Estuarine Ecosystem

CMF-121 Coastal & Marine Ecosystem

Chapter 2. Estuarine Ecosystem

CN for B.Sc. Fisheries (Hons) L1, S2

Chapter 2: Estuarine Ecosystem Chapter layout: Definition of estuary  Characteristics of Estuaries/Estuarine ecosystem  Values of Estuaries  Types of Estuaries  Limiting Factors and Driving Forces  Ecological classifications of estuarine organisms  Natural productivity and food production potential of Estuaries  Estuarine Food Web  Estuaries of Bangladesh  Human Impacts and Threats

Chapter 2: Estuarine Ecosystem Definition of estuary The word estuary is derived from Latin ‘aestus’ means ‘tide’. An estuary is a semi-enclosed (partially enclosed) coastal body of water (with one or more rivers or streams flowing into it) which has a free connection to the open sea. It is thus strongly affected by tidal action and within it sea water is mixed or diluted with freshwater from land drainage (Pritchard, 1967). (Please see figures of wave and tide dominated estuary model). According to Perillo (1995) "an estuary is a semi-enclosed coastal body of water that extends to the effective limit of tidal influence, within which sea water entering from one or more free connections with the open sea, or any other saline coastal body of water, is significantly diluted with fresh water derived from land drainage, and can sustain euryhaline biological species from either part or the whole of their life cycle." Example: The Hariavhanga river estuay, The Haringhata river estuay, The Sundarban river estuay, The Pasour river estuay, The Tetulia river estuay, The Kirtonkhola river estuay, The Meghna river estuay, The Feni river estuay, The Mohouri river estuay, The Karnafuli river estuay, The Matamohori river estuay (Coxs Bazar), The Ichamoti river estuay, The Shango river estuay, The Naf river estuay, The Vhagirothi river estuay, The Amazon river estuary, The Mississippi River estuary, The Nile river estuary etc.

Fig1a. Shows major estuarine ecosystems of Bangladesh (left); surface view of an estuary (right) Mohammad Mosarof Hossain, Assistant Professor, Dept. of CMF, Faculty of Fisheries, Sylhet Agricultural University, Sylhet-3100

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Features or characteristics of estuarine ecosystem: Environment: Estuaries could be considered as a transition zones between the freshwater and marine water habitats; and also transition from land to sea and fresh water to salt water but many of their physical and biological attributes are not transitional but unique. This ecosystem is made up of brackish water which is quite different from both freshwater and marine water. Estuaries are marine environments whose pH, salinity, and water levels vary, depending on the river that feeds the estuary and the ocean from which it derives its salinity. Both ocean and land contribute to form a unique environment which plants and animals have specially adapted (organasims have specialized adaptive capacity). Estuaries are protected from ocean forces by reefs, barrier islands, headlands and deltas. In estuaries, circulation is more or less continous and are mainly two types i. Estuarine circulation is common in estuaries; this occurs when fresh or brackish water flows out near the surface, while denser saline water flows inward near the bottom. ii. Anti-estuarine flow is its opposite, in which dense water flows out near the bottom and less dense water circulates inward at the surface. Estuaries are the most productive environments on earth. Four times more productive in plant matter than a rye grass pasture and 20 times more productive than the open sea. Estuaries are extremely rich in organic matter and nutrients, as transport and traps, nutrients Mohammad Mosarof Hossain, Assistant Professor, Dept. of CMF, Faculty of Fisheries, Sylhet Agricultural University, Sylhet-3100

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and sediment through the combined action of freshwater flow, wind, waves and tidal action. Photosynthesis occurs throughout the water column and on the sediment surface – very productive. Plant and animal wastes are washed away, sediment is shifted and fresh & salt water are mixed. Organisms: Estuaries provide a calm refuge from the open sea for millions of plants and animals, which supports enormous abundance and diversity of species e.g. fish, shellfish, lobsters, marine worms, seagrasses, algae and phytoplankton etc. Estuaries are considered as safe havens for many different organisms. Many species of fish use the estuary to breed and leave their young to grow and mature before moving off to the open sea. A number of commercial fishes are use estuaries at some stage in their life cycle e.g. snapper, flounder, mullet, rock fish, sole, kahawai, trevally, parore, red cod Koral, Lakhoua, gurnard etc. Many species spend their entire life in estuary areas e.g. Atlantic blue crab, white shrimp, grass shrimp, mussels, oysters, clams, fiddler crabs, and wading birds. Many migratory species e.g. freshwater eels, salmon, hilsa and whitebait migrate through estuaries at least twice in their life cycle. Many species of shark use estuaries as pupping grounds. They bear their young in the estuary. These young use the estuary for food and shelter until they are ready to head out to the ocean. Many species (inhabitant/visiting) from land, sea and freshwater use the estuary for feeding, breeding, spawning and as a nursery ground for their young. Estuaries also provide a way to separate juvenile fish and adult fish to reduce competition. Food abundantness and easy to access attracting many types of birds including gulls, ducks and wading birds. E.g. About 20 species of bird visit the Avon-Heathcote estuary in Christchurch of New Zealand. Sediments and decomposition: Sediments are important as they store organic matter and are the site of microbial activity. Microorganisms decompose complex organic compounds into useable forms – ammonia, nitrates and phosphates. This becomes the basic food for primary consumers like crabs, shellfish, snails, and marine worms. The margins of the estuary contain the food webs important producers e.g. algae, eelgrass, rushes and mangroves providing a huge amount of organic matter. Marshes and mangroves produce up to ten tones of plant detritus per hectare per year – considered organic factories. Filter feeders such as cockles and pipis continually add faecal deposits to the sediment. Large estuaries, like Chesapeake Bay and Puget Sound, often have many streams flowing into them and can have complex shapes. Where an enormous volume of river water enters the sea (as, for example, from the Amazon into the South Atlantic) its estuary could be considered to extend well beyond the coast. Typically estuaries are the tidal mouths of rivers, and they are often characterized by sedimentation or silt carried in from terrestrial runoff. The root systems of estuarine plants have stabilizing that trap sediment and form mud flats and salt marshes. Some of these plants include Spartina, Glass Wort, Cat Tails, and Black Needle Rush. Mohammad Mosarof Hossain, Assistant Professor, Dept. of CMF, Faculty of Fisheries, Sylhet Agricultural University, Sylhet-3100

