We’ll notice pond water color turning a yellow-green when pollen is shed. We looked at a pond last week that had the same color but was result of phytoplankton bloom instead. UGA Extension Aquaculture Specialist Dr. Gary Burtle says this about algal blooms:
“The yellow-green color is normal for a mix of plankton found in fish ponds. The bloom shows signs of fertility, either from addition of fertilizer or from watershed runoff.
Aerial images look blue due to the reflection of light. Blue is a shorter wavelength and is not absorbed as much as red and yellow, so is reflected. Particles in the water may scatter light so that a different color is observed. However, when the particles are deep in the water, aerial photos do not capture their effect, only the blue or blue-black color. Water color can also be different from different angles, due to refraction.
The aerial photos posted on the web are taken at different seasons. One taken in the winter may not indicate a pond algal bloom, but one taken in midsummer might capture the bloom.
We recommend that visibility between 12 and 18 inches is optimal for sport fish pond management. Dense algal blooms and scums may occur when visibility is less than 12 inches. Measurements of visibility are taken at the pond surface rather than from a distance. Efforts to correlate light reflection to water quality have shown some interesting correlations, but require special filters and photographic techniques.”
Although fertilization can increase fish production significantly, it is not the best management practice for every pond. Here is a publication on Pond Fertilization & Liming from UGA Warnell School of Forest Resources.