Succession in aquatic ecosystems

  • Communities develop over time
  • In facilitation, each succeeding species makes its habitat more favorable for subsequent species
  • In inhibition, each species inhibits the species that try to succeed it
  • In tolerance, a species neither facilitates nor inhibits its successors
  • Species richness usually increases during succession
  • Biotic processes, such as herbivory, competition, and mycorrhizae, can deflect the path of succession
  • Succession: gradual changes in a community that are predictable and orderly
  • Primary succession: when plants invade an area in which no plants have grown before
  • Secondary succession: a modification of longer-term primary succession. It does not occur on virgin previously un-inhabited aquatic ecosystems.

Effects aquatic pollution on organisms, colonization and succession

  • Changes in water quality affect the entire biosphere. i.e. Plants, animals and other organisms living in water may suffer physiological changes.
  • The specific effects of aquatic pollution vary depending on the type of pollutant that enters the environment.
  • Sometimes water pollution, e.g. eutrophication causes the excessive proliferation or growth of an invasive or local plant/organism. For example, harmful algal blooms (HABs) are promoted by increased levels of nitrogen and phosphorus in the environment.
  • The excessive growth of algae and cyanobacteria changes the succession and community structure of phytoplankton and macrophytes in the aquatic environment, a process that can sometimes have devastating consequences on the environment.
  • HABs are usually composed of algal and cyanobacterial species that produce deadly toxins and are sometimes known as “red tides” or “brown tides” for their appearance in water.
  • The toxins produced and released by HABs can kill fish, marine mammals and seabirds and harm humans.
  • The death of algae and cyanobacteria species that make up the blooms, bacteria may use up all the oxygen from the water as the algae and cyanobacteria decompose, leading to a condition called hypoxia.
  • Hypoxia creates a “dead zone” where fish cannot live, and more than 400 areas around the world have been identified as experiencing eutrophication and 169 of them are hypoxic.

Adaptations to Aquatic Environments

Aquatic plants evolved from terrestrial plants. Like whales and other marine mammals, aquatic plants evolved from land back to aquatic habitats. Aquatic plants modified terrestrial features to withstand emerged, submerged, or floating conditions.

plant adaptations to aquatic environment:

Emergents plants:

Aeration of Roots:

  • Formation of aerenchyma: large open spaces between cells, which is important to carry oxygen down to the root zone. 
  • Formation of prop roots (Red Mangrove)
  • Formation of pnuematophores (Black Mangrove)
  • Anaerobic respiration: plants will form ethylene, then more aerenchyma tissue and adventitious tissue. The plant elongates and forms fatty acids from the ethylene. Ethylene is a common gas in swamps due to decay.

Reproduction: Sexual reproduction is rare, more commonly used methods are:

  • Fragmentation, pieces break off and float away to another location where they get embedded in the substrate.
  • Rhizomes: underground stems send up shoots to start a new plant.
  • Stolon: same as rhizomes except these are above ground stems which form into shoots and start a new plant.

Seed germination:Plants have different strategies for seeds:

  • Timing of seed production to occur during the non-flood season either by delayed or accelerated flowering.
  • Production of buoyant seeds that float on high unflooded ground.
  • Seeds germinate while still attached to the plant.

Photosynthesis: Gas exchange: As the water gets deeper, the wavelength of light gets shorter until its gone. The red and blue wavelengths are lost, and the green (not so good for photosynthesis) remains. Adaptations include:

  • Wetland plants often use C4 biochemical pathway of photosynthesis instead of C3.
    -C4 provides a possible pathway for recycling CO2 from cell respiration
    -plants using C4 have low photorespiration rates and the ability to use even the most
    intense sunlight efficiently.
    -C4 plants are more efficient than C3 plants in the rate of carbon fixation and amount of water used per unit carbon fixed.


  • Barriers prevent or control the entry of salts -root and leaf cell membrane act like ultra-filters.
  • Organs specialized to excrete salts -selectively remove certain ions from the vascular tissue of the leaf.

