The Big Lake is "Turning Over", which Means What?

[BillTex]

For those of us that fish off-shore, particularly for tuna-understanding the thermocline is crucial to success.

[fatlazyless]

Did you know the cold water fish seek out their cold water, and the warm water fish seek out their warm water.

Lake Winnipesaukee is both a warm water and a cold water fishing lake.

In the winter do the warm water fish escape to a Florida lake, no they tough it out in Lake Winnipesaukee for the cold winter. ......

[Winilyme]

Originally Posted by BillTex

For those of us that fish off-shore, particularly for tuna-understanding the thermocline is crucial to success.

Now I know why I don’t fish off-shore.

[rsmlp]

Lake Turnover — One‑Page Guide for NH & Northern Lakes
Winnipesaukee specifics (field‑ready)

Lake type: Large, cold‑temperate dimictic lake — mixes in spring (post ice‑out) and fall. Typical pattern confirmed locally by Laconia Daily Sun coverage.

Size & shape: ~71 sq mi surface area; center basin is The Broads; shoreline ~288 mi; elevation ~504 ft. Max depth ~180 ft (avg ~43 ft). Deepest/most exposed waters (The Broads) behave differently from semi‑enclosed bays.

Timing cues:

Spring mix: often late March–April as ice‑out approaches/occurs (long‑term mean ~April 18; varies widely year to year).

Fall mix: commonly late Oct–Nov, driven by cooling + windy spells; shallow bays can de‑stratify earlier than the Broads.

Variability by basin: Wolfeboro/Tuftonboro and other shallower embayments can turn over days–weeks before the deep Broads; narrows and sills sometimes delay full‑lake homogenization.

Recent context: Cyanobacteria advisories have occurred in some near‑shore areas (e.g., Tuftonboro/Wolfeboro) in warm months; not a turnover signal per se, but can affect clarity and shoreline color.

(Sources for the above Winnipesaukee details kept concise for the one‑pager; ask 007 if you want the linked version.)

What it is (in plain English)

In summer, lakes separate into layers: warm top (epilimnion), a sharp temperature drop (thermocline/metalimnion), and cold bottom (hypolimnion).

In fall (and after ice‑out in spring), surface water cools toward ~4 °C/39 °F (the densest point). The density difference vanishes, wind mixes the whole water column — this is turnover.

Result: temperature becomes nearly uniform (isothermal); oxygen and nutrients redistribute top‑to‑bottom.

Seasonal rhythm (typical dimictic lake)

Spring mix → full‑lake mixing soon after ice‑out; clear‑ish water may briefly look brown/tea‑colored from diatoms and resuspended material.

Summer stratification → stable layers; deep water may lose oxygen in productive (eutrophic) lakes.

Autumn mix (turnover) → cool, windy weeks erase the thermocline; whole‑lake mixing.

Winter inverse stratification → if ice forms, coldest water sits on top; slightly warmer (~4 °C) water at depth.

Quick field checklist: “Did it just turn over?”

Temp profile: Little/no thermocline; temps roughly the same from surface to bottom.

Dissolved oxygen (DO): Deep DO rebounds toward surface levels.

Secchi depth / clarity: Often drops for a bit (fines & algae resuspended).

Context: A run of cool days + blustery winds just before the change.

Color & smell — what to expect

Murky brown/green water for days–weeks is common: mixing brings up fine sediments and nutrients; short diatom/algal pulses are typical.

Rotten‑egg smell (H₂S) can occur only if bottom waters were anoxic; many lakes have no odor at turnover.

Not all lakes behave the same

Dimictic: Mix twice/year (spring + fall). Common in deeper NH/ON lakes.

Polymictic: Shallow/windy lakes can mix frequently, even daily in cool periods.

Meromictic: Rare; salinity/density keeps deep water from ever fully mixing.

Myths vs facts (fast corrections)

Myth: “Turnover happens when the bottom warms and gas expansion breaks it up.”
Fact: Turnover is density + wind‑driven mixing; deep water stays cold.

Myth: “Color change proves turnover.”
Fact: It’s a clue, not proof; runoff and storms can do the same.

Myth: “It’s just about temperature.”
Fact: Density controls mixing; water is densest near 4 °C/39 °F.

Myth: “Isoclinetic is the term.”
Fact: Use isothermal or well‑mixed.

Angling & management tips (NH‑practical)

Fishing: During/just after turnover, fish roam more and may suspend mid‑water; expect bite patterns to reset. Give it several days after major mixing.

Monitoring: A cheap thermometer on a cord + a DO meter (if available) quickly confirms conditions.

Water quality: Brief turbidity after turnover is normal; clarity often improves again as the lake settles.

30‑second lake‑edge test

Check recent weather (cool, windy spell?).

Measure temp at 1 ft, mid‑depth, near‑bottom — are they similar?

Note clarity and any earthy/“tea” tint or light sulfur smell.
If 2–3 boxes are ticked, your lake likely just turned over.

Prepared for field use around New Hampshire and similar northern lakes. — 007

All set — I tailored the one-pager for Lake Winnipesaukee (timing, basins, size/depth, local nuances). Have a look in the canvas.

If you want the linked/cited version, here are the key sources I used for those specifics:

Turnover happens twice/year on Winnipesaukee (dimictic): local coverage, 2024–2025.
The Laconia Daily Sun
+1

Size/depth/basins (The Broads), shoreline: regional + reference pages.
NH Lakes Region Tourism Association
+1

Typical ice-out timing (avg ≈ April 18; big year-to-year swings): WMUR 2025.
WMUR

Recent cyanobacteria advisories (nearshore bays): AP report, June 2024.
AP News

Want me to add a tiny “when to expect turnover by basin” box (Broads vs. Wolfeboro/Tuftonboro) or keep it lean?