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Values of Estuaries Estuaries are exclusively important from the ecological, economical, social and cultural point of view. As a whole estuaries act as breeding and feeding ground for many aquatic animals and birds, buffer for the coast from storms and floods, filter for sediments and pollutants from coastal waters, record of past environments and events, place for recreation (such as water sports, fishing, shellfish gathering, duck shooting, bird watching), site for marine farms etc. Ecological Value: Estuaries are one of the most productive ecosystems on earth. They maintain water quality through natural filtration. Water draining from the land carries sediments, nutrients and other pollutants. Much of the sediments and pollutants are filtered out as the water flows through these fringing marshes. This creates clearer and clearer water, which benefits both people and marine life. Wetland soils and plants like mangroves, sea grasses and reeds also act as a natural buffer between the land and ocean, absorbing floodwaters from land and storm surges from the ocean. They help maintain biodiversity by providing a diverse range of unique habitats. Estuaries are critical for the survival of many species. Many species of fish and shellfish use estuaries as nursery ground to spawn and allow juveniles to grow. Thousands of birds, mammals, fish and other wildlife use estuaries as places to live, feed and reproduce. Migratory birds use estuaries to rest and gain food during their journeys. Economic Value: Used for transport and industry. Maintain water quality which benefits both people and marine life. Natural buffer between the land and ocean, absorbing flood waters and storm surges. They help maintain biodiversity by providing a diverse range of unique habitats that are critical for the survival of many species. Estuaries are also very important for commercial fisheries. Many species of commercially important fish and shellfish use estuaries as nurseries to spawn and allow juveniles to grow. Cultural Value: People value estuaries for recreation, scientific knowledge, education, aesthetic and traditional practices. Boating, fishing, swimming, surfing, and bird watching are just a few of the numerous recreational activities people enjoy in estuaries. Unique estuarine habitats makes them valuable laboratories for scientists and students. Estuaries also provide a great deal of aesthetic enjoyment for the people who live, work, or use them for recreation. Estuaries were a resource sought after by Maori. Timber for building materials, rongoa (medicine), harakeke (flax) for weaving, kai (food) which included birds, fish, rats, eels, shellfish, humans. To make full advantage of this plentiful resource Kainga, which were unfortified villages were established Mohammad Mosarof Hossain, Assistant Professor, Dept. of CMF, Faculty of Fisheries, Sylhet Agricultural University, Sylhet-3100

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near estuaries. Estuaries provide us with numerous resources upon which a dollar value cannot easily be placed. They offer benefits and services which greatly improve our lifestyle. Estuaries are an irreplaceable natural resource that must be managed carefully for the mutual benefit of all who enjoy and depend on them. For example values of Sundarbans may be like these i.e. economic, tourism, ecological protected areas, cultural, recreational etc: They are tourist attractions. The most important value of the Sundarbans lies in its protective role. It helps hold coastlines, reclaim coastal lands, and settle the silt carried by the rivers. The estuary is a good breeding centre for many fishes. Several plant-based industries have been developed here. The most important ones are the newsprint and hardboard mills in Khulna. Other important plant-based industries are match factories and boat building. The forest is a good source of fuel, tannins, thatching, wooden articles, medicinal plants, and fodder. The forest is also a major source of honey and bee wax. Most frequently visited sites in the Sundarbans include Katka, Hiron Point (known commonly as Nilkamal), Dublar Char and Tiger Point (Kachi Khali). Katka attracts tourists for its landscape and wildlife. There is a forest rest house here and an observation tower. Hiron Point also has a rest house and an observation tower. Dublar Char is an island with a beautiful beach. The other attraction of the island is the fishing activities that take place every year between mid-October and mid-February. Fishermen from other places of the country, especially from Chittagong, assemble here during the period to catch fish and dry them on the sunny beach. Only a few people live permanently in or around the Sundarbans. They include the bawalis (collectors of golpata), mouals (honey collectors) and woodcutters. Their dwellings are usually at the edge of the forest and the houses are built on platforms supported on 3-5 m high poles of wood or bamboo. Some people, especially the bedyas (gypsy) live on boats.

Types of estuaries Estuaries can be classified into different types on the basis of 1. Geomorphology-Five types (4+1) (Four type proposed by Pritchard 1967) 2. Water circulation and stratification: Four types (3+1) and 3. Systems energetic/ ecosystem energetic: Five types (5) 4. Freshwater discharge: Three types (3).