Submerged plants:


  • Vallisneria produce a coiled peduncle (female), which straightens out so the stigma can reach above the water surface. The spathe (male) also straightens out so its petals float on the surface. Its three leaves and anthers form a sailboat. The spathe floats along until hopefully it bumps into a stigma.
  • Ceratophyllum: uses a strategy of hydrophily: the male releases pollen into the water where it floats until it sinks again, hopefully landing on a female plant.
  • A chinese lotus can lay dormant for over 1,000 years.     


Algal blooms can block the sunlight and nutrients to submerged plants.

Aeration of Roots –

Oxygen is transmitted from the leaves to the roots and rhizomes by lacunae (air spaces forming channels in leaves, stems, and roots). Lacunae also have a structural role. Lacunae take up about 60% of the plants volume.

An experiment was done to demonstrate the oxygen gradient in plants.  It was found that a plant has 20% oxygen in its leaves, 15% in its stem,10% in the root parts, and only 2- 5% in the root hairs. The oxygen is taken in from the air by photosynthesis and travels through the plant and out of the root hairs.

When low oxygen levels are present, plants use other mechanisms to adjust for respiration. Aquatic plants can respire anaerobically. This has been shown experimentally by bubbling N2 or O2 into the water with rhizomes, and then measuring the ethanol production. At <3% O2 , ethanol is produced by Typha, Scirpus, Nuphar, and others. Some aquatic plants have developed air roots along their stems for respiration in water. Aquatic trees have developed pnuematophores, which are extensions of the root system reaching above the water level. Pnuematophores take in oxygen through small holes at their tips.

Other challenges that aquatic plants must adapt to include: flooding, desiccation (drying out) nutrient uptake, and vegetative reproduction.

Adaptation of animals in aquatic habitat

  • Changes in body organization to exploit water as a habitat are known as aquatic adaptation.
  • All classes of vertebrates have their representatives partially or totally adapted to aquatic life.
  • An aquatic animal should have the ability to swim to overcome the resistance of the surrounding medium.
  • Therefore, they must have a streamlined body with an organ or ability to float.
  • The animal should also have to overcome the problem of osmoregulation.
  • There are two types of animals living in the present day water, which have undergone aquatic adaptation.
  • According to their origin, they are primary and secondary aquatic animals.
  • Primary aquatic animals are those animals whose ancestors lived in water and they are still living in the water. Therefore, primary aquatic animals never had a terrestrial ancestry. They exhibit perfect aquatic adaptions. All fishes are primary aquatic animal.
  • Secondary aquatic animals are those whose ancestors were lung breathing land animals, migrated to the water for some reason and ultimately got adapted to live in aquatic habitat.
  • Some secondary animals live partially while others live totally in the water.
  • All aquatic reptiles, aves and mammals are representatives of secondary aquatic animals.
  • Amphibians are in a transitional form between primary and secondary aquatic life.
  • Respiratory organs are the gills in perfectly adapted aquatic forms such as fishes but in the air breathing forms nostrils are located near the top of the head to enable them to go to surface frequently to inhale air.
  • Locomotary organs are developed as the fins to swim in water easily. There are different types of fins including dorsal fin, ventral fin, caudal fin, pectoral fins and pelvic fins.
  • All the fins help in swimming but the caudal fin helps them to balance the body in water.
  • Some aquatic forms like amphibians have a thin fold of skin in between the digits of the hind limbs which are called web. Web helps to increase the surface area for swimming.
  • Aquatic animals like turtles have fin like organs called paddles for swimming and whales have the flippers as the swimming organ.
  • Body is covered by scales which make the body soft and slippery so as to escape from the enemies and also helps them to protect the internal soft organs of the body.
  • Some fishes have a hydrostatic organ called air bladder for adjusting them in the different depths of water according to their need by increasing the amount of gas or by decreasing the amount of gas inside the air bladder.