AQUATIC ENVIRONMENT

 

An aquatic ecosystem includes freshwater habitats like lakes, ponds, rivers, oceans and streams, wetlands, swamp, etc. and marine habitats include oceans, intertidal zone, reefs, seabed. The aquatic ecosystem is the habitat for various animals, plants, and microbes.

 

FRESHWATER ENVIRONMENTS

Fresh water habitats are classified based on their chemical and physical properties. Those with standing water such as lakes and ponds are called lentic habitats and those with running water are lotic habitats.  Most fresh water is locked up in ice sheets, glaciers, or is in ground waters such as lakes and rivers. These provide microbial environments that are different from the larger oceanic systems in many important ways. 

    For example, in lakes, mixing and water exchange can be limited. This creates vertical gradients over much shorter distances. Changes in rivers occur over distance and/or time as water flows through river channels.

 

Lakes

Lakes vary in nutrient status. Some are oligotrophic or nutrient poor; others are eutrophic or nutrient-rich. Nutrient-poor lakes are oxygen saturated, has low organic matter and a low microbial population.  Nutrient-rich lakes have bottom sediments that contain organic matter and oxygen poor bottoms.

            When large amounts of nutrients are added to water, eutrophication (nutrient enrichment) takes place and stimulates the growth of plants, algae, and bacteria. Because nitrogen and phosphorus frequently limit microbial growth in freshwater habitats, the addition of nitrogen and phosphorus compounds has a particularly large impact on freshwater systems. Both cyanobacteria and algae can contribute to massive blooms in strongly eutrophied lakes.

Based on penetration of the light, lakes are divided into three zones

 

 

1)          Littoral zone- where light penetrates into the bottom- occupied by submerged or partially submerged higher plants and attached filamentous and epiphytic algae.

2)          Limnetic zone – area of open water away from the shore. 

Littoral and limnetic zone together form the euphotic zone, where photosynthesis occurs. Limnetic zone extends to a level known as compensation depth.

The compensation depth is the lowest level having effective light penetration. Photosynthetic activity balances respiratory activity, here.

 

3)      Profundal zone- the area of deeper water beyond the depth of effective light penetration. Absent in shallow lakes. In deeper lakes, it extends from the light compensation level to the bottom.

Littoral and limnetic zone form the euphotic zone. Below euphotic zone is the aphotic zone with no light penetration.

Euphotic zone mainly has primary producers such as phototrophs. Profundal zone has secondary producers who depend on the transport of organic compounds from the upper zone. The lake bottom called, benthos is the interface between hydrosphere and lithosphere. The lithosphere seen at the bottom of lake is called sediment and is occupied by many microorganisms.

 

Thermal stratification of Lake

Lakes have different layers/strata depending on the temperature of water. This thermal stratification of a lake according to temperature and depth is seasonal. There is an aerobic  epilimnion (warm, upper layer) and usually anaerobic hypolimnion (deeper, colder, bottom layer) if the lake is nutrient-rich). The epilimnion and hypolimnion are separated by a one of rapid temperature decrease called the thermocline, and there is little mixing of water between the two layers.

 

Epilimnion- warm, oxygen rich with vigorous photosynthesis

Hypolimnion – low temperature, low oxygen, poor light penetration, restricted photosynthesis 

 

         As seasons change, the aerobic surface water and the anaerobic subsurface water will turn over as the result of differences in temperature and specific gravity. After such mixing occurs, motile bacteria and algae migrate within the water column to again find their most suitable environment.

In addition to temperature and light, concentrations of organic and inorganic nutrients, oxygen, salt concentrations and acidity affect microbial growth and distributions in lake.

 

 

 

Freshwater Microflora- Freshwater Microbial Diversity

            Freshwater environments provide excellent habitats for microorganisms. Large numbers of microorganisms in a body of water generally indicate high nutrient levels in the water. Water contaminated by inflows from sewage systems or from biodegradable industrial organic wastes is relatively high in bacterial numbers. Freshwater environments are highly variable in the resources and conditions available for microbial growth. Both oxygen producing and oxygen consuming organisms are present in aquatic environments, and the balance between photosynthesis and respiration controls the natural cycles of oxygen, carbon, and other nutrients (nitrogen, phosphorus, metals).

 

Neuston

Neuston is the uppermost layer of hydrosphere-it is the interface between hydrosphere and atmosphere and is occupied by phototrophic microorganisms. Organisms float on the top of the water -(Epineuston) or live right under the surface - (Hyponeuston).  Primary producers are abundant here because of the availability of unrestricted light, carbon dioxide & mineral nutrients. Secondary producers also proliferate here. Microbial numbers in the surface layer are 10 to 100 fold higher than the underlying water column. Bubbles arising from the neuston layer burst out liberating bacteria and other microorganisms to air.  