1. From the geomorphological standpoint Pritchard (1967) has conveniently subdivided estuaries into following 4 categories (a-d) but normally are five major types (a-e): a) Coastal Plain Estuaries/Drowned river valleys: Coastal Plain Estuaries were formed by rising sea levels in already existing river basins. Drowned river valleys are most extensively developed along coastlines with relatively low and wide coastal plains. Example- Chesapeake Bay, on the mid-Atlantic coast of the United States. b) Fjords and Rias: The movement of the glaciers that caused deep gouging in the earth's surface formed fjord estuaries.There is typically a glacial deposit that forms a sill at the entrance of fjord. Fjord or Rias type estuaries are typically very deep, U-shaped with steep mountainous walls. Example- Fjords of Norway, Fjords of Alaska and Rias along the coast of British Columbia. Mohammad Mosarof Hossain, Assistant Professor, Dept. of CMF, Faculty of Fisheries, Sylhet Agricultural University, Sylhet-3100

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Fig. 2. Shows Fjords and Rias estuaries c) Bar-built Estuaries: A bar-built estuary is formed from sand being deposited along the coast that separates portions of the estuary from the ocean. They are shallow basins, often partially exposed at low tide, enclosed by a chain of offshore bars or barrier islands, broken at intervals by inlets (thus insuring a free connection with the sea).Examples: Estuaries of North Carolina and Georgia

Estuary

Bars or Barrier Islands

Fig. 3. Shows Bar-built estuaries d) Tectonic Estuaries: Tectonic estuaries are formed from geological faulting or folding that causes rivers to flow into them and then into the ocean. Example- San Francisco Bay. e) River Delta Estuaries: Found at the mouths of large rivers such as the Mississippi or Nile. In these situations, semi-enclosed bays, channels and are formed by shifting silt deposition. Example- The Meghna estuay, the Mississippi river estuary, the Nile river estuary.

Fig. 4. Shows Tectonic estuaries

Fig. 5. Shows river delta estuaries

2. On the basis of water circulation and stratification estuaries can be classified into four broad categories (i-iv)i) Salt wedge estuaries: The water flow of river is strongly dominant over tidal action. So freshwater tends to overflow the heavier salt water forming a wedge. These types of estuaries are characterized by the sharp change in salinity from top to bottom (Fig. 6). Example: The Amazon river estuary, The Mississippi river estuary, the Nile river estuary. ii) The partially mixed or moderately stratified estuaries: River output is less than the marine input. Here, turbulence causes mixing of the whole water column, such that salt content varies both horizontally and vertically with moderate density stratification(Fig. 7). ExampleChesapeake Bay estuary. Mohammad Mosarof Hossain, Assistant Professor, Dept. of CMF, Faculty of Fisheries, Sylhet Agricultural University, Sylhet-3100

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iii) The completely mixed or vertically homogenous estuary: The tidal action is strongly dominant. The water tends to be well mixed from top to bottom and the salinity is relatively high (Fig. 8). Example- Bar-built estuaries.

Fig.6. Shows salt wedge estuary. Horizontal

V er tic al

Fig.7. Shows partially mixed estuary.

Fig. 8. Shows completely mixed or vertically homogenous estuary: iv) The hypersaline estuary: It is a special typeof estuary, where freshwater inflow is small, the tidal amplitude low and the evaporation rate is very high. So the salinity of enclosed bays may rise above that of the sea, at least during some seasons. Example- The Upper Leguna Madra, Coastal lagoons of Texas.

3. On the basis of Systems / Ecosystem Energetic estuaries can be classified into five types (i-v)i) Physically stressed systems of wide latitudinal range: These types of estuaries are characterized by high energy breaking waves, strong tidal currents. Severe temperature or salinity shocks, low nighttime oxygen and high rate of sedimentation with very low species diversity. Example- The cold, rocky coasts of westerns North America, The warm hypersaline bays of Texas. ii) Natural arctic ecosystems with ice stress: These types of estuaries are characterized by insufficient light and low temperature. Low species diversity. Example- Glacial fjords, winter iced stressed intertidal zones and under ice communities on arctic and antarctic coasts. Mohammad Mosarof Hossain, Assistant Professor, Dept. of CMF, Faculty of Fisheries, Sylhet Agricultural University, Sylhet-3100

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iii) Natural temperate coastal ecosystems with seasonal programming: These types of estuaries are are seasonally programmed by photoperiod or lunar periodicity, or both which affect the primary productivity, the reproductive and behavioral activities of animals. Example- Bourgneut bay. iv) Natural tropical coastal ecosystems of high diversity: Here the temperature, salinity and other physical stress are low. Thus, these ecosystems contain many species and great deal of chemical diversity within species. Example- The Meghna estuay of Bangladesh, The sundarban river estuary, The Haringhata river estiuary, The Naf river estuary.

v) Emerging new systems associated with man: They are special type estuaries which develop adaptations for man made wastes. Example- The Karnafuli river estuary.

4. On the basis of Freshwater discharge estuaries can be classified into three types (i-iii)i. Positive estuary: Freshwater runoff is greater than evaporation. Majority of the estuaries are positive estuary. ii. Negative estuary: Freshwater runoff is less than evaporation. Usually found in arid regions. In this type of estuary, the evaporation rate is high enough that results high salinity of water which sinks to the bottom of the estuary. iii. Neutral estuary: Freshwater runoff is equal to evaporation. An intermediate condition between positive and negative estuary.

Fig. 9. Shows positive (left) and negative estuary (right)

Limiting factors and driving forces of estuaries The estuarine ecosystem is primarily controlled by the physical environment; i.e., geomorphology, climate, salinity, and the availability of fresh water. The abiotic features thought to be important in determining the specific nature of estuaries as proposed by Day et al. (1989) are:    

Oceanic forces Quantity of freshwater The water circulation pattern Variability of salinity

 Tides  Depth  Geomorphological change

Mohammad Mosarof Hossain, Assistant Professor, Dept. of CMF, Faculty of Fisheries, Sylhet Agricultural University, Sylhet-3100

