The seasonal considerations are the same as when you fertilize the lawns. If the water temp is too cool to grow planktonic algae, then there is no need to fertilize. In other words, don't fertilize if the plants aren't growing because the phytoplankton won't be growing that fast either. If you DO fertilize then, and the water warms up, you could have too dense of a bloom and create a fish kill. That is all dependent on the alkalinity of the water too.
No, I don't believe there is any benefit to fertilizing the pond if the clarity is less than 36" due to suspended sediment from bottom feeders. The bloom won't receive enough sunlight to really grow - diminished returns for the $$ spent.
CityDad, there are no hard and fast rules for ponds, i.e. "if the clarity is 35" don't fertilize", but without checking the alkalinity first and making sure it is high enough that it allows the fertilizer to be used, it's a waste of $$. Sort of like fertilizing your crops without getting a soil test done. You can pour a ton of it on them, but if the pH isn't right the crops cannot utilize the nutrients.
