Thank you for your reply, Butrflynet. on two counts :
I'd read this some years ago and it was where I first had come across the Prandtl Boundary ; later I wanted to refer someone to the article but (of course!) I'd forgotten to make a note of it, so couldn't!
Secondly, although the article doesn't contain the information that I'm after, I have attempted to e-mail its author ; it hasn't yet come back as 'undeliverable' so I'm hoping that I may receive a response in due course!
Also, in looking up that author, I stumbled upon a research team working on the biology of non-native (to UK) aquatic plants and have e-mailed them with my query : it was generated because I'm building a biotope based on a blackwater creek/pool in Kalimantan, Borneo, so they may be familiar to some extent with the particular ecology.
So - thanks again, B. : all other responses gratefully received!