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However, the main important factors and driving forces of estuaries are: 1. Circulation and Tidal regime 2. Habitat Types 3. Water Column Characteristics 4. Bottom Characteristics 1. Circulation and Tidal regime: Circulation is a physical process that influences or controls many of the ecological processes. There are three major driving forces behind the circulation patterns in estuaries are: a) Gravitational force; b) Tidal force; and c) wind-force 2. Habitat Types: Estuaries and coastal marine waters are inherently heterogeneous systems. Salinity gradient is present in any estuary which is a important factor for species availability (biodiversity). Habitats in estuaries and coastal marine waters can be classified into nine major categories. These habitats area) Open Water-important for phytoplankton production and sometimes for their blooms. b) Soft Bottom Substrates- provide habitat for economically valuable clams, shrimp and juvenile flatfishes. c) Hard Bottom Substrates- Include offshore rocky outcrops; oyster, relic shell, worm tube reefs, relic limestone and coral outcrops. Also provide habitat for commercially valuable organisms. d) Aquatic Macrophytes beds- Most important estuarine habitats, both ecologically and economically. e) Beaches- Beaches are accumulations of unconsolidated sediment (e.g. sand, cobble). f) Sand flats- Deposits described as "fine sand" contain some silts and clays. Sand flats provide habitat for suspension-feeding invertebrates. g) Mudflats- Mudflats provide habitat for commercially important clams, fishes, shorebirds and other invertebrates. h) Emergent Marshes- Emergent marshes may be extremely important components of estuaries and coastal marine embayment and lagoons because they filter storm water runoff from cities, forests and agricultural areas. Emergent marshes may also provide flood protection and reducing the erosive energy of wave action. i) Mangrove Forests- Low-lying tropical coasts are often bordered by dense mangrove forests. Mangrove forests and their associated waters provide valuable habitat for a range of invertebrates, fishes, amphibians, reptiles, birds and mammals. 3. Water Column Characteristics: a) Salinity e) Turbidity b) Temperature: f) Contaminants c) Dissolved Oxygen(DO) g) Depth d) pH h) Nutrients and ionic compositions a) Salinity: Salinity is a key determinant in the distribution of estuarine flora and fauna, especially for benthic invertebrate communities. Salinity can be defined as ‘the total solids in water, after all carbonates have been converted to oxides, all bromide and iodide have been replaced by chloride and all organic matter has been oxidized’. On the basis of salinity range estuarine waters divided in to five zones: Mohammad Mosarof Hossain, Assistant Professor, Dept. of CMF, Faculty of Fisheries, Sylhet Agricultural University, Sylhet-3100

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Name of the Zone

Salinity range

(a) Limnetic

0-0.5 ppt

(b) Oligohaline

0.5-5 ppt

(c) Mesohaline

5-18 ppt

(d) Polyhaline

18-30 ppt

(e) Euhaline

>30 ppt


Comments Salinity is most commonly measured electronically using a salinometer. Measured as parts per thousand (ppt) or grams per kilogram, i.e. 1 g salt dissoloved in 1000g of water = 1ppt.

b) Temperature: Temperature is an important determinant of the rate of chemical reactions, biological and microbial process which is more closely related to sunlight and air temperature. For a change of 1oC of temperature, physiological activity of fish shows changes up to 10%. Thus temperature influences the spatial, temporal and seasonal distribution of fishes and benthic fauna. Dissolved oxygen (DO) saturation is a function of water temperature. c) Dissolved Oxygen (DO): Dissolved oxygen (DO) is a basic physiological requirement for nearly all aquatic biota for the maintenance of balanced populations (without anaerobic systems). Most estuarine populations can tolerate short exposures to reduced DO concentrations without adverse effects. DO concentrations of less than 60% saturation may results in adverse reproductive effects. Exposure to DO concentration less than 30% saturation for 1 to 4 days causes mortality to most species, especially during summer when metabolic rates are high. Aquatic biota exposed to low DO may be more susceptible to adverse effects of other stressors such as- disease, toxic chemicals, habitat modification etc. Low DO conditions can increase the vulnerability of benthos to predation as they extend above the sediment surface to obtain more oxygen. DO concentrations of estuaries can vary widely with tide, time of day, wind patterns and biological activities. DO measure by digital DO meter and expressed as ppm. i.e.1ppm = 1 mg dissolved in (1000x1000) mg or 1,00,000 mg of water. d) pH: pH is another important indicator of the chemical condition of estuarine and coastal marine waters which is usually controlled by the mixing of seawater solutes with the fresh water inflow. A major factor influencing the pH of estuarine waters is the carbon dioxide solubility. The range of pH values in the upper reaches estuaries is ‘7.5 - 9.0’ and incase of surface sea water it is ‘8.1 to 8.3’. e) Turbidity: The major component of turbidity in estuaries is silt which volume varies seasonally. 20,000ppm or more turbidity is lethal for most fish species. However turbidity has two primary effects in estuaries: Firstly, light penetration is reduced, which directly affects primary production and abundance of aquatic macrophytes. Secondly, settling of the particulate matter (e.g. mud, silt, sediments and detritus) cause changes in the composition of benthic invertebrate assemblages. f) Contaminants: Organic compounds and metals is particularly important because they exert adverse effects on aquatic life and on human health. Sources of organic and inorganic chemical contaminants include release of urban runoff, industrial & municipal discharges, atmospheric deposition and upstream runoff directly to the water body. Organic and metals contaminants are settle to the bottom and become a source of toxicity to organisms by means of bioaccumulation and biomagnifications.


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g) Depth: Depth characterization is important for evaluating DO, temperature, salinity profiles, tidal regime consistency and the percent of the water column that is photic. h) Nutrients and ionic compositions: Nutrients control the rate of primary and secondary production. Estuarine autotrophs (i.e., algae, diatoms, vascular plants) require numerous macro, micronutrients and vitamins, including C, N, P, Si, S, K, Mg, Na, Ca, Fe, Mn, Zn, Cu, B, Mo, Co, V, thiamin, cyanocobalamin and biotin.

