Discriminate between the immediate and chronic effect of water pollution and explain why chronic pollution is a more serious long term threat.

We first need to define 'acute' and 'chronic' pollution. Acute pollution occurs at high doses and affects organisms over short periods because it is more lethal. So we are typically looking at biological endpoints of lethality. Chronic pollution occurs at much lower doses, and effects in organisms would be evident over the long term, with typical effects being reduced growth and reproduction. I won't argue that 'chronic pollution is a more serious long term threat' because this displays very limited understanding of how aquatic ecosystems work. Even if a pollution event is at a high dose over a short period of time (acute), this may result in the death of all the organisms in a river or stream. Is this better than the slow inhibition of growth and reproduction of a biological community? And what of the lingering effects of the acute pollution event as the pollutant moves into the sediment? Effectively the acute pollutant becomes a chronic pollutant anyway.  

Exposure to solar flares would be the correct answer