I was reading a paper about eutrophication and there was a term "deep winter mixing" in which an abundant supply of phosphorus is released to the surface waters. I searched a lot but nowhere I could find a definition for that.
The winter mixing of stratified layers in lakes and seas due to temperature changes can vary in depth, so scientists sometimes say "deeper winter mixing" and "deep winter mixing". Scientists measure the water arising from 300 or from 800 meters depending on the year, so the deeper winter mixing is a relative term. i.e.:
1992, associated with more stormy weather and higher precipitation than usual. This anomaly was the cause for the deeper winter mixing in the Cyprus Eddy (>500 m) than the typical 350 to 400 m reported by Brenner et al. (1991). Exceptionally deep vertical mixing (>850 m) was reported that winter also in the Gulf of Aqaba https://www.int-res.com/articles/meps/167/m167p047.pdf
Here is basic description of winter mixing: https://teara.govt.nz/en/diagram/11817/water-circulation-in-a-lake
and here is a science paper: https://www.jstor.org/stable/2259960?seq=1
A wiki page on lake stratification: https://en.wikipedia.org/wiki/Lake_stratification