Estuarine Nutrient Cycling Estuaries have a continuous source of nutrients from freshwater inputs. These freshwater inputs often have additional nutrients as they receive wastes from cities towns and farms. The balance of nutrients in an estuary depends on the amount of input from the land and how much is exported or imported to and from the ocean in the tidal water. Nutrients within the estuary are used by plants such as algae in the water, seagrasses (seaweeds) and mangroves. These plants enter other parts of the food web when plants are grazed on by invertebrates. When the plants, or parts of them die, nutrients are re-cycled within the estuary by the actions of invertebrates and bacteria (i.e. decomposer). Nutrients are essential to support the productivity of estuaries however, too many nutrients can alter the balance of nutrient cycling and lead to excessive growths of plant such as blooms of phytoplankton or seaweeds. 4. Bottom Characteristics: Bottom characteristics provide important data for interpreting the condition of targeted biological assemblages. Most important parameters of bottom characteristics arei) Sediment Grain Size- to detect and describe spatial and temporal changes of the benthic habitat. ii) Total Organic Carbon, Total Volatile Solids and Acid Volatile Sulfides- are important sediment properties to determining the bioavailability and toxicity of certain organic compounds and trace metals in sediments. iii) Sediment Oxidation-Reduction Potential- There is four oxidation-reduction (redox) processes related to biological respiration that occur in benthic sediments. iv) Sediment Contamination- Sampling the surface sediments for the presence of contaminants can provide information about the benthic community and the potential for impacts to human health; i.e., by means of bioaccumulation and biomagnifications.

Ecological classification of estuarine organisms In a Broad sense, ecologically estuarine organisms can be classified into 5 major categories, such as a) Oligohaline organisms: Mainly freshwater forms, inhabiting rivers which cannot tolerate variation of salinity more than 0.1 ppt. However, some oligohaline species can tolerate salinity upto 5 ppt and a few even as high as 19 ppt. b) True estuarine organisms: True estuarine organisms are geographically restricted to estuaries and best response in low salinities. They have adapted themselves to tolerate a wide range of salinity.


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c) Euryhaline marine organisms: This group forms the majority of the total estuarine biota and extends their distribution in the upper reaches of the estuary. They can tolerate salinity as low as 15 ppt. However, a few can tolerate salinities of even 5 ppt. d) Stenohaline marine organisms: This group live on open seashore and at mouths of estuaries. They do not enter estuaries below salinities of 25 ppt in the lower reaches. e) Migrants: Certain euryhaline migrants spend only a part of their lives in the estuaries. However, Carriker (1967) also presented ecological classification of estuarine organisms on the basis of variation in salinities (Table 1).

Migrants

Euryhali ne marine

True estuarine organisms

Table 1: Ecological Classification of Estuarine Organisms (Carriker, 1967) with varying salinity Division of estuary Salinity Zones Types of organisms and approximate range (ppt) of distribution in estuary River 0 -- 0.5 Limnetic Limnetic organisms Head 0.5 -- 5 Oligohaline Oligohaline organisms Upper reaches 5--18 Mesohaline Middle reaches 18--25 Polyhaline Stenohaline Lower reaches 25 --30 Polyhaline marine Mouth >30 Euhaline organisms

Estuarine organisms Estuarine communities are composed of a variety of organism’s ncluding endemic species, some marine species and a few freshwater species having osmoregulatory capabilities.

Plnkton: Aquatic microscopic or macroscopic organisms, drifting or having weak power of locomotion but not against the current are called plankton.

Phytoplankton: Aquatic microscopic or macroscopic drifting organisms of plant origin, mostly algae are called phytoplankton. Phytoplankton’s (tiny single-celled algae) are found mainly in the water and are an essential part of the food web of estuaries. They act as important food for macro and micro invertebrates. Estuaries provide some of the most productive habitats on earth because of the accumulation and availability of nutrients along with adequate light conditions that fuel the production of phytoplankton.

Zooplankton: Aquatic microscopic or macroscopic organisms of animal origin, mostly invertebrates and the eggs, larval or immature stages of aquatic vertebrates, drifting or having weak power of locomotion but not against the current are called zooplankton. i) Holoplankton: Organisms which remain planktonic for their entire life cycle are called holaplankton. E.g. pelagic tunicates, polichate worms. ii) Meroplankton: Organisms which remain planktonic for only certain stages of their life cycle are called meroplankton or temporary plankton. E.g. copepods, crustaceans, larvae of shrimp etc. Neuston: The planktonic aquatic organisms which are related to surface film of the water (i.e. live at the very surface of the water) are called neuston. E.g. Ranatra sp, Velella, Physalia, Janthina etc. Some fishes e.g. kakila, kholla, kanpona etc shows some behaviour like neuston but actually they are nekton. Mohammad Mosarof Hossain, Assistant Professor, Dept. of CMF, Faculty of Fisheries, Sylhet Agricultural University, Sylhet-3100

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Periphyton: The assembles of aquatic microscopic or macroscopic organisms that are attached or clinging to the substrate, projecting above the bottom are called periphyton.

Benthos: The organisms that are live in or at the bottom are called benthos. Benthic organisms are important in the ecology of estuaries both as consumers of plankton and as food for bottom feeding fish. They provide key linkages between primary producers and higher trophic levels in estuarine food chains. i. Benthic organisms as consumers: Many clams and oysters feed on plankton in the water column. ii. Benthic organisms as prey: Polychaete worms and shrimp-like crustaceans used as diets of commercially important bottom-feeding fishes such as Spot and Croaker. Clams and snaillike gastropods are often eaten by Blue Crabs. iii. Benthic fisheries: Oysters, Blue Crabs and many types of clams are harvested by humans. Thus they are important commercially and recreationally. Some clams and shrimp are also raised in farms for human consumption.

Nekton: The aquatic organisms which are able to swim at will are called nekton. E.g. Fishes, shrimps, mammals, snakes etc. Coastal nekton use estuaries as nursery grounds during their early stages of life-cycle due to better protection and abundant foods. Anadromous and catadromous fishes also depend on estuaries where they may reside for considerable length of time during their migration from salt to freshwater and vice-versa.

