At that time I had what can only called' Pea Soup' in both the large ponds. Incidentally, the small one which is not in the system was crystal clear, don't ask me why - I don't know, except it's not so deep and the plants were growing more vigorously (presumably there was less light loss in the shallow water) and using up all the available nutrients - something which the algae was doing in the deeper ponds. However, they did eventually clear.

What is algae? Answer -plants! The next logical questions is - What is a plant? So sit back and read. Let's have a quick biology lesson. Perhaps those of you that did '0' level Biology many years ago will know, or at least remember.
I must confess to not being able to remember fully and have therefore had to look it up, ie, do some revision.

Firstly what is the difference between plants and animal life forms?
Answer: all living things can be divided into two groups - Plants and Animals.
How can you tell the difference and which group should a living thing be placed in?

Animals move about from place to place, whilst plants remain rooted to the ground. But always there are exceptions. Corals for instance remain fixed in place whereas some small algae species swim in the water. So just how do we distinguish between animals and plants? There are six rules.

1. The cells of plants are surrounded by a cell wall, which is made of a dead     substance which is rigid. This is not so in animals.
2. Plants usually contain CHLOROPHYLL but this is not found in animals.
    Again there are the usual exceptions, bacteria and fungus do not contain     chlorophyll. Green Hydra, Hydra viridissima, on the other hand, does, being an     animal containing it. However it is not contained in the cells of the Hydra, but     in small round cells which are really tiny plants living between the cells of the     Hydra.
3. Plants that contain chlorophyll are able to make their own food.
    They convert carbon dioxide and water into starch in the presence of sunlight.
    This food together with the minerals taken up by the roots is changed into     more complex substances. Animals, however, cannot make their own food.     They eat the foods that have been made by plants, either by eating the plants     or by eating other animals that have eaten the plants.
4. Animals usually move from place to place in search of food, whereas plants     remain in one place as they can make their own food.
5. Animals grow to a limited size and the number of parts of their body is fixed.     Plants continue to grow since their size is not limited.
6. Growth in animals goes on all over the body. In plants there are special     growing regions, which are at the tips of the roots or shoots, or just beneath     the bark in the stems and the roots.

Plants can be divided into two groups:
Flowerless plants that do not produce seeds and those that do.

Algae fall into the first group and there are hundreds of plants within this family. They are simple in their structure, and nearly all live in water.
Some of them are very big, although their structure is simple, but many are so tiny they can only be seen through a microscope.

There are many kinds of pond algae, all of which play an important part in providing food for the other pond inhabitants. They are mostly one-celled, or thread-like, filamentous plants. They are usually found floating on the sunlit surface of the pond although some attach themselves to rocks or the stems of larger plants.

Many freshwater algae belong to the primitive group known as Cyanophyceae (Blue-green Algae), others to groups known as the Chlorophyceae and Bacillariophyceae. The latter includes the Diatoms.Together with vegetable matter such as leaves that fall into the water, algae forms an important part of the food chain in all waters of the world including our ponds.

Like all photosynthesising plants they produce sugars by using the energy of the sun for their growth. This is then often eaten by animals, which derive their energy from them. Some of the larger algae contribute to this food chain indirectly when they die by falling to the bottom, where they become part of the debris, which provides food for animals such as Gammarus Shrimp as well as bacteria.

Starting at the microscopic end we find Diatoms which consist of either a single cell enclosed by a cell wall of silica, or a group of cells with their cell walls joined in various shapes, such as

Asterionella joined in a
star-shape.

Fragilaria and rectangular Tabellaria form zig-zag chains.
Pinnularia has single boat shaped cells.

All of these can only be observed with a microscope. Their skeletons fall to the bottom when they die.

In some parts of the world their fossilised remains are found in deposits of thousands of tonnes. These are mined being called diatomaceous earths or more commonly 'Fullers Earth'; they are used as ingredients in paints, polishes and toothpastes and industrial filters. There used to be a filter on the market for aquariums which used them as the filter medium, I don't know if it's still around. Diatoms can be found in aquariums from time to time. You know the brown film that appears on rocks and plants or occasionally on the glass? This is a deposit ot Diatoms.

Sharing the surface with Diatoms you will find green algae, known as Desmids. These are also found on the surface of the mud forming a green film.
They are also very variable in shape such as Staurastrum which have long, spiky projections, others such as Micrasterias, are smooth in outline whilst species such as Desmidium have the cells joined in short filaments.

Staying at the microscopic end we also find species which are collectively called the 'swimmers and rollers'. One of the simplest of the green algae in this group is known as Chlamydomonas. It consists of one cell, which is pear shaped and contains a large chloroplast, containing chlorophyll, which is used in photosynthesis. These very tiny algae are able to swim about in the water by means of two whip-like hair projections, called CILIA or FLAGELLA.
Given the right conditions and bright sunlight they can multiply rapidly making the water turn green (my 'pea soup') in a matter of days.

Many of these green algae band together in groups or colonies, all with very definite shapes.

One such shape known as Volvox consists of thousands of cells arranged around the outside of a hollow sphere, usually held together by a strand of protoplasm. These colonies are about the size of a small pinhead and can just about be seen with the naked eye, under magnification they can be seen to move by slowly rolling along, and rotating as it does so.

Not all green algae are planktonic as those previously mentioned or colonial.

Two of the Filamentous Green Algae which we are all aware of and curse from time to time are Spirogyra, which consists of tangled masses of long green filaments, which are covered in mucilage, making them slimy to the touch.
They are often attached to the bottom but can also be found floating.

A very similar-looking algae which we all love to hate is Cladophora or 'Blanket Weed'. Its filaments are not slimy but masses of tangled filaments and is always found floating. They reproduce by adding cells to the ends, if pieces break off these can quickly grow into long filaments by this means.

Spirogyra has another means of reproducing - two strands come side by side the mucilage sticks them together, cells produce bumps down the strands opposite each other. These lumps join the contents of these cells then shrink from the cell walls and round themselves off. Those in one strand pass into the other, where they join together and become surrounded by a thick-walled cell called a ZYGOSPORE. These fall away and drop to the bottom in a dormant state until the conditions are right (next Spring) and we have a new growth of algae just when we thought we had got rid of the darn stuff - a good reason to clear out the debris from the pond in Autumn!

Finally, Blue-Green Algae; these are amongst the most simple and primitive organisms living. They can be unicellular or colonial. Not only do the cell walls stick together but they can also be enveloped by a thick gelatinous envelope. Oscillatoriois a filamentous form, which grows as a blue-green slime on stones, or can be found moving about in the water. Other forms can be found in gelatinous masses floating in the water. This Blue-green Algae is the form which has been causing the problems in our reservoirs in recent years; if they increase uncontrollably they can choke the water system and cause anaerobic (oxygen deficient) conditions; in turn, these conditions allow the bacteria to increase rapidly which produce the highly toxic gas, Hydrogen sulphide.

So the best advice I can offer is control algae growths of whatever form by whatever way you are happiest with. Be it adding proprietary chemicals, mechanically by filtration or by possibly the best way - BY HAND.

Bibliography: TF:H., Modern Science.