Green Water Pond Algae
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The proper name for this type of algae is Planktonic Algae and will bloom if your pond has an increased level of nutrients and carbon dioxide present in the garden ponds water.
This combined with sun light and warm weather will cause planktonic algae to bloom.
As a result of this combination you can end up with green water in your pond.
The primary nutrients or food source is nitrogen and phosphorus.
Nitrogen is a by-product of decomposed fish waste along with uneaten fish food. The fish waste and uneaten fish food will settle and create sludge across the bottom of the pond.
Phosphorus is an essential nutrient for all life forms and pond sediments will contain phosphorus. This phosphorus is in fish food and fish waste along with anything that may enter the water like dead bugs, or frog waste.
Algae are free-floating so they must get those nutrients from the water and not from the bottom of the pond. But if the bottom of the pond is sturred up this may cause a bloom of algae in your pond.
Other common sources of nutrients are fertilizing of the grass or plants around your pond or from dying aquatic vegetation. More excess nutrients added to the ponds water, the more likely the resulting bloom will be a nuisance.
The Chlorophyll in algae absorbs sunlight and uses it to convert carbon dioxide and other nutrients into new cells through photosynthesis.
Chlorophyll is green so the more algae you have the greener the water.