Direct answer: Lake turnover is the seasonal mixing event in which the entire water column of a stratified lake becomes vertically homogeneous, redistributing oxygen, nutrients, and temperature throughout the lake. In temperate North America, dimictic lakes turn over twice annually — at spring ice-out and again in fall as surface water cools.
How Turnover Works
Turnover is driven by water density. Water is densest at 4°C; both warmer and colder water is less dense. As surface water cools in autumn from summer temperatures toward 4°C, surface density increases, eventually exceeding the density of the underlying hypolimnion. The now-denser surface water sinks, displacing deeper water upward, and wind-driven turbulence completes the mixing. Once temperature is uniform throughout the water column, wind continues to maintain a fully mixed state until winter ice cover (or, in non-freezing southern lakes, until the next spring warming).
The timing of fall turnover varies by latitude and lake morphometry. Northern Minnesota lakes typically turn over in late September to mid-October; lakes in the mid-Atlantic states turn over in November; deep southern reservoirs may turn over in December or not fully turn over in mild winters. Spring turnover in dimictic lakes occurs immediately after ice-out — a brief but ecologically critical event as the lake reorganizes from winter inverse stratification through a uniform 4°C profile before establishing the summer thermal stratification regime.
Ecological Effects of Turnover
Fall turnover redistributes the accumulated chemistry of summer stratification throughout the lake. Anoxic hypolimnetic water — rich in dissolved phosphorus, ammonia, iron, manganese, and hydrogen sulfide — mixes with the surface layer. The sudden whole-lake oxygen redistribution typically restores hypolimnetic dissolved oxygen to near-saturation within days, but the simultaneous nutrient mixing can trigger short-lived algal blooms ("autumnal turnover blooms") that consume the redistributed nutrients.
The most severe ecological effect of turnover is the potential for fall fish kills. When stratification breaks down rapidly under storm wind conditions, anoxic hypolimnetic water can mix into the surface layer faster than reaeration restores oxygen — producing brief whole-lake oxygen depletion that kills fish. Hydrogen sulfide and ammonia released from the previously anoxic hypolimnion are directly toxic to fish at high concentrations. Eutrophic lakes with deep, severely anoxic hypolimnia (described in the oxygen depletion guide) are at highest risk for turnover-associated fish kills.
Turnover and Aquatic Weed Management
Fall turnover is a critical timing consideration for late-season aquatic weed treatments. Herbicide applications in early autumn, before turnover, may concentrate in the upper water column and produce uneven distribution. Applications after turnover distribute more uniformly through the mixed water column but may be less effective against actively growing plants that have entered fall senescence. State permit requirements often constrain late-season treatment windows for these reasons — see permits and regulatory considerations.
The nutrient pulse from fall turnover also influences spring aquatic weed productivity. Phosphorus mixed from the hypolimnion in fall becomes available for the following spring's plant and algal growth, contributing to early-season productivity that gives invasive species their seasonal competitive advantage. Source-control programs targeting watershed nutrient inputs reduce the long-term phosphorus pool that drives this annual cycle.
Stratification and Mixing Regimes
Not all lakes turn over twice annually. Limnologists classify lakes by their mixing regime: dimictic lakes mix twice yearly (spring and fall — the typical temperate pattern); monomictic lakes mix once yearly (warm-monomictic lakes in subtropical regions that do not freeze; cold-monomictic Arctic lakes that remain ice-covered year-round); polymictic lakes mix many times per year (shallow lakes where stratification is repeatedly broken by wind); and meromictic lakes never fully mix (their deep bottom waters remain permanently isolated, often due to salinity or extreme depth). The mixing classification predicts the lake's vulnerability to internal nutrient loading and the appropriate management strategies.
Frequently Asked Questions
What causes lake turnover?
Lake turnover is caused by changes in water density that overcome stratification. In fall, surface water cools toward 4°C (the density maximum), becomes denser than underlying water, and sinks — initiating whole-water-column mixing that is completed by wind turbulence. In spring after ice-out, ice melt water at 0°C warms toward 4°C, briefly producing uniform density throughout the lake that allows wind to fully mix the system.
When does fall turnover happen?
Fall turnover timing varies with latitude and lake depth. Northern Minnesota and similar lakes typically turn over in late September to mid-October; mid-Atlantic and Midwest lakes turn over in October–November; southern reservoirs may turn over in November–December or skip turnover entirely in mild winters.
Does turnover cause fish kills?
Fall turnover can trigger fish kills in severely eutrophic lakes when anoxic hypolimnetic water mixes rapidly into the surface layer faster than reaeration restores oxygen. Hydrogen sulfide and ammonia released from the anoxic hypolimnion are also directly toxic to fish. Healthy oligotrophic or mesotrophic lakes rarely experience turnover fish kills.
How do I know if my lake has turned over?
The most reliable indicator is a uniform temperature profile from surface to bottom — easily measured with a thermometer on a weighted line at multiple depths. Other indicators include sudden surface clarity changes, distinctive odors from previously anoxic hypolimnion water reaching the surface, and a temporary algal bloom in the days to weeks following turnover.
Do southern lakes turn over?
Most southern U.S. lakes turn over at least once annually as winter cooling reduces surface temperatures sufficiently to break summer stratification. Some shallow southern reservoirs are polymictic (mix many times per year). A small number of deep southern reservoirs in mild winters may not fully turn over annually — these tend to develop chronic hypolimnetic water quality problems.
Should I treat aquatic weeds before or after fall turnover?
Most aquatic weed treatments are most effective during active spring and summer growth, well before fall turnover. Late-season treatments after turnover are less effective because target plants have entered senescence and are no longer actively metabolizing herbicides. Permit windows often close before fall turnover. Consult your state aquatic weed permit guidance for specific timing.
References
- Wetzel, R.G. (2001). Limnology: Lake and River Ecosystems, 3rd ed. Academic Press, San Diego.
- Hutchinson, G.E. (1957). A Treatise on Limnology, Volume 1: Geography, Physics, and Chemistry. John Wiley & Sons.
- Kalff, J. (2002). Limnology: Inland Water Ecosystems. Prentice Hall, Upper Saddle River, NJ.
- Magnuson, J.J., et al. (2000). Historical trends in lake and river ice cover in the Northern Hemisphere. Science, 289, 1743–1746.
- Cooke, G.D., et al. (2005). Restoration and Management of Lakes and Reservoirs, 3rd ed. Taylor & Francis.
Ten-Year Lake Management Plan: Lake Wingra, WI
Lake Wingra, a 342-acre urban lake in Madison, WI, developed a comprehensive 10-year management plan coordinating the City of Madison, University of Wisconsin, and adjacent neighborhood associations. The plan addressed Eurasian watermilfoil, curly-leaf pondweed, and purple loosestrife through an integrated approach including targeted herbicide treatment, mechanical harvesting, native plant restoration, and public education.
Key outcome: The structured multi-agency planning process secured consistent funding across multiple budget cycles, a key advantage over ad hoc management. Native plant restoration efforts showed measurable progress in designated restoration zones within three years of initiation.
The ecological impact section helped our team explain to county commissioners why early intervention matters. The oxygen depletion data alone secured funding for our early-detection monitoring program.
Donna Whitfield State Wildlife Biologist, GA · Okefenokee regionWe used the integrated management framework from this site to structure our Eurasian watermilfoil control program. After three seasons we've reduced lake-wide coverage by 78% on our 340-acre water body.
Susan Thibodeau Lake District Manager, MN · Crow Wing County