Salinity – Substantial river discharges and relatively shallow near-shore water often result in large fluctuations and strong gradients in salinity. Reduced salinity is associated with finer substrates. Salinity of estuaries usually increases away from a freshwater source, although evaporation sometimes causes the salinity at the head of an estuary to exceed seawater.
Oxidation – Much of the organic matter carried by the river to the estuary, produced by phytoplanktons or derived from marshes, is deposited on the surface of the sediment. Oxygen is the most important electron acceptor for organic matter decomposition. But in anaerobic sediments, sulphate becomes more significant as an electron acceptor. The major product of sulphate reduction is H2S, which gives estuaries and salt marshes a pungent smell. The environment is more oxidizing near the sediment-water interface and reduced deeper in the sediment.
Nutrients – Autotrophic nutrients are important because they are the raw materials for primary producers. Their concentration keeps changing because of the mixing of river and ocean water. Microbial heterotrophic activity and primary production play a vital role in formation and turnover of organic matter in eutrophic estuaries. Higher microbial uptake and respiration rates happen when there is high organic nutrient input.
Food Web – In this ecosystem, plants and algae convert light energy into living biomass. Detritus feeders, plant grazers and zooplanktons are the primary consumers. Secondary and tertiary consumers include estuarine birds, ducks, invertebrate predators and fish. Excreta and detritus pass to the decomposer trophic level where microorganisms break down the material. At each trophic level, matter and energy are consumed and some of it is excreted as waste or converted into body mass or heat after respiration.
Microorganisms – Estuaries have a unique microbial community. This is partly due to the dynamic nature of estuaries and the heavy influence on estuarine populations by those organisms that wash in from adjacent environments. Most of the bacteria in a typical estuary are closely related to the surrounding freshwater or marine bacterial groups. Some phyla isolated are – Proteobacteria and Bacteroidetes. Acinetobacter has also been found.
Importance of Estuary
Estuary serves as habitat to a diversity of species due to its calm and nutrient-laden waters. Thousands of species of fish, birds, marine mammals, clams, shellfish and other life forms survive in and around estuary.
Many fishes and shellfish species depend on the waters of estuaries as a home to spawn and for their offspring to grow and live. Several migratory birds also use estuaries as an ideal place for resting and reproduction.
It also provides valuable environmental service. The water flowing to the ocean carries sediment, organic and inorganic nutrients and pollutants such as PAH (polycyclic aromatic hydrocarbon) along with it. Much of these residues are filtered out when they flow through wetlands, swamps and salt marshes, depositing harmful pollutants in these transition zones. This creates an ideal environment for microbial bioremediation.
Estuarine plants can also absorb tide and storm surges, providing stable and peaceful habitat for wildlife. This natural buffer helps to prevent soil erosion.
They also provide aesthetic enjoyment for people who live, work or recreate in and around such areas.
