical lakes might experience less frequent or partial mixing events driven by strong winds.
Impacts on Lake Ecosystems
Lake turnover influences the chemical and biological characteristics of the lake ecosystem. A primary impact is the redistribution of dissolved oxygen. During stratification, oxygen can become depleted in the deeper, colder hypolimnion due to decomposition of organic matter and lack of atmospheric exchange. Turnover replenishes these deep waters with oxygen from the surface, creating more habitable conditions for fish and other aquatic organisms that require oxygen. This oxygenation helps prevent the accumulation of anaerobic conditions at the lakebed.
Turnover also plays a significant role in nutrient cycling. During stratification, nutrients like phosphorus and nitrogen can accumulate in the hypolimnion, often released from bottom sediments under low-oxygen conditions. When the lake mixes, these accumulated nutrients are brought to the surface waters. This influx can stimulate primary productivity, potentially leading to increased phytoplankton growth, including algal blooms, in the epilimnion. While this nutrient redistribution supports the base of the food web, excessive nutrient loading can lead to detrimental blooms.
The mixing event homogenizes the lake’s temperature, eliminating sharp gradients from stratification. This uniform temperature influences the metabolic rates and habitat selection of aquatic species. Turnover resets the lake’s internal chemistry, creating a more uniform environment that supports diverse biological communities.
Lakes That Don’t Turn Over
Not all lakes experience regular, complete mixing. Meromictic lakes, for instance, exhibit permanent stratification, where their water columns never fully mix. This occurs due to specific physical or chemical conditions that prevent seasonal turnover. Factors contributing to meromixis include extreme depth, which can make complete mixing energetically unfavorable, or protected basins that shield the lake from significant wind action. A strong density gradient, often caused by high concentrations of dissolved salts in the deeper waters, can also create a permanent barrier to mixing.
In meromictic lakes, the deepest layer, called the monimolimnion, remains isolated from the upper layers and the atmosphere. This isolation leads to a persistent lack of oxygen in the monimolimnion, creating an anoxic environment. Unique microbial communities adapted to these conditions thrive in these deep layers. The upper, oxygenated layers, known as the mixolimnion, behave more like a typical lake, but the continuous separation of the water column leads to distinct ecological characteristics and biogeochemical processes compared to regularly mixing lakes.