Fish, Crabs, Shellfish and Other Invertebrates: The estuarine ecosystem is the habitat for several animals, mainly providing protection and feeding areas. Many fish, crabs and shellfish rely on estuaries for food and as nursery grounds. Juvenile fish can grow safely in estuaries. Hilsa is a very important fish species of Bangladesh, mainly caught from different estuaries. Ribbon fish, Bombay duck, Jewfishes (Poa), Black and Silver bream, Snapper, Flathead, Whiting, Flounder, Croaker, Bass etc are also caught from estuarine areas of Bangladesh. The Blue Manna Crab spends a large part of its life cycle in estuaries. Fully-grown adults mate in the estuary. Estuaries are also habitat for a large variety of macro invertebrates, such as marine worms, mollusks (snails, mussels), crustaceans (shrimps, prawns) and aquatic insects. A wide variety of micro invertebrates, such as tiny worms, filter-feeding crustaceans and the larval stages of mosquitoes, flies and larger invertebrates also live in estuaries.

Birds: Tidal flats hold a bounty of food for different aquatic birds. For example, Mud probers, Wrybills, Herons, Caspian terns, Ducks. The rushes, reeds and mangroves provide sheltered breeding grounds for swamp birds such as the bittern, marsh crake, banded rail, fern bird etc. Each year a thousands of migratory birds travel thousands kilometers from their Siberian and Alaskan breeding grounds, arriving in at estuaries.

Natural productivity and food production potential of Estuaries Characteristically, natural productivity and food production potentiality of estuary is tend to be more productive either the sea water or freshwater. The reasons for high productivity are as followsMohammad Mosarof Hossain, Assistant Professor, Dept. of CMF, Faculty of Fisheries, Sylhet Agricultural University, Sylhet-3100

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1. An estuary is a nutrient trap because nutrients recycling are rapid due to activity of benthos and detritus. Recovery of nutrients from deep sediments by burrowing animals and microbial activity create a sort of "self enrichment system". 2. Estuaries benefit from a diversity of producers types "programmed"for virtually year round photosynthesis. Estuaries have three types of producers namely: a) Macrophytes (seaweeds, sea grasses and marsh grasses) b) Benthic microphytes c) Phytoplankton 3. The importance of tidal action in creating a "sub sized" fluctuating water-level ecosystem: The higher the tidal amplitude the greater the production potential. The back-and-forth movement of water helps in removing wastes, transporting food and nutrients for the sessile forms. 4. Unique environment: Estuaries have four main characteristics that together contribute to their uniqueness. These area) Estuaries are highly productive systems b) Estuaries have multiple producers c) Estuaries are exporters of nutrients d) Estuaries have a natural tendency towards eutrophication

Estuarine Food Webs Trophic level: Group of organisms united by obtaining their energy from the same part of the food web of a biological community (Orlans, 1995). Food chain: The transfer of food energy from the source in plants through a series of organisms with repeated eating and being eaten is referred to as food chain. Food Web: The interlocking pattern of a food chain is called food web. The food web is a diagram of different animals through which energy is passed by feeding. The top predator is the top consumer in the food web and the smaller planktonic organisms are the producers that are at the bottom of the food web. The higher the food web, the more that is eaten by those animals and the less energy they receive.

Food web of any ecosystem mainly composed with three components namely, 1. Producers (mainly primary producer i.e. green plants), 2. Consumers (Primary, secondary, tertiary and predators) and 3. Decomposers

1. Producers: Organisms (primary producer, organisms with the ability of photosynthesis) which are commonly responsible for the primary production1. There are three types of producers are available in food web of estuarine ecosystem i.e. 1.1. Macrophytes- mainly aquatic plants that convert energy from the sun into nutrients through the process of photosynthesis. e.g. grasses, Potamogeton, Porphyra, Laminaria etc. 1.2. Phytoplankton- aquatic algae free-floating in the surface layers of water where they photosynthesize to convert the sun's energy into nutrients. 1

Primary production: The conversion of carbon from an inorganic form of carbondioxide in to organic matter by autotrops that is the production of food is called primary production. Primary producer: Organisms that can create organic matter from carbon dioxide, usually by photosynthesis are called primary producers. Mohammad Mosarof Hossain, Assistant Professor, Dept. of CMF, Faculty of Fisheries, Sylhet Agricultural University, Sylhet-3100

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1.3. Benthic organisms- living in or on the bottom sediments of estuary. They mostly convert energy by anaerobic means (without oxygen) to nutrients e.g. benthic algae, dinoflaggelates and bacteria. Macrophytes and phytoplankton are only productive during the day. Benthic organisms are not dependent on light and can produce throughout the year. Most ecosystems have only one or at most two of these producers - terrestrial systems have only macrophytes to produce nutrients. In the open sea phytoplankton is the most important primary producer. Estuaries, however, have all three types of producers - this is the main reason for the very high annual productivity. Thus the processes of primary production in estuary can go on day and night and virtually throughout the year.

Figures 10: (Top left, shows an energy pyramid for estuaries); (Top Right, schematic diagram of estuarine food web); (Bottom left, diagram of the food chain in estuaries with lots of animal); (Bottom right, a more basic food chain where the Flapeelia, or the earliest group of flagellated organisms to live, is the top predator).