Autochthonous (native) neuston microbiota include algae, bacteria, fungi and protozoa. Common bacteria are Pseudomonas, Caulobacter, Achromobacter, Flavobacterium, Alcaligenes etc. Gram positive and negative, pigmented and non-pigmented, motile and non-motile, rod and cocci, stalked and un-stalked forms seen. Common blue green algae or Cyanobacteria include Anabaena & Microcystis. Filamentous fungi like Cladosporium and various yeasts, algae like Nautococcus, Chromulina, and protozoa like Vorticella, Arcella etc. are present in neuston.

 

Other Freshwater Microflora           

 

A variety of microorganisms live in fresh water. The region of a water body near the shoreline (the littoral zone) is well lighted, shallow, and warmer than other regions of the water. Photosynthetic algae and bacteria that use light as energy flourish in this zone. Further away from the shore is the limnetic zone. Areas of the limnetic zone with sufficient oxygen contain bacteria like Pseudomonads and species of Cytophaga, Caulobacter, and Hyphomicrobium. Photosynthetic algae are also located in the limnetic zone. 

Deeper waters of the profundal and benthic zones have low oxygen concentrations and less light. Algal growth near the surface often filters the light, and photosynthetic microbes in deeper zones  use different wavelengths of light from those used by surface-layer photosynthesizers. Purple and green sulfur bacteria are found in the profundal zone. These bacteria are anaerobic photosynthetic organisms that metabolize H₂S to sulfur and sulfate in the bottom sediments of the benthic zone. Finally, at the bottom of fresh waters is the benthic zone containing the sediments, where few microbes survive. Bacteria that can survive in the absence of oxygen and sunlight, such as methane producing bacteria, thrive here. Clostridium species are common in bottom sediments and may include botulism organisms, particularly those causing outbreaks of botulism in waterfowl. 

Microbial photosynthesizers mainly include algae and cyanobacteria . Others feed on these organisms, forming the next link in the food chain . Plant material from the land also enters lakes and streams at their edges, providing an important nutrient source for many water bodies. Decomposers form an especially important part of fresh-water ecosystems because they consume dead bodies of plants, animals, and other microbes. These microbial agents of decay are an important part of the ecosystem because they convert detritus (dead and decaying matter) and organic materials into needed nutrients, such as nitrate, phosphate, and sulfate. Decomposers  are essential to the major biogeochemical cycles by which nutrients are exchanged between the various parts of the ecosystem, both living and nonliving.

 Aerobic decomposers in water need oxygen to survive and do their work which is ensured by the flowing water and waves. If there is not enough oxygen in the water, many parts of the system suffer-the aerobic decomposers cannot digest plant matter, insects cannot develop and mature, and the fish cannot grow properly. Eventually, the stream or pond will be changed, starting at the microbial level. Human interaction can jeopardize parts of this system in a variety of ways.

Fresh water is host to numerous microorganisms that affect human health directly.  Polluted drinking water is a major source of illness and death throughout the world, particularly in developing countries. Some common microbes in lakes and streams that are responsible for disease include:

·                  The protozoa Giardia lamblia, found in fresh-water bodies throughout the world. Giardiasis is a common waterborne illness.

·                  The bacterium Vibrio cholerae, remains a significant source of disease and death .

·                  The bacterium Escherichia coli, is a very common waterborne pollutant. Humans have a large and harmless population of E. coli in their large intestines, and bacteria make up a large fraction of the volume of human feces. When released into drinking water or recreational water sources, E. coli can be ingested causing diarrhea. 

Thus, many microorganisms are found naturally in fresh water including bacteria, cyanobacteria, protozoa, algae and tiny animals such as rotifers. These can be important in the food chain that forms the basis of life in the water.

 

OCEAN ENVIRONMENT

 

During the slow evolution of our planet, plate tectonics has continuously changed the positions of continents and oceans. At the time of the Pangea, there was only one large ocean surrounding this one continent. Today, according to the classification of the International Hydrological Organization (IHO), there are three oceans.

The Pacific Ocean is the largest, its surface area is about half that of the oceans as a whole, and it alone covers one-third of the Earth’s surface. because of its predominance on the surface that the median meridian of this ocean was chosen as the date change line. The Atlantic Ocean is the second largest by area, accounting for about 30% of the total. It is much better supplied with fresh water than other oceans, since it receives flows from large rivers such as the Amazon, Congo and St. Lawrence. The Indian Ocean, the third largest by area, accounts for about 20% of the total. It is almost entirely located in the southern hemisphere, between Asia, Africa and Australia.

