Mendota turned over? Maybe not...

[rk_diver]

It's my understanding that, by definition, turnover isn't complete until the Disosolved Oxygen (DO) level on the bottom is rejuvenated by the highly oxygenated surface/upper water column. There's something funky with the recording buoy's DO readings, if they are refering to the DO on the lakes bottom in approx. 60 FOW, they're no where close to being right. Before turnover the DO levels are typically less then 1 ppm (quite a bit less). So, just because the water column has a consistent temperature, doesn't mean the lake has rolled over yet, it only means it's ready to roll over, and that'll happen with some high windy days. When the lake does roll there should be nasty stinky stuff from the bottom now on the surface (decaying plant matter for example). The water clarity will then be terrible for a couple weeks, as the crap settles back down to the bottom. I haven't been on Mendota since last Sunday, where the water was clearing up, but hadn't rolled over yet. Looking at the chart below, the last column show Limnology's manual DO readings throughout the summer, up to 9/30/09. These DO levels look as expected, on Sept. 30th the DO level was .8 ppm. Compared to the readings shown by the buoy's auto measuring equipment.

Note; the table below doesn't come out formatted properly so it's hard to read (it's fine when it was typed in). The first set of numbers are the date the readings were taken, then next set is the water temp (in °C) measured at 20 meters, the next is the DO level reported by the real time buoy system in Mendota, and the final set shows the readings taken "manually" by Limnology staff.

Buoy Limn.
YYYY-MM-DD Temp @20.0M DO PPM DO ppm
5/27/2009 10.87 10.69 7.5
6/9/2009 11.57 10.429 4.4
6/23/2009 11.57 11.889 1.1
7/6/2009 11.72 13.884 0.5
7/20/2009 11.95 14.465 0.6
8/5/2009 12.16 12.636 0.8
8/19/2009 12.45 11.029 0.4
9/2/2009 12.25 9.577 0.5
9/30/2009 12.6 8.244 0.8

10/14/2009 13.04 8.183

So, unless the DO level is back to spring time levels, it hasn't turned yet. I'm on vacation next week and plan on doing some deep water dives on Mendota. I'll post a report then on how everything looks. Pleas note, I'm not trying to sound like a "know-it-all", I'm far from an expert. But I've read a lot of info about rollover, thermoclines, etc. because it was mysterious to me, especially as a diver.
Rick