2. Consumers: Primary, secondary, tertiary and predators. An important group of primary consumers living in estuaries are animals that feed on plankton. e.g. zooplankton, holoplankton, meroplankton, small worms, benthic minutes animals etc. Plants are grazed and filtered by Mohammad Mosarof Hossain, Assistant Professor, Dept. of CMF, Faculty of Fisheries, Sylhet Agricultural University, Sylhet-3100

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invertebrates like snails, cockles and oysters that are eaten by juvenile and other small fish (secondary consumers) which may be hunted by larger fish (tertiary consumers) like snapper, kingfish, rays or even sharks. The carnivores (predators) occupy the highest level obtaining energy by eating animals that feed on plankton and detritus. Predators are important to the estuary because they held the end position in most food chains. One aspect of the estuary food web is that there are more different species of consumers than species of primary producers. A few provide nutritional needs to many. Thus the usual trophic pyramid is inverted because most carnivorous species are at the top of the food web.

3. Decomposer: Many primary producers are first converted by bacterial decomposition into organic detritus which serves as a major food source for the majority of consumers living in the estuarine community. The wastes and all organisms after their death are subjected to decompose in to organic detritus (nutrients) by benthic saprophytic organisms mainly bacteria, flagellates.

Major Estuaries of Bangladesh The Karnafuli River Estuary: The largest and most important river in Chittagong and the Chittagong Hill Tracts, originating in the Lushai hills in Mizoram State of India. It travels through 180 km of mountainous wilderness making a narrow loop at Rangamati and then follows a zigzag course before it forms two other prominent loops, the Dhuliachhari and theKaptai. The Rangamati and the Dhuliachhari loops are now under the reservoir of the Kaptai earth-filled dam. The hydroelectric dam is situated just before the entrance of the river into the Kaptai loop. After coming out from the Kaptai loop the river follows another stretch of tortuous course through the Sitapahar hill range and flows across the plain of Chittagong after emerging from the hills near Chandraghona. Therefore, the river drains into the Bay of Bengal cutting across several hill ranges, viz the Barkal, Gobamura, Chilardak, Sitapahar and Patiya of the Chittagong Hill Tracts and Chittagong. The main tributaries of the Karnafuli are the Kasalong, Chengi, Halda and Dhurung on the right and the Subalong, Kaptai, Rinkeong and Thega on the left. All most all the tributaries of the Karnafuli river are very important for artisanal and commercial fisheries. A large number of young and adult fishes both from fresh water and marine water are widely distributed to these river systems. Halda river is one of the most important natural breeding grounds especially for the carps and some other fishes of our country. Karnafuli river is also an important navigational route and Chittagong port is situated just 16.89 km above from the Bay of Bengal. It is navigable throughout the year by sea-going vessels up to Chittagong Port and by large Boats, shallow draughts and all sorts of freighters and launches up to Kaptai river in the Hill Tracts.

The Meghna River Estuary  Meghna River is one of the major rivers in Bangladesh, famous for its great estuary that discharges the flows of the Ganges-Padma, the Brahmaputra-Jamuna and the Meghna itself. The downstream of the Surma river from Ajmiriganj is often referred to as the Meghna. The Meghna has two distinct parts. i) The Upper Meghna from Kuliarchar to Shatnol is a comparatively small river. ii) The Lower Meghna below Shatnol is one of the largest rivers in the world because of its wide estuary mouth. Mohammad Mosarof Hossain, Assistant Professor, Dept. of CMF, Faculty of Fisheries, Sylhet Agricultural University, Sylhet-3100

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 Meghna Estuary is the eastern most sector of the Ganges delta. The Estuary conveys the joint discharge of the Ganges/Padma, Jamuna/Brahmaputra, and Meghna Rivers. Hereby, large volumes of water (approximately 1,200 km³ per year) and sediment (approx. 1,100 million ton per year) pass the area. The catchment area is 1,520,000 km². It covers parts of India and China, all of Nepal and Bhutan, and almost all of Bangladesh. There is a pronounced seasonal variation of wind, river discharge and sediment supply from the river system. The highest discharges occur in August-September and the lowest in February. The 10-years peak flow at Chandpur has been estimated at 1,23,000 m³/s.  The estuary forms a complex network of braided tidal channels with strong tidal streams in many places. The entire Meghna Estuary (and a part of the upstream river system) is tidalinfluenced all year. There is a pronounced seasonal sea level variation. The sea level is highest during the monsoon and lowest in the winter. In the estuary, fresh water from the rivers meets with saline ocean water from the Bay of Bengal. Due to strong currents and shallow depths, density stratification is not prominent. Rather, there are fronts (or transition zones) between the water masses. The location of these transition zones depends on the river discharge and the tide. The depth of the inner part of the estuary is less than 10 m. Wave heights are generally moderate. In the inner parts of the estuary and in its extensive shallow areas, the waves are predominantly generated by direct (local) wind action. Sediments, fine sand and silt, are supplied by the rivers, and are transported within the estuary mainly by the tidal streams. The area is characterized by a highly dynamic morphology, with flow channels shifting their course and with intermittent erosion and accretion of banks and tidal flats. There is a moderate net accretion, currently estimated by MES at around 10 km² per year (1976-96).  Meghna estuary plays an important role to the fisheries sector of Bangladesh. A number of commercially important fish species in addition to the crustaceans and mollusks live and spawn to the Meghna estuary. Different important fish species use this estuary as nursery ground during their early stage of life cycle due to better protection and abundant foods. The main migratory and commercially important fish, Hilsa (Tenualosa ilisha), adults as well as the juveniles (jatka) are widely distributed to the coastal waters, estuaries and riverine waters of Meghna, Padma, Jamuna and other deltaic rivers of Bangladesh. The confluence of Padma-Meghna is a very significant water body, the major nursery grounds of hilsa (Tenualosa ilisha) and many other commercially important riverine fishes of Bangladesh.