Despite this official classification, we mention 5 oceans on our planet with the Antarctic Ocean to the south, which surrounds the Antarctic continent up to about 60 degrees and whose area represents about 6% of the total, and the Arctic Ocean to the north, which is bordered by the lands of Siberia, Scandinavia, Greenland and North America, whose area represents about 4% of the total.

The seas are the marine sub-domains, of relatively small sizes- Mediterranean Sea, North Sea Baltic Sea Caribbean Sea and the English Channel.

The salinity, or mass fraction of salt is expressed in g/kg (gram of salt per kilogram of sea water). In lakes and rivers, salinity is almost zero, rarely exceeding a few units. It can reach and sometimes exceed 50 g/kg in the seas, its average value is around 35 g/kg. It is 12 g/kg in the Black Sea. In the Dead Sea, its very high value, close to 275 g/kg, practically prohibits any animal or plant life.

In summer, the surface water of the warmest seas can reach temperatures of 26 to 30°C, which often leads to cyclones. In the upper layers of the marine environment, surface water, heated by solar radiation, is subjected to constant thermal exchanges by conduction and convection with the atmosphere. Agitation by waves and turbulence then manages to homogenize the temperature in the first tens of meters (between 0 and -50 m). At great depths (below -120 m), exchanges are limited and become much weaker yielding a resting marine environment.

Between these two areas, a relatively thin layer (between -50 and -120 m) called the thermocline, where the temperature can vary by about ten degrees between the water above and the water below. The temperature of the water above the thermocline experiences significant seasonal variations, due to variations in sunlight, without any change in the temperature of the deep layers.

There is a vertical movement of water which plays a key role in the deep life of the marine environment by regenerating oxygen by supplying surface water and also bring nutrients from the seabed to the surface. In the Mediterranean, this phenomenon of deep convection is mainly located in the Gulf of Lions, making it a major center of biological activity. However, this convection only occurs when the conditions are adequate.

The Black Sea lacks these which this prevents the penetration of oxygen beyond a depth of 200 m. Only very specific species can live in this marine environment under these anoxic conditions.

The amount of sunlight that reaches the water in ocean depend mainly on two factors: distance from shore and depth of water. Oceans are divided into zones based on these two factors. The ocean floor makes up another zone (benthos) .

 

Horizontal Divisions- Zones Based on Distance from Shore

 

The ocean is divided horizontally by distance from the shore. There are three main Horizontal Divisions -the intertidal zone, neritic zone, and oceanic zone.

·                  Nearest to the shore lies the intertidal zone (also called the littoral zone), the region between the high and low tidal marks. The important feature of the intertidal is change: water is in constant motion in the form of waves, tides, and currents. The land is sometimes under water and sometimes exposed.

·                  The neritic zone is from low tide mark and slopes gradually downward to the edge of the seaward side of the continental shelf. Some sunlight penetrates to the seabed here.

·                  The oceanic zone is the entire rest of the ocean from the bottom edge of the neritic zone, where sunlight does not reach the bottom.

 

Vertical Divisions-  Zones Based on Depth of Water

    The vertical extent of ocean water is the water column Two main zones based on depth of water and based on light penetration, vertically are the photic zone and aphotic zone. 

    Sunlight only penetrates the sea surface to a depth of about 200 m, creating the photic zone ("photic" means light). Organisms that photosynthesize depend on sunlight for food and so are restricted to the photic zone. Tiny photosynthetic organisms, known as phytoplankton, supply nearly all of the energy and nutrients to the rest of the marine food web and they occupy photic zone. In the aphotic zone there is not enough light for photosynthesis. The aphotic zone makes up the majority of the ocean, but has a relatively small amount of its life, both in diversity of type and in numbers. Photic zone is further divided into epipelagic, mesopelagic and bathypelagic zones, while, aphotic zone consists of abyssal pelagic and hadal zones, based on the depth.

 Composition of seawater

The chemical composition of seawater is quite complex. Most of the chemical elements are found in solution in the form of a complex mixture of anions, cations and molecules.

