I have recently started fishing at a 40 acre stillwater owned by a well known angling club. It is stocked with rainbows. Having fished it a few times already and catching a few skinny looking stockies I met and chatted to a fisherman there yesterday who explained to me that the water used to have v prolific buzzer hatches and good general fly life but now seems to be devoid of all insect life save that which is blown onto the water from the surrounding farmland. What could have changed this so dramatically ?...the guy I spoke to seems to think that some chemical or other has been added to the water to combat the previously prolific weed growth in summer (this too has died off to some extent) Anyone any idea how this could happen ?

A lot of waters have weed or algal problems and try to combat it with chemical treatment. Unfortunately it can wipe out the entire algal or weed population if not treated in small areas at a time (which is difficult due to currents and water flow) If that happens you remove the entire bottom of the food chain and it has a knock on effect on the rest of the aquatic life.
That is the only reason I can come up with off the top of my head as to why there is no buzzer life

__________________
My Blog: http://duellingdialysis.wordpress.com/

There is also the slight possibilty of insecticide finding its way into the water due to run-off from farmers fields....

__________________
My Blog: http://duellingdialysis.wordpress.com/