Re: Calcium and aquatic plants

The table printed in the last APD suggested an 'average' Ca content in 
aquatic plants of 650 mg/kg. This is useless information. In my 
experiments I have found for at least one plant (Lemna trisulca) a 
minimum internal concentration of about 8000 mg/kg. For other plants the 
mean Ca content ranges from 1 to 3%. Hutchinson (1975) lists the mean 
content of Ca in aquatic plants as 1.70% or 17,000 mg/kg. The point is that 
taken as a whole, the range of Ca contents is large so that an average 
value tells you nothing about the requirements of the particular plants 
you are interested in. Furthermore, Ca is unlike most other nutrients in 
that there is an external requirement that is independent of the 
amount stored within the plant. Data from my research indicates that 
aquatic plants can thrive at external concentrations far lower than those 
found in natural environments. As an example, Potamogeton pectinatus is 
never found in waters with a Ca content less than 40 ppm but will thrive 
at concentrations as low as 2ppm in the lab. The reason is that Ca 
content is correlated with other parameters in natural environments but 
not in defined media. The suggestion that 90 ppm is required is therefore 
extremently misleading. The suggestion that Ca 'disappears' in soft water 
is also erroneous. In Canada we have some of the softest, most 
oligotrophic waters in the world and the Ca is still there at about 1 to 
4 ppm ... high enough to support the growth of even the most calciphilic 
aquatic plant species.

I cant understand where these data come from. They are clearly not 
based on anything currently extant in the scientific literature.