Secchi readings. Any help to the diver?

[Bindlestitch]

Hey guys,
I stumbled onto these secchi readings and was wondering if they ring true for diving visibility.
Here's a link.http://www.lmvp.org/lakes.htm
Just click on the lake and scroll down the page to find the secchi readings.
For instance secchi readings for the lake ozarks in 2003 show 6'-10' vis in the main lake chanel. But I've heard from divers that the lake has near zero vis.
What gives?
Stitch
P.S. If this is in the wrong forum I appologize. Wasn't sure exactly where it fit best.

[Atlaua]

Hey guys,
I stumbled onto these secchi readings and was wondering if they ring true for diving visibility.
Here's a link.http://www.lmvp.org/lakes.htm
Just click on the lake and scroll down the page to find the secchi readings.
For instance secchi readings for the lake ozarks in 2003 show 6'-10' vis in the main lake chanel. But I've heard from divers that the lake has near zero vis.
What gives?


I took a look at this site. I looked at Bull Shoals for 2001 & 2003 (there was no 2002 value which is when I was there last).

The maximum values for those years was 11 & 14 feet respectivly. When I was there in 02, we had 20+ feet below the thermocline.

I'm not sure exactly what these values represent so I popped a message off to a marine biologist I know. I'll let you know when I get a response.

James


P.S. Great Site!

[Atlaua]

Ok here is what I got:



Sometimes it's spelled "secci". I've never found out the true spelling, maybe its lost in time.

Anyway, a secci "measure" is simply an underwater measure of length, using a secci disk. Disks vary somewhat, but are generally a white disk with black countershading. From the surface of a body of water, you drop the disk (attached to a metered rope) down into the water. When you can no longer differentiate the white from the black on the disk, you record the depth. This is the most widespread way to get a secci measure. SOme folks use the value at which they can no longer make out the disk from the ambient water; that's more in use with the disks that are uniformly white (commonly used in freshwater systems and/or Fore-Ule water colour measurements).

Secci measures are the most commonly encountered scientific measure for determining water clarity. They're also the crudest. Nowadays researchers use turbidometers or their equivalents, and secci's are relegated for basic survey work. Their cheapness and durability simply can't be beat. I've got three of the things in my teaching lab. They're easily adapted by scuba divers to measure horizontal visibility. Done properly, accuracy can be much improved over the ambiguous "eyeballing" of the diver.


So it sounds like they are surface readings and won't really be usefull on measuring visiblity below thermoclines.

James

[Bindlestitch]

Good point James. And thanks for the info.



Later,

Stitch

[Tabasco1955]

Ok here is what I got:




So it sounds like they are surface readings and won't really be usefull on measuring visiblity below thermoclines.

James


James,

Unless I misunderstood, it sounds to me like they "could" be used by divers at depth too if you wanted to know the visibility bad enough to carry one with you on a dive and actually use it.

[Atlaua]

James,

Unless I misunderstood, it sounds to me like they "could" be used by divers at depth too if you wanted to know the visibility bad enough to carry one with you on a dive and actually use it.

No doubt, if they were being used horizontally at depth. However, I'm pretty sure that's not the way they are being used on the website above :D.

James