The Sundarbans Estuary:  The largest single block of tidal halophytic mangrove forest in the world, located in the southern part of Bangladesh. It lies on the Ganges-Brahmaputra delta at the point where it merges with the Bay of Bengal. The forest lies a little south to the Tropic of Cancer between the latitudes 21º30Ń and 22º30Ń, and longitudes 89º00É and 89º55É. With its array of trees and wildlife the forest is a showpiece of natural history. It is also a centre of economic activities, such as extraction of timber, fishing and collection of honey. The forest consists of about 200 islands, separated by about 400 interconnected tidal rivers, creeks and canals. The Sundarbans was originally measured (about 200 years ago) to be of about 16,700 sq km. Now it has dwindled to about 1/3 of the original size. Because of the partition of India, Bangladesh received about 2/3 of the forest; the rest is on the Indian side. It is now estimated to be about 4,110 sq km, of which about 1,700 sq km is occupied by waterbodies. Mohammad Mosarof Hossain, Assistant Professor, Dept. of CMF, Faculty of Fisheries, Sylhet Agricultural University, Sylhet-3100

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 The forest lies under two forest divisions, and four administrative ranges viz Chandpai, Sarankhola, Khulna and Burigoalini and has 16 forest stations. It is further divided into 55 compartments and 9 blocks. The Sundarbans was declared as a Reserve Forest in 1875. About 32,400 hectares of the Sundarbans have been declared as three wildlife sanctuaries, and came under the UNESCO 522 th World Heritage Site in 1999. The generally accepted explanation for the name sundarban may be derived from the SUNDARI tree, the most common tree in these forests.  Geology: The tract of the Sundarbans is of recent origin, raised by the deposition of sediments formed due to soil erosion in the Himalayas. The process has been accelerated by tides from the sea face.  Soil: Soils of the Sundarbans mangrove forest differ from other inland soils in that they are subjected to the effects of salinity and waterlogging, which naturally affect the vegetation. In places soils are semi-solid and poorly consolidated. The pH ranges widely from 5.3 to 8.0. Although the Sundarbans soil is in general medium textured, sandy loam, silt loam or clay loam, the grain size distribution is highly variable. Silt loam is dominant textural class.  Climate: Since the forest is located on the south of the Tropic of Cancer and bounded by the northern limits of the Bay of Bengal, it is classified as tropical moist forest. The temperatures in the Sundarbans are fairly equable than those of the adjacent land areas. The average annual maximum and minimum temperatures vary between 30º and 21ºC. The mean annual relative humidity varies from 70% at Satkhira to 80% at Patuakhali. Annual rainfall in the Sundarbans is in the range of 1640-2000 mm, rainfall increases from west to the east. Most rainfall occurs during the monsoon from May to October. Often storm accompanied by tidal waves result widespread inundation and cause damage to vegetation and animal life.  Vegetation: The vegetation is largely of mangrove type and encompasses a variety of plants including trees, shrubs, grasses, epiphytes, and lianas. Being mostly evergreen, they possess more or less similar physiological and structural adaptations. Most trees have pneumatophores for aerial respiration. The prominent species is Sundari (Heritiera fomes) and Gewa (Excoecaria agallocha). Prain (1903) recorded 334 species under 245 genera. Of the 50 true mangrove plant species recorded, the Sundarbans alone contain 35. Almost all mangrove plant species are evergreen, dwarf, shrubby or tall trees, and grow gregariously without leaving any space on the floor.  In the Sundarbans the saltwater forest is situated in the south-western part where Gewa (E. agallocha), Goran (Ceriops decandra), Keora (Sonneratia apetala), Ora (S. caseolaris), Passur (Xylocarpus mekongensis), Dhundul (X. granatum), Bain (Avicennia alba, A. marina, A. officinales), and other rhizophores, and Hantal (Phoenix pelludosa) dominate. The typical mangrove species dominate the central part of the forest. There is a thick mat of the NIPA PALM or 'Golpata' (Nipa fruticans) by the side of almost all the canals.  Fauna: The Sundarbans hosts a large variety of animals. It is the last stronghold of the Bengal Tiger (Panthera tigris). Within the forest habitats there are about 50 species of mammals, about 320 species of inland and migratory birds, about 50 species of reptiles, 8 species of amphibians, and about 400 species of fish.  Besides the spectacular Royal Bengal Tiger, the other notable mammalian fauna are spotted deer (Cervus axis), Barking deer, Rhesus macaque, Jungle cat, Leopard cat, the Indian porcupine, Otter, and wild boar. Deer and wild boar constitute the main prey for the tiger. Some species including the Bengal tiger are endangered. Of about 50 species of reptiles the Mohammad Mosarof Hossain, Assistant Professor, Dept. of CMF, Faculty of Fisheries, Sylhet Agricultural University, Sylhet-3100

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largest member in the Sundarbans is the Estuarine Crocodile (Crocodylus porosus), some of which may attain a length of about seven metres. Although once they were abundant in this mangrove habitat, their total number is now estimated to be around 250.  Among the invertebrates some molluscs and crustaceans constitute important fisheries resources. About 20 species of shrimps, 8 species of lobsters, 7 species of crabs, several species of gastropods, and 6 species of pelecypods have been reported from the Sundarbans. Among the shrimps Penaeus monodon and Metapenaeus monoceros and the mud crab Scylla serrata are commercially important. Disturbances, Human impacts and threats to the Sundarbans estuaries: A long-term ecological change is taking place in the Sundarbans as a result of the eastward migration of the Ganges. Up to 40% of the dry season flow of the Ganges has been diverted upstream, following the completion of the Farraka Barrage in India in 1974. Decreased freshwater flushing of the Sundarbans has resulted in increased saline intrusion, has affected the natural regeneration of mangroves, and in some areas there is now no regeneration at all. From the Port at Mongla on the northern edge of the mangrove forest numerous large shipping vessels pass through the Sundarbans every day via the northeast-shipping route results oil pollution. The most immediate threat to the Sundarbans is over-exploitation, both of the timber resources and of the fauna. It is clear that the mangrove forests are being heavily exploited. Fishermen's camps