Aquatic Biomes

There are two main types of aquatic biomes, defined according to their salinity. A distinction is thus made between marine biomes and freshwater biomes. Marine biomes include all the environments where there is what is known as salt water, in other words, bodies of water where the salinity is greater than $10\ \text{g/L}.$ The average salinity of these biomes is approximately $35\ \text{g/L}.$ Freshwater biomes are characterized by a salinity of around $1\ \text{g/L}.$ Each of these biomes is home to unique species that are adapted to the salinity of the water.

Despite occupying less than $1\%$ of the Earth's surface, they are still home to a great diversity of species. They are extremely influenced by their neighbouring terrestrial biomes. Freshwater biomes include the regions described below.

They are, therefore, strongly influenced by the vegetation and the type of soil that surrounds them. Lakes are divided into different zones according to their depth. The littoral zone, which corresponds to the submerged shores of the lake, provides a living environment for several aquatic plants and algae, either fixed or floating. The superficial zone, far from the shores, is well lit by sunlight, which allows it to shelter plankton and many fish. The deep area is dark since it is below the limit of light penetration. In addition, this area is cold and poorly oxygenated. Many microorganisms and decomposer invertebrates can be found there.

They include, among others, ponds, marshes, swamps, and peat bogs. Wetlands play an important ecological role since they retain water and filter it through their abundant vegetation, consisting of mosses, algae, and plants such as cattails and cedars.

The flora and fauna found there vary according to the nutrient content of the waterway. In still waters, communities of living organisms are similar to those found in lakes. In flowing waters, the strength of the current limits the presence of plants and plankton. Some fish can swim against the current, such as trout and salmon, to reach quieter areas.

These areas are very rich in nutrients and sediments, which make these feeding and breeding grounds for many species, in both fresh and sea water.

They are more abundant than freshwater biomes since they cover nearly $71\%$ of the Earth's surface. The distribution of living organisms in these biomes is extremely variable since it depends both on the water depth and its proximity to the coasts. In marine biomes, there are generally five zones, corresponding to oceanic divisions, which are described below.

It is permanently subjected to the movement of the tides and the waves. Depending on the composition of the littoral bottom (rocky, sandy, or muddy), this zone is inhabited by stationary animals (mussels, barnacles) or burrowers (worms, clams), by fixed algae and semi-aquatic plants.

This area rests on the continental shelf of the Earth's crust. It can be covered with algae and seaweed up to about $100\ \text{m}$ deep since a sufficient amount of light penetrates to this depth. Rich in nutrients due to the proximity of the continents, coastal waters are particularly rich in plankton, invertebrates (crabs, lobsters, squid, etc.), and fish (herring, sardines, cod, etc.). These waters contain $90\%$ of all marine species, making them important fishing grounds.

They form a separate biome often located near volcanic islands. Reefs are underwater limestone structures built by coral colonies, marine invertebrates that live in symbiosis with microscopic algae. Coral reefs provide a habitat for a great diversity of invertebrates and fish. Their biodiversity is comparable to that of tropical rainforests.

Despite this geographical abundance, it contains only $10%$ of marine species, mainly concentrated in the surface area, where phytoplankton grows as a result of solar ray penetration. The phytoplankton of the oceanic pelagic zone provide nearly $40\%$ of the biosphere's photosynthesis. The deep and darker part of the oceanic pelagic zone is low populated, although some species of fish pass through or live there.

Most of this area is below $1\ \000\ \text{m}$ in depth, in complete darkness. Varied in relief, the seabed is covered with a thick layer of mud and sediment. Microorganisms and certain invertebrates adapted to extreme conditions (cold temperatures and very high pressures) can be found here, feeding mainly on detritus coming from the pelagic zone.