Pond Scum

Floating mats of pond scum are an annual feature in Stonebridge Pond and on the allotment watercourses, where they tend to accumulate upstream of the two sluice outfalls. I thought I would take a closer look to see what the scum consists of. The samples which I took showed that the majority of the scum consists of two species of green filamentous algae, Spirogyra species and Vaucheria sp that form threads or mesh like filaments, with lesser amounts of other algae and diatoms. What we see with the naked eye as one strand are several microscopic strands lying adjacent. The strands shown below are 77 -90 microns in width. 1micron (µm) is 1/1000 of a millimetre.

Spirogyra low magnification. Vaucheria and Spirogyra high magnification

The algae are important because they produce oxygen and food for animals that live in the pond. These primitive plants grow under water lying on or loosely attached to the silt, especially in shallow water where the light penetration is high; but they can break free and form the floating mats of algae. The algae photosynthesise, producing oxygen and bubbles get trapped in the mats, causing them to float to the surface. This is when they can also cause problems such as clogs and stagnancy and create a sometimes unpleasant stench as the clumps decay.

Are clumps of filamentous algae unhealthy?

Most filamentous algae do not produce toxins that are harmful to humans and wildlife, but some of the bacteria and other pathogens living on the algal mats and assisting with the breakdown the algae may do so. Filamentous algae do not have roots. They get their nutrients directly from the water, meaning that their growth and reproduction are entirely dependent on the nutrients in the water. Accumulations of filamentous algae are caused by an overload of nutrients in the pond or water channels, especially nitrogen or phosphorus. Excess nutrients can unintentionally enter the water bodies as run off from fertilised crops, wildlife waste from ducks and gulls, storm water inflows and road run off. There are many sources of nutrients in residential areas. Disturbance of the silt may also contribute to the nutrient load because it may release chemicals bound to the silt.

Is the scum a problem?

1. Apart from large concentrations of scum that cause an unpleasant smell when it decays, the mats of scum are not hazardous to people or wildlife per se.

2 . If there was a toxicity issue we would quickly see multiple fish kills and sick and dying waterfowl.

3. If the source of nutrients is animal waste – and we don’t know that it is – it is likely that bacteria and other pathogens are living on the algae mats. Plot holders who come into contact with the scum, should wash their hands thoroughly or use sanitiser, as a precaution.

4. Physical removal of mats near to the sluices is undertaken and helps break up the mats clogging the outfall.

5. A reconfiguration of the outfall on the North Stream by the construction of a weir, as on the main outfall, would allow the algae to flow over the weir and into the creek.

6. Aquatic vegetation should be encouraged along the margins of the watercourses because the plants help the uptake of excess nutrients such as phosphates and nitrates and oxygenate the water. Much of the food for waterfowl, ducklings especially and other wildlife is in this marginal habitat..

There are other types of bacteria, known as cyanobacteria, formally known as blue-green algae, which can produce toxic chemicals that are very harmful to the health of people and animals. Cyanobacteria are particularly a health risk during hot summers. They also form scums on the surface and often accumulate along shorelines. They look like green or turquoise, wispy paint and leave smear trails across the water surface, when for example a stick is dragged across the surface. We have, to date, not suffered from this bloom in the allotments. The photographs below show cyanobacteria, Microcystis on a lake near to Faversham.

Algal growth is a natural occurrence and Nature’s way of capturing nutrients. The algae play an important role in the pond and allotments water and support a whole ecosystem, teeming with life, producing oxygen and food for animals that live in the water body. Much of the life is microscopic and invisible to the eye and is in turn a food source for larger animals. I have attempted to show some of it in the photos and videos below taken through the microscope. There is a large fauna of unicellular protozoans and other organisms, many of which feed on the algae and bacteria and help break down the scum. These are just a few microns in length, but even though single celled, some have very intricate organelles. All the organisms depicted were in one drop of water about 5mm in diameter! The grey background and halos around the organism are due to phase contrast illumination that is used to show otherwise transparent objects.

Click on the images to play videos.


Amoeba creep over surfaces using pseudopodia and change shape continuously by a process of cytoplasmic streaming as they wrap around prey and engulf it.

Stilonychia between decaying strands of algae.
Watch the middle one catch and engulf a free swimming algal cell. the small black dots are bacteria being wafted towards the animal by the beating cilia.

The above three are Ciliate protozoans. Ciliates have small hairs on their body to propel themselves through the water. Often there are specialised groups of cilia around the mouth opening which beat to draw in water and contents which are then ingested, as shown above. Others such as the species immediately above have cilia fused together into stiff bristles or cirri , mainly on the underside and the animals run about using the cirri as “legs”.

All photos and videos by Bob Gomes