"P van Rijckevorsel" wrote in message .. .
Jelena schreef
Besides, in a surface layer of lake sometimes
I got negative values of concentration of chl a (?) and I am wondering
how it is possible to get negative value, minimal concentration should
be zero, not less then zero.

+ + +
Obviously, if you are measuring negative concentrations you are doing
something wrong. More likely in your calculations than in your measurements,
but who knows ...
PvR

That is first what I had in my mind - wrong calculation, but formula
is OK actually, it is ISO method that I use. Unbelivable things
sometimes can happen in science, isn't it? Thanks anyway, if you have
more sugestions you are very welcome, I really need help!