Life in Aquatic Ecosystems
Organisms living in aquatic ecosystems are dependent on the resources of their environment. Biological communities including the types of animals present and their relative abundance are also shaped through the interactions with other organisms.
This section explores in greater detail the relationships between living organisms and the relationship of living creatures with their environment, which include the following;
The Building Blocks of Life
All living organisms need water, energy, carbon, nutrients and oxygen to stay alive, grow and reproduce. Living organisms differ in their specific requirements (e.g., by life stage or activity) and in the processes they use to secure these essentials.
1. Water
Living organisms are primarily composed of water and cannot function without it, although the resting stages for some organisms can survive with very little of this substance.
In aquatic habitats, water is a source of oxygen (i.e., dissolved oxygen) and food (e.g., suspended particles of organic matter).
2. Energy
Almost all energy used by organisms is derived, directly or indirectly, from the sun; the exception includes some bacteria that derive energy from chemical sources (e.g., by oxidizing inorganic compounds such as sulphide).
Plants use energy from sunlight to manufacture a range of sugars by the chemical process of photosynthesis. When animals eat plants, they make use of the energy ‘fixed’ by the plant.
Organisms who cannot manufacture their own food using the sun’s energy must consume other organisms to obtain carbon, energy and nutrients.
3. Carbon
Carbon is a building block in the sugars, proteins, and fats that make up the tissues of all organisms. In plants, carbon dioxide and water, together with energy derived from sunlight, are incorporated into sugar molecules during photosynthesis.
The sugars are stored in the plant body in the form of starch, but can be combined with other chemicals to form different types of molecules (such as protein). A schematic diagram of the carbon cycle is shown on the right.
4. Nutrients
Nitrogen and phosphorus are the most important nutrients for the growth of algae and aquatic plants, as they are often in short supply relative to the needs of these organisms.
Other nutrients, such as potassium, iron, sulphur, and selenium, are also required, though these are usually abundant relative to the amount that algae and plants require.
Within aquatic environments, nutrients are derived from the erosion of minerals and soils within the basin, organic matter, or from human inputs.
Read Also : Aquatic Organisms and their Groups in Fresh Water Ecosystems
The addition of nutrients to aquatic systems for example, from industrial outputs, sewage or agricultural runoff can have major impacts on aquatic systems, sometimes leading to eutrophication (excess nutrients leading to excessive plant growth).
5. Oxygen
Oxygen is a basic requirement for most organisms, although there are some microorganisms that can grow in (or even require) environments without oxygen, while others can tolerate very low levels. Organisms that spend their entire life in water ‘breathe’ oxygen dissolved in the water.
6. Energy and Food
Every organism must acquire energy to live, grow and reproduce. In aquatic ecology, biologists often classify organisms according to how they obtain energy.
Because sunlight is the ultimate source of energy used by organisms on the earth’s surface, a basic distinction lies between those who use its energy directly autotrophs and those who receive it indirectly by consuming other organisms heterotrophs.
7. Autotrophs
Autotrophs, or producers, are organisms that can manufacture their own organic material from inorganic sources.
Most autotrophs carry out this process using photosynthesis, the process by which plants and algae use solar energy to combine carbon dioxide with water to produce starch, sugars and oxygen.
Photosynthesis is the most important biological process on the planet, and its products drive the biological activity of nearly all ecosystems, including aquatic environments.
The oxygen produced is available to be used by other organisms, making photosynthesis an important controller of carbon dioxide and oxygen in the environment.
Photosynthesis in aquatic systems is carried out by a wide variety of autotrophs, which range in size from microscopic single-celled organisms to large aquatic plants called macrophytes.
Autotrophs are primary producers, because they produce the first level of organic carbon from inorganic compounds. Ultimately, all other types of organisms (heterotrophs) are dependent on the organic carbon produced by autotrophs
Because photosynthesis depends on sunlight, the distribution of autotrophs is reliant in part on the amount of light available in an aquatic ecosystem.
In shallow, stony rivers, periphyton (or biofilm) especially diatoms and cyanobacteria are the main source of primary production, but shade from riparian vegetation can limit photosynthesis; nutrients may also be in short supply in these habitats.
In wider rivers, reduced shading from riparian vegetation allows the river surface to receive more light. However, in deep or turbid sections, light penetration may be insufficient to sustain growth of autotrophs.
Read Also : Aquatic Organisms: Invertebrates
8. Heterotrophs
Heterotrophs, or consumers, are organisms that must obtain energy by consuming other organisms (autotrophs or other heterotrophs) as food.
From the perspective of energy flow in ecological systems, heterotrophs can be classified according to what they eat:
Herbivores are called primary consumers because they eat only plants.
Carnivores are called secondary consumers because they feed on other animals.
Omnivores feed both on autotrophs and on other heterotrophs; that is, they eat both plants and animals. Many aquatic organisms, including fish, are omnivorous.
Detritivores consume dead organic matter (detritus). Detritivores include many bacteria and fungi, invertebrates such as worms and insects, and some scavenging vertebrates. Aquatic insects, for instance, shred dead leaves, but also consume bacteria and fungi growing on the leaves.
Heterotrophs can also be classified according to how they obtain food energy (i.e., functional feeding groups), and by their specific roles in the aquatic ecosystem (Cummins and Klug 1979):
The grazer-scraper category includes herbivores that feed on periphyton and biofilm. Shredders are detritivores feeding on coarse organic particles, especially leaf litter derived from the riparian zone.
Collectors eat fine organic particles and can be subdivided according to whether the food particles they collect are suspended in the water (e.g., filtering-collectors or filter-feeders), or have been deposited on the substratum (collector-gatherers).
Deposit-feeders ingest fine bottom sediments and the organic material that they contain. Predators are species that eat other animals.
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