Below is the standard advice. While good I don't follow it exactly based on experience.

Because we have an ongoing year to year fertilizer program you can't know the exact fertility level of the pond when cold temps stop the bloom for winter. There is residual fertility remaining the next spring. So I wait to see how the pond responds to natural warming and residual fertility before I start. Then I start slowly adding fertilizer at low rates until the bloom starts. When it starts to go from green to brown I add small amounts of fertilizer to try to keep both a phyto and zooplankton bloom ongoing. I usually try to keep our visibilities between 18 and 36in. For those who have not done this before be careful as you can easily over fertilize. Often it is not the lack of fertilizer that is suppressing the bloom but may be temp or light conditions or rain etc. When the limiting conditions are removed the bloom can take off and get out of control if it is overly fertile.



http://msucares.com/pubs/publications/p1428.pdf pg 18



When To Apply Fertilizer

Begin fertilization in late winter

or early spring, but definitely by

the time water temperatures have

stabilized at 60 �F or higher. Usually

this temperature occurs

around March 15 in south Mississippi

and April 1 in central and

north Mississippi. Early fertilization

will shade the pond bottom

and help control filamentous

algae, a common problem in Mississippi

ponds in spring.

Make the first three applications

of fertilizer two weeks

apart; then apply whenever you

can see your hand clearly with

your arm under water at elbow

depth. By fertilizing only when

water is clear at18 to 24 inches,

you will fertilize the pond at approximate

3- to 5-week intervals

from spring through September.