Lake turnover is a natural process driven by seasonal temperature changes. It involves the mixing of a lake’s distinct water layers, altering the aquatic environment. These shifts profoundly impact aquatic life, particularly fish, as their habitats and physiological conditions change rapidly. Consequences for fish range from stress and behavioral adjustments to mortality.
Understanding Lake Turnover
Lakes develop distinct thermal layers, known as thermal stratification, during warmer months. The uppermost layer, the epilimnion, is warm, less dense, and well-oxygenated by atmospheric exchange and photosynthesis. Below this, the thermocline (or metalimnion) is a middle layer with a rapid temperature decrease, acting as a barrier to mixing. The deepest layer, the hypolimnion, contains colder, denser water with lower dissolved oxygen due to decomposition and limited atmospheric contact.
Seasonal temperature changes disrupt this stratification. As air temperatures cool in fall, surface water loses heat and becomes denser. This cooler, denser water sinks, displacing warmer, less dense water below. In spring, as ice melts and surface water warms to approximately 4 degrees Celsius (39 degrees Fahrenheit), it reaches maximum density and sinks. Wind aids this process by creating circulation that mixes the water column.
This mixing, or “turnover,” redistributes water, breaking down thermal layers. It circulates dissolved oxygen from the surface throughout the water column, including previously oxygen-depleted bottom layers. Simultaneously, nutrients accumulated in the hypolimnion are brought to the surface, influencing the lake’s chemical properties.
Immediate Effects on Fish
Lake turnover introduces immediate physiological stress for fish due to rapid environmental changes. A primary challenge involves dissolved oxygen levels. During stratification, the hypolimnion can become severely oxygen-depleted, approaching anoxic (zero oxygen) or hypoxic (low oxygen) conditions. When turnover occurs, this oxygen-poor water mixes throughout the lake, causing a sudden reduction in available oxygen. Fish require dissolved oxygen levels between 5-6 parts per million (ppm) for optimal health and growth; levels below 3 ppm can be stressful, with mortality occurring at concentrations below 2 ppm for most species.
Fish subjected to low oxygen may exhibit distress, such as gasping at the surface (“piping”), lethargy, and reduced metabolic function. Prolonged exposure to insufficient oxygen can lead to gill damage and oxidative stress. Turnover can also expose fish to sudden temperature shifts. For instance, cold, oxygen-depleted bottom water mixing with warmer surface water can cause thermal shock, compromising fish health.
Chemical changes also contribute to stress. Decomposition of organic matter in the oxygen-depleted hypolimnion can lead to accumulation of toxic gases like hydrogen sulfide, methane, and ammonia. When these gases are brought to the surface during turnover, they can pose an additional threat; hydrogen sulfide is particularly harmful to fish.
Fish Behavior and Survival
Fish exhibit various behavioral responses to environmental shifts caused by lake turnover, seeking more favorable conditions. When oxygen levels drop, fish may initially increase activity, swimming rapidly or erratically to escape hypoxic zones. If low oxygen conditions persist, they reduce activity significantly to conserve energy and decrease oxygen demand. Some species may resort to aquatic surface respiration, positioning themselves just below the water’s surface to breathe from the oxygen-rich film in contact with the air.
Habitat relocation is a common survival strategy, with fish seeking pockets of water with higher dissolved oxygen or more suitable temperatures. For example, during low oxygen events, fish might move to shallower areas or gravitate towards inflowing streams where oxygen levels are higher. However, such movements can increase vulnerability to predation, as being near the surface or in unfamiliar areas can expose them to aerial or terrestrial predators.
Severe turnover, especially that leading to widespread and prolonged oxygen depletion, can result in large-scale fish mortality, commonly known as fish kills. The susceptibility to these events varies among species; some fish, like northern pike and fathead minnows, demonstrate a higher tolerance to low oxygen than others, such as coldwater species like trout and salmon. While some fish can adapt physiologically by increasing red blood cell count or gill surface area, survival depends on the severity and duration of hypoxic conditions.
Long-Term Consequences for Fish Populations
Beyond immediate impacts, repeated lake turnover can have lasting consequences for fish populations. Chronic stress from frequent or severe turnovers can negatively affect fish growth rates and reproductive success over time. Fish may experience reduced feeding and impaired physiological functions, leading to slower development and decreased fitness. This can influence offspring numbers and juvenile fish survival rates, as juveniles are often more sensitive to environmental stressors than adults.
These environmental pressures can also alter the distribution of certain fish species. Over time, species less tolerant to fluctuating oxygen and temperature may decline, while more resilient species might thrive, leading to shifts in fish community structure. Such changes can impact the lake’s biodiversity and ecosystem balance. For recreational or commercial fisheries, these long-term alterations can affect the abundance and availability of target species, impacting fishing opportunities and yields.