Microscopic life in freshwater habitats

Stephen Durr

Many people enjoy sitting beside a pond or lake during the summer months and enjoy watching the birds, insects and plants going about their business. These same people will probably never give a second thought to the numerous microscopic plants and animals found beneath the surface of the pond. The green tiny single-celled plants called algae are some of the most beautiful sights in nature and would be in everyone’s garden were they the same size as the larger and more familiar plants that we see around us. To view this largely unseen world, all you need is a microscope and a willingness to learn.

The first person to bring the attention of such creatures to the unsuspecting scientific community was a Dutch draper by the name of Antony van Leeuwenhoek, after reading Robert Hooke’s Micrographia, he began making microscopes in his quest for knowledge. He saw many small creatures in the numerous ponds and dykes that occur in the Netherlands. He was the first person to record the minute bodies that we call bacteria, indeed an excellent feat for someone who had no scientific training, just an enquiring mind.

Even a tiny shallow body of water, such as a garden pond, consists of various habitats where individual organisms live at different times of the year. For freshwater algae and protozoa, the three main areas of the pond are the margins of the lake, the open water, and the intricate network of plants that live within the water. Many of these creatures are sensitive to sunlight, wind, oxygen levels and chemicals; a slight imbalance of any of the above properties can spell disaster for the organism. The main physical properties of the pond are the epilimnion which is just a few centimetres below the surface in most lakes and where most of the photosynthesis occurs. In the summer months, good circulation, carried out by the effects of sunlight and wind action, keep the pond alive. The bottom layer of the pond, called the hypolimnion, can be cold and dark during the winter months; depending on how much dead plant and animal material accumulates at the lake’s bottom, oxygen can be in short supply this period.

Sitting between these two outermost layers is what is known as the thermocline, which is a thin band of water that separates the two different temperature gradients of the epilimnion and the hypofimnion. These two layers are not always distinct regions within the pond or lake; wind, bird and animal actions can quickly destroy the separate layers. The lakes that became established in the low-lying areas are eutrophic and are rich in nutrients. Chemicals in use by the farmer run off the land into the water; animals such as cattle, water birds and sheep also enrich the pond with their manure. Each unique habitat is populated by organisms selected through the evolutionary process of natural selection for characteristics that ensure an optimal chance of survival in a particular environment. As ponds undergo both seasonal and short term (rain, drought, waterfowl etc.) changes that impose selective pressures on the inhabitants, they offer a rich opportunity for the study of a variety of ecological issues.

Various methods can be used by the amateur microscopist to make the observation of living cells more revealing and also more educational, without harming or distorting the cells in any way. Darkfield, phase contrast and Nomarski are excellent tools, but can also be very expensive to buy new. Detail within the cell body can be revealed and structures such as cilia, flagella etc. are all made visible by using the above methods. If you are serious about your hobby then it is worth investing in a darkfield condenser and possibly a phase outfit when the funds become available. The human eye or camera cannot detect phase differences, but can detect differences in amplitude and this is where the phase contrast outfit comes in handy.

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