Some ions come from the dissolution of continental rocks by rivers that carry them to the oceans, where they stay for very long periods of time and where evaporation of water increases their concentration. A significant part of the cations comes from the original ocean floor. And the origin of the chloride ion is often attributed to the degassing of hydrogen chloride from volcanoes, which is soluble in water

In addition to water and salts, there are also various low-concentration molecules, such as boric acid (0.0198 g/kg) and carbon dioxide (0.0004 g/kg), as well as nitrogen and oxygen. The amount of carbon dioxide in seawater is much greater than in the air- the possibility of sequestering carbon dioxide in the oceans, with a view to reducing the content of this greenhouse gas in the atmosphere is a much discussed topic now.

 
Biodiversity of the marine environment

On Earth, formed 4.6 billion years ago, life appeared in the oceans about 3.8 billion years ago. And it was only very recently, about 400 million years before our era, that it conquered the land. As a result, extremely diverse lifestyles have developed in the oceans, where light only penetrates the upper layers, gradually adapting to the specific conditions of this marine environment. Most of the biodiversity on our planet is found in the marine environment but most of these species, especially those living in deep water, are still unknown.

 

Marine Microflora

Neuston and pleuston are organisms that live near the surface of a water body. The small aquatic organisms inhabiting the surface layer or moving on the surface film, are neuston and the organisms that live at the air-water interface is the pleuston. These organisms are exposed to harsh environmental conditions such as high-temperature variations, high-light variations, including UV irradiation, marine, and aerial predators, etc. 

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Marine bacteria

 

Pelagibacter ubique and its relatives may be the most abundant organisms in the ocean, and they are possibly the most abundant bacteria in the world. They make up about 25% of all microbial plankton cells, and in the summer they may account for approximately half the cells present in temperate ocean surface water.

The largest known bacterium, the marine Thiomargarita namibiensis, can be visible to the naked eye and sometimes attains 0.75 mm (750 μm).

Marine bacteria perform all kinds of chemical processes in the open ocean, including most of the steps in nitrogen cycling. Cyanobacteria are a large group of photosynthetic bacteria, they “fix” nitrogen, converting nitrogen gas into more biologically useful compounds.

Trichodesimium is one of the most important and well-studied nitrogen-fixing cyanobacteria found in open-ocean areas such as Station ALOHA. It is one of the few organisms involved in the oceanic nitrogen cycle that is visible to the naked eye. Other cyanobacteria in open-ocean areas is Richelia, which is found living inside diatoms, a type of marine algae. Uncultivated cyanobacteria group A (UCYN-A) is a group of nitrogen-fixing cyanobacteria that cannot perform photosynthesis and possibly form “partnerships” (symbioses) with other photosynthetic organisms.

Proteobacteria are an extremely diverse group of bacteria. Some alphaproteobacteria and gammaproteobacteria fix nitrogen. Other important ones are the ammonium oxidizing bacteria (AOB) such as betaproteobacteria and gammaproteobacteria involved in nitrification and nitrite oxidizing bacteria including Nitrobacter, Nitrospira, and Nitrospina etc. Photosynthetic cyanobacteria which do not fix nitrogen include Prochlorococcus, Synechococcus.

Marine archaea are also common in oceans, including ammonium oxidizing archaea (AOA) in the group Crenarchaeota. These archaea are common in the deeper parts of the open ocean, where there are little light and oxygen concentrations are relatively low.

 Archaea are extremophiles living in harsh environments, such as the yellow archaea in a hot spring, but they are also found in a much broader range of habitats. Archaea are particularly numerous in the oceans and may play roles in both the carbon cycle and the nitrogen cycle. Thermoproteota (also known as eocytes or Crenarchaeota) are a phylum of archaea thought to be very abundant in marine environments and one of the main contributors to the fixation of carbon. Eocytes may be the most abundant of marine archaea. Halobacteria, found in water near saturated with salt, are now recognised as archaea. Methanosarcina barkeri, is a marine archaea that produces methane. Marine thermophiles, such as Pyrolobus fumarii, survive well over 100 °C.

 

 

Redtide

 

Red tide is a colloquial term used to refer the natural phenomena harmful algal blooms (HABs), (or excessive algae growth) that causes negative impacts to other organisms by production of toxins, mechanical damage to other organisms, or by other means. After the bloom dies, the microbes that decompose the dead algae use up more of the oxygen, generating a "dead zone" which can cause fish death. When these zones cover a large area for an extended period of time, neither fish nor plants are able to survive. HABs are induced by eutrophication, which is an overabundance of nutrients (nitrogen and phosphate) in the water. The excess nutrients are emitted by agriculture, industrial pollution, excessive fertilizer use in urban/suburban areas, and associated urban runoff.

