Estuaries

An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and opening to the sea.

Estuaries are transition zones between rivers and the sea and differ from both in abiotic and biotic factors. They are also among the most highly productive ecosystems on the earth. The estuarine environment is characterized by a constant mixing of freshwater, saline seawater, and sediment, which is carried into the estuary from the sea and land. 

The mixture and fluctuation of salt and freshwater impose challenges to the animals and microbes. The salinity ranges from full strength seawater to freshwater. Associated change is sedimentary conditions from fine sediment to coarse sediments. Other changes include nutrient input, pollutant and chemical concentration along with estuarine flows.

The productivity and variety of estuarine habitats support a wonderful abundance and diversity of species. Thousands of species of fish, migratory birds, shore birds, marine mammals, clams, shellfish and other wildlife survive in and around estuarine habitats. In addition to serving as important habitats for wildlife, estuaries also provide valuable environmental services. The water flowing to the ocean carries sediments, organic and inorganic nutrients, and pollutants. The harmful pollutants deposited creates an environment for microbial biodegradation of these sediments. Estuaries also provide a great deal of aesthetic enjoyment for the people who live, work, or recreate in and around them. 

The activities of microorganisms dominate the functions and nutrient cycling of estuarine ecosystems. Large numbers of bacteria, fungi and protozoa have been found in estuaries and benthic sediments. Their distribution, species abundances and activities interact with their physical and chemical environment.

 

Microbial communities

Bacteria

Bacteria are the most numerous organisms in the estuary. Sediments and salt marsh soil generally harbor more bacteria per unit volume than does the water column. Aerobic and facultative anaerobic bacteria are most common, and Pseudomonads and Vibrio are the most often isolated species. Higher bacteria densities have been found in most estuaries than in nearby coastal seawater and river water 

Fungi

The number of fungi living in estuaries is extremely large. Some of fungi are unique in estuaries, while others have a broader range of habitats. Aquatic fungi and yeast dominate species in aquatic environment, few of fungi associate with particles or solid matters in the water. In sediments, the active species of fungi primarily are found in surface aerobic zones. The densities of fungi decrease rapidly with soil depth, but the spores of fungi are found throughout sediments 

Most of the bacterioplankton are closely related to surrounding freshwater or marine bacterial groups and belong to the phyla Proteobacteria, Bacteroidetes, and Actinobacteria. Cyanobacteria play an important role as primary producers,  Oscillatoriales, chroococcoid colonies and  Synechococcus-like Cyanobacteria are prevalent. Methanogenic Archaea are important for the mineralization of organic matter in anoxic estuarine environments. Sulfate-reducing bacteria often outcompete methanogens for hydrogen and acetate in estuarine sediments.

 

Carbon & Nitrogen cycling

Bacteria show a variety of metabolic pathways related to carbon flow and cycling. Photosynthesis is mainly carried out by algae and phytoplankton in estuarine. As many of the sediment and water-logged soils of estuaries are anoxic, anaerobic decomposition is important. Complex organic matter is used by the fermenters and dissimilatory nitrogenous oxide reducers.

Nitrogen is a major limiting nutrient for primary production in estuaries. The N-cycling processes that are dominated by microbial activity include nitrification, dissimilatory nitrous oxide reduction, and nitrogen fixation. Nitrogen cycling in estuaries is related to the water mixing and microbial community dynamics.

Water movement is the dominant controlling factor in estuarine ecosystem. Circulation stimulates fluxes of dissolved constituents and particulate materials such as sediments, detritus, bacteria, and plankton. In Estuaries, salt water mixes with water derived from land drainage. The estuarine circulation movements are the primary mechanism of mixing. 

Much of the organic matter carried to an estuary by rivers, produced by phytoplankton, or derived from marshes, is deposited on the sediment surface. Oxygen is the most important electron acceptor in organic matter respiration, but at the water column of anerobic estuarine or saturated sediment sulfate become more significant electron acceptors. The major product of sulfate reduction is hydrogen sulfide, which gives the habitat a pungent smell. 

Autotrophic nutrients are important for the functional estuarine ecosystems, because they are the raw materials for the primary producers. The concentrations of these nutrients change in estuaries due to the mixing of river and ocean water. Microbial heterotrophic activity and primary production play very important roles in the formation and turnover of organic matter in eutrophic estuaries