Owing to its location between the freshwater biome and the marine biome, the estuary biome is usually considered to be the transition zone between these aquatic biomes. Even though you are likely to come across various sources stating that the estuary biome is a part of the broader marine biome - or the freshwater biome for that matter, it is technically incorrect as you get to see several differences in the biotic and abiotic factors in them.
Estuary Biome Facts
An estuary is a partially enclosed body of water along the coastline which is typically characterized by the fact that it is connected with one or more rivers as well as the sea. There exist different types of estuaries which are either classified on the basis of their geomorphology or on the basis of their water circulation. The lagoon-type estuaries - which are semi-isolated from the ocean by barrier beaches, fjord-type estuaries - which are formed as a result of erosional activity by glaciers, or those estuaries which are formed as a result of tectonic movement of the Earth's crust, are some of the most popular types as far as geomorphological classifications of estuaries in concerned.
On the other hand, the salt wedge type estuary - wherein the river output greatly exceeds the marine input, inverse type - wherein evaporation exceeds the inflow of fresh water by a significant extent, partially mixed type - wherein tidal forces reduce the freshwater output and add to the marine output, etc., are examples of the water circulation based classification of estuaries. Even though they do not fit into the aforementioned definition of an estuary, features like bays, inlets and lagoons, are also referred to as estuaries at times. Both, geomorphology and water circulation combine with other abiotic factors of the estuary biome to determine the level of biodiversity that you get to see here.
Being the transition zone, this biome is influenced by the abiotic factors which influence the freshwater biome - such as fresh water flow and sedimentation, as well as those which influence the marine biome - such as tides and waves. A high degree of variation can be seen in terms of salinity within the estuary biome. If the salinity levels at the point wherein the river ends is zero, the same at the mouth of the estuary can be as high as 30 percent or - though rarely, more.
Similarly, the sediment brought in by the river tends to settle at intertidal mudflats, thus making it difficult for various species of plants and animals to survive here. As the levels of dissolved oxygen here are pretty low, the development of hypoxic or anoxic zones cannot be ruled out. Coming to a consensus on estuary biome climate is not an easy task, as estuaries are not restricted to a particular geographical region, and the climatic conditions of each of these regions vary.
Biotic Factors: Flora and Fauna
Even though problems like salinity, sedimentation, and low levels of dissolved oxygen do exist in the estuary biome, many species of plants and animals have adapted themselves to the seemingly harsh conditions that prevail here. While this biome may not boast of a great deal of biodiversity as such, there do exist some species which are found in large numbers in this biome. The list of plants includes species of algae as well as sea weeds and marsh grass. Though mangroves form an important constituent of the vegetation here, they are only found in abundance in the estuaries of the tropical regions of the world.
Estuaries also act as breeding grounds and nurseries for various marine animal species. In fact, the depleted levels of dissolved oxygen in the estuary water can be attributed to the presence of certain species of bacteria which require a significant amount of oxygen to survive. Estuary biome is home to species like crabs, shellfish, clams, lobsters, shrimps, fish, sea snakes, etc. In a broad sense - on the other hand, the list of animals would also include aquatic birds, reptiles, marine mammals, as well as terrestrial mammals, which inhabit the areas around the estuary.
As in case of other biomes of the world, even the plants and animals of this biome come together to form a complex food web. At the base of the estuary biome food web you find detritus, i.e. the non-living particulate organic material, which happens to be the primary source of food for many species found in this biome. At the top of the food web in the estuary biome, you find some mammalian species which burrow around the estuaries, and / or some species of fish which come here during the breeding season.
Though not as diverse as the rainforests or the marine biome, the estuary biome is important - not just for various species which are an integral part of its food web, but also for various species which are not directly related to it - humans being the best example of the same. Some of the largest cities of the world - including New York City, are situated along the estuaries, and this very fact hints at the importance of this water body for humans. While estuaries have been beneficial for mankind since quite some time now, we humans seem to be hellbent on wiping them off the planet - at least the number of anthropogenic activities that harm estuaries hints at this very fact.