The harmful effects from such blooms is due to the toxins they produce or from using up oxygen in the water which can lead to fish die-offs. Some only discolor water, producing a smelly odor, or adding a bad taste to the water. There are three main types of phytoplankton which can form into harmful algal blooms: cyanobacteria, dinoflagellates and diatoms.  Some cyanobacteria, such as Microsystis, can produce hazardous cyanotoxins such as microcystins, which are hepatotoxins that harm the liver of mammals. Other types of cyanobacteria can also produce hepatoxins, as well as neurotoxins, cytotoxins, and endotoxins. Diatoms and dinoflagellates (in marine coastal areas) also cause HAB. Diatoms produce domoic acid, another neurotoxin, which can cause seizures in higher vertebrates and birds as it concentrates up the food chain. Domoic acid  accumulates in the bodies of shellfish, sardines, and affect the nervous system of the consumers such as sea lions, otters, cetaceans, birds or people causing serious injury or death. Blooms of harmful algae can have large and varied impacts on marine ecosystems, depending on the species involved, the environment where they are found, and the mechanism by which they exert negative effects.

Redfield ratio

 In 1934, Alfred Redfield discovered that the ratio of carbon to nitrogen to phosphorus is a nearly constant 106:16:1 throughout the world's oceans, in both phytoplankton biomass and in dissolved nutrient pools. Redfield noticed that the ratio between the quantities of Carbon, Nitrogen and Phosphorus constituting the healthy oceanic phytoplankton, as well as the Nitrogen and Phosphorus in the waters of healthy seas remained close to this value. Thus, "Redfield Ratio" is maintained as the optimal ratio between Carbon, Nitrogen and Phosphorus in natural aquatic ecosystems.

 

 

Marine viruses

Marine phages parasite marine bacteria and archaea, such as cyanobacteria. They are the most abundant biological entity in marine environments. Tailed phages of the order Caudovirales, non-tailed viruses, Phages belonging to the families Corticoviridae, Inoviridae, Microviridae are also known to infect diverse marine bacteria. There are also archaean viruses which replicate within archaea and giant viruses such as the giant mimivirus and the largest known virus, Tupanvirus, as marine microflora.

Microorganisms make up about 70% of the marine biomass. There are 15 times as many viruses in the oceans as there are bacteria and archaea. The viruses kill 20% of microbial biomass and harmful algal blooms. Viruses are an important natural means of transferring genes between different species, which increases genetic diversity and drives evolution.

 

Marine protists

Protists are eukaryotes that cannot be classified as plants, fungi or animals. They are usually single-celled and microscopic. Common examples are red and brown algae, diatoms, some dinoflagellates, foraminiferans, radiolarian, some marine amoebae, ciliates and flagellates, slime moulds and slime nets. Diatoms have glass like cell walls made of silica. Diatoms generate about 20% of world oxygen production. Coccolithophores are minute unicellular photosynthetic protists with two flagella for locomotion. Most of them are protected by a shell covered with  circular plates or scales called coccoliths, made from calcium carbonate.

 

Single-celled alga, Gephyrocapsa oceanica,and groups of small-sized algae- Zoochlorellae or zooxanthellae -that live inside freshwater protozoans and invertebrate hosts like coral are common in marine environments. They have a symbiotic association with the host and use carbon dioxide, nitrogenous, phosphorous waste, and provide oxygen and essential nutrients to the host. Most protists are single-celled and microscopic. Some single-celled marine protists are macroscopic. Spiculosiphon oceana, a unicellular foraminiferan has an appearance and lifestyle that mimics a sponge. Xenophyophore, another single-celled foraminiferan, lives in abyssal zones. Giant kelp, a brown algae, is not a true plant, but is multicellular and can grow to 50m.

 

Marine fungi

 

Over 1500 species of fungi are known from marine environments. These are parasitic on marine algae or animals, or are saprobes feeding on dead organic matter from algae, corals, protozoan cysts, sea grasses, and other substrata. Marine fungi can also be found in sea foam and around hydrothermal areas of the ocean. Many unusual secondary metabolites is produced by marine fungi. The aquatic fungi play a significant role in heterotrophic mineralization and nutrient cycling. Lower fungi  adapted to marine habitats include mastigomycetes: oomycetes and chytridiomycetes. Higher fungi are filamentous hyphomycetes, ascomycetes, basidiomycetes.

 Lichens are mutualistic associations between a fungus, usually an ascomycete, and an alga or a cyanobacterium. Several lichens are found on rocks in marine environments or covering sea snails. Fossil marine lichens 600 million years old have been discovered in China

 

 

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