Lake turnover is a natural phenomenon occurring seasonally in many lakes, involving the mixing of their water layers. This process is fundamental to aquatic ecosystems, yet it raises questions about its effects on fish populations. Understanding this transformation is important for grasping its impact on fish. This natural mixing event can sometimes create challenging conditions for aquatic life.
The Mechanics of Lake Turnover
Lakes in temperate regions commonly exhibit thermal stratification, where water forms distinct layers based on temperature and density. During summer, the uppermost layer, known as the epilimnion, is warm and less dense due to solar heating. Below this lies the metalimnion, or thermocline, a transitional zone characterized by a rapid decrease in temperature with increasing depth. The deepest layer is the hypolimnion, which remains cold and dense, largely unaffected by surface temperatures.
Water’s unique property of being densest at approximately 4 degrees Celsius (39 degrees Fahrenheit) drives this stratification and subsequent mixing. As seasons change, the temperature gradients within the lake shift. This equalization of temperature and density throughout the water column allows for the breakdown of stratification.
With the aid of wind, the entire lake mixes from top to bottom, a process known as fall turnover. A similar event, spring turnover, occurs as ice melts and surface waters warm to 4 degrees Celsius, leading to another full circulation. This cyclical mixing redistributes oxygen and nutrients throughout the lake.
Consequences for Fish Health
Lake turnover introduces physiological shifts and stress for fish due to the disruption of stable environmental conditions. As the lake layers mix, dissolved oxygen levels can fluctuate rapidly. Water from the deep hypolimnion, often low in oxygen, can be brought upwards and mixed with more oxygenated surface waters. This sudden redistribution can reduce overall dissolved oxygen available to fish.
Temperature also experiences sudden changes during turnover, which can be unsettling for fish accustomed to stable thermal zones. The mixing can stir up sediments and alter the distribution of nutrients and pH levels throughout the water column. These environmental stressors can cause fish to experience discomfort, disorientation, or reduced feeding activity. Prolonged exposure to such conditions can increase a fish’s susceptibility to diseases.
When Turnover Becomes Lethal
Lake turnover can lead to fish mortality under severe conditions. The primary cause of fish kills during turnover is often a drastic reduction in dissolved oxygen. During stratification, particularly in nutrient-rich lakes, the hypolimnion can become severely depleted of oxygen, reaching anoxic (zero oxygen) or hypoxic (low oxygen) conditions due to organic matter decomposition at the lakebed.
When turnover occurs, this oxygen-depleted water from the bottom is mixed throughout the water column. This rapid infusion of deoxygenated water can lower the oxygen concentration to levels insufficient for fish survival, leading to suffocation. The impact is often greater in smaller lakes or those with substantial organic sediment accumulation.
Anoxic conditions in the hypolimnion can lead to the accumulation of toxic gases, such as hydrogen sulfide, ammonia, and methane, produced by anaerobic bacteria decomposing organic material. When these gases are brought to the surface during turnover, they can poison fish. Hydrogen sulfide, recognizable by its rotten-egg smell, is particularly harmful as it interferes with a fish’s ability to utilize oxygen at a cellular level, even at low concentrations. Rapid temperature changes during turnover can induce thermal shock in sensitive fish species, contributing to mortality.
Fish Adaptation and Survival
Not every lake turnover event results in significant fish mortality, and fish species exhibit varying degrees of tolerance to the changes. Fish species possess inherent physiological adaptations that allow them to cope with fluctuations in oxygen and temperature. Some species are naturally more tolerant of lower oxygen concentrations than others, enabling them to endure the temporary reduction in dissolved oxygen during turnover.
Fish may also display behavioral responses to mitigate the effects of changing water conditions. For instance, they might reduce their activity levels to conserve energy and oxygen, or engage in aquatic surface respiration, which involves gulping water from the surface layer where oxygen is more abundant. The characteristics of the lake also play a role in influencing fish survival. Deeper lakes with less organic matter or those with constant inflows of fresh, oxygenated water may experience less severe turnover events.
Shallow lakes, particularly those frequently mixed by wind, may not stratify significantly and thus experience less pronounced turnover. The presence of cold-water refugia or areas with consistent oxygen levels can provide sanctuaries for fish during periods of stress. While turnover is a natural and sometimes stressful event, fish populations often have mechanisms and environmental conditions that support their survival.