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Lecture notes – The aquatic ecosystems
Biology (C100)
University of Salford
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Lecture notes – The aquatic ecosystems
Microbes drive marine and aquatic ecosystems. They are the most diverse and abundant organisms in these habitats. The importance of microbial activities in oceans, rivers, and lakes has been brought into sharp focus by global climate change. To understand the capacity of microbes to influence the worldwide flux of nutrients and greenhouse gases, we must first consider marine and aquatic habitats.
The nature of water as a microbial habitat depends on a number of physical factors such as temperature, pH, and light penetration. One of the most important factors is dissolved oxygen. The flux rate of oxygen through water is about 10,000 times less than its rate through air. However, in some aquatic habitats, the limits to oxygen diffusion can be offset by the increased solubility of oxygen at colder temperatures and increasing atmospheric pressures.
Water as a Microbial Habitat
For instance, in the very deep ocean, the dissolved oxygen concentration actually increases with depth, even though the airwater interface can be literally miles away. On the other hand, tropical lakes and summertime temperate lakes may become oxygen limited only meters below the surface.
Light is also critical for the health of marine and freshwater ecosystems. Like all life on Earth, all organisms in these environments depend on primary producers-autotrophic organisms to provide organic carbon. In streams, lakes, and coastal marine systems, macroscopic algae and plants are the chief primary producers. Organic carbon also enters these systems in terrestrial runoff. The situation is very different in the open ocean, where all organic carbon is the product of microbial autotrophy. In fact, about half of all primary production on Earth is the result of this microbial carbon fixation.
Solar radiation warms the water, and this can lead to thermal stratification. Warm water is less dense than cool water, so as the sun heats the surface in tropical and temperate waters, a thermocline develops. A thermocline can be thought of as a mass of warmer water “floating” on top of cooler water. These two water masses remain separate until there is either a substantial mixing event, such as a severe storm, or in temperate climates, the onset of autumn.
As the weather cools, the upper layer of warm water becomes cooled and the two water masses mix. This is often associated with a pulse of nutrients from the lower, darker waters to the surface. This pulse of nutrients can trigger a sudden and rapid increase in the population of certain microbes, and
a bloom may develop. This is considered more fully in the discussion of microorganisms in coastal marine systems (p. 663) and lakes (p. 675).
Lastly, CO2 plays many important roles in chemical and biological processes. The pH of unbuffered distilled water is determined by dissolved CO2in equilibrium with the air and is approximately 5 to 5. The pH of freshwater systems such as lakes and streams, which are usually only weakly buffered, is therefore controlled by terrestrial input (e., minerals that may be either acidic or alkaline) and the rate at which CO2 is removed by photoautotrophic organisms such as diatoms; CO2 fixation can increase the pH of the water.
Lecture notes – The aquatic ecosystems
Module: Biology (C100)
University: University of Salford
