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Life in Freshwater

Plankton and Blooms

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Introduction
Phytoplankton
Zooplankton
Plankton Blooms

Introduction

Plankton refers to organisms that live in open water and have little or no control over where they go. Fish are not planktonic as they can move and determine which way to go. Animals of open water that have the ability to swim purposefully and powerfully are called nekton. Most plankton is microscopic, especially that found in freshwater. Marine plankton have exclusively whole groups of organisms that are very visible to the naked eye, e.g. Ctenophores (sea gooseberries).

All plankton is divided into the plant-type plankton, the phytoplankton, and the animal plankton, zooplankton. The latter can be further sub-divided into the temporary and permanent zooplankton. The temporary tends to be those which are larval forms of adults living on the seashore, e.g. barnacle larvae called nauplii. In freshwater the temporary plankton is very limited.

As planktonic creatures are so small they will get everywhere. Phytoplankton will even stick to stones in a river bed, growing on them to become an important food source (see primary production).

Phytoplankton

These are the producers of energy and may be the base of many food webs. The principle component is the diatom, a form of alga. The basic form is a single-celled organism with the outer wall made from a high level of silica. This is one of the most important nutrients needed for growth. They multiply by cell division and if there is sufficient silica in the water (see below) the cells will grow and then split again. Sometimes the cells do not separate and remain together so that chains of diatoms form. Those like Asterionella form a circle and are quite distinctive under the microscope. Phytoplankton is very productive. Although at any one time the amount of primary production is limited (called standing crop) over a period like a year the net productivity can be higher than a meadow. This is down to the ability for diatoms to quickly multiply, less than half an hour to double. Only a few diatoms need survive and they can quickly reproduce.

Diatoms
Collection of various diatoms x200

Diatoms have diurnal rhythms in the water column. As soon as the sun is up they commence photosynthesis. The process generates oxygen and so the diatom becomes buoyant and begins to rise in the water. This causes it to move closer to the surface and gain more light for maximum photosynthesis. Of course, when the sun sets food production halts, no oxygen is formed and is used up. Buoyancy is lost and it starts to sink. If it sinks below the lit zone before photosynthesis starts the next day it will continue to sink and dies on the bottom.

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Zooplankton

The animal plankton will need to follow the phytoplankton. Some of these animals will be herbivores eating the diatoms. Daphnia moves using its antennae but the degree of jerky movement is quite limited and so is referred to as plankton. To help them with their buoyancy during summer months they develop an enlarged head! Other animals in the plankton are carnivores, e.g. Cyclops, and will eat the small Daphnia. The density of zooplankton is linked to the density of diatoms and often the release of animal young is synchronised with the time of year and blooms.

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Plankton Blooms

The amount of plankton is far from constant. In fact there tends to be two peaks in a year. The first and largest occurs in spring and the second smaller one is in the autumnal. These peaks coincide with prolonged wind blowing in the same direction over the large pond or lake.

As mentioned above the growth and development of the phytoplankton is dependent upon nutrients like silica to be available in the water column not to mention that the diatoms (or their propagules) need to be in the lit zone so that photosynthesis can occur. Both diatoms and nutrients will be lying on the bottom if the water body is large and, the chances are, the bottom (the benthos) will be in the dark well away from being able to develop. If the plankton are to bloom they need to be moved up to the lit zone and this needs a major disturbance. A lake will have a thermocline in winter but the sudden appearance of strong March wind in the spring will cause it to break down. Strong wind blowing perpetually for several days across a water body will cause the surface to shift with the wind. It has been shown that when the wind stops the water "slops" back. A shift in the surface like this is called a seiche. Both this and the break up of the thermocline draws material off the bottom. The result is that some will enter the lit zone and diatoms are triggered into life. They photosynthesise and the subsequent growth and multiplication over weeks will cause the water to change colour as the plankton bloom. This is likely to be green but in some cases it may become a "red tide" of species that can be harmful. Any bloom is not to be encouraged in reservoirs but a toxic bloom could be a serious problem.

Plankton bloom
There are three lakes here but only the large, lower one has a bloom present

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