Lakes by Warwick F. Vincent

Author:Warwick F. Vincent [Vincent, Warwick F.]
Language: eng
Format: epub
ISBN: 9780191079344
Publisher: OUP Oxford
Published: 2017-11-25T00:00:00+00:00

a diatom, is eaten by a rotifer, which is eaten by a copepod, which is eaten by a cladoceran, which is eaten by a whitefish, which is eaten by a pike, which is eaten by an otter or by a human.

Glass is a heavy substance, and diatoms are therefore at the mercy of gravity, many of them sinking out of the photic zone and accumulating in the sediments. The best time of year for their growth is during spring and autumn, when the full mixing of the lake keeps these heavy cells up in suspension, and when there is adequate sunlight and nutrients for their photosynthetic production. There are long-term diatom records for many lakes of the world, including Lake Windermere, that show the regular rise and fall of diatoms every year. Their seasonal collapse is typically due to the onset of stratification, and the cessation of full mixing. Their rapid demise through sinking may also be hastened by grazing by zooplankton, or by parasitic attack.

Non-swimming green algae are also common in the phytoplankton, and vary enormously in size and shape. The smallest are less than 2 or 3μm in size and are referred to as ‘picoeukaryotes’. These can be present in high abundance, but usually require DNA techniques to identify. At the other extreme, some species of green algae form large colonies of cells. For example, a beautiful lace-like species found only at Lake Biwa, Japan, Pediastrum biwae variety triangulatum, has a colony diameter of almost a tenth of a millimetre. A common species found in many lakes of the world, the gelatinous colonies of Sphaerocystis schroeteri, is of a similar or larger size. These dimensions make the particles too big for zooplankton to readily feed on, and large colony size is an effective defence against grazers.

The third group of phytoplankton comprises photosynthetic species that can swim: ‘phytoflagellates’ that belong to many algal phyla and encompass a great variety of species, both in terms of ecology, and pigmentation. These motile cells propel their way through the viscous liquid environment with flagella, some with one large and one very small flagellum such as the golden-brown species Dinobryon divergens that produces tree-like colonies of beating cells (Figure 20), others with two equal flagella such as the green alga Chlamydomonas. A mammoth among these motile phytoplankton is the brown-coloured dinoflagellate, Ceratium hirundinella, found in many lakes throughout the world; the cells are up to a quarter of a millimetre in length (250μm) and are able to swim up and down in the epilimnion each day.

20. The colonial phytoflagellate, Dinobryon divergens. The individual cells are 10–15μm in length and are mixotrophic: they feed on bacteria as well as capture sunlight for photosynthesis.

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