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

[muskychuck]

Rick, will you let me know when the lake has flipped please. And were you out diving yesterday at 4:30? I saw a diver over by the Commodore.
Chuck

[rk_diver]

Right now that's the $64,000 question! According to an expert I spoke with earlier he explained that the lake hasn't rolled over until the DO was renewed at the bottom of the lake at all depths. Now I've been corresponding with Limnology and they say since the water temp is the same throughout the water column it has indeed turned over. But she said this is a matter of debate among their groups. I think a large part of it depends on your definition of what turnover is. To me, in my layman mindset, it hasn't turned until there's sufficient oxygen throughout the water column to substain fish life. If you follow the other Limnology website they record readings every couple of weeks. In Sept. 30th the DO level was <.8 (going from memory). In the future you'll see that level go up substancially, which means fish can now survive at depth for as long as they want to stay down there.

As a fisherman you're looking for fish, and if you try finding walleyes in 60 FOW with a DO level of <.8, they shouldn't be able to survive very long. As eveyone I've dealt with on this has told me, it's not cut and dry, there's a lot of differing opinions and it's a difficult thing to understand.

I was out on Mendota yesterday in that area, but I didn't dive at all. Hoping for a break in the weather so I can get out. Don't like getting the boat soaked this time of the year. The carpet gets saturated and then it'll freex up, making for hazordous walking around! If I ever get a definite answer to the roll over I'll let you know. Rick

[muskychuck]

Thanks for the reply. I'm thinking that the winds we had Thursday may be enough to roll the lake. I've lived on Mendota for 60 years, house is for sale now, and I think I know what turnover should look like, and it's ugly. I think the fish are waiting for it too 'cause I've never seen fish sooooo negative. Thanks again and if all works out the way I want it too I'll be fishing either Tues or Thurs next week. Alumacraft with a 75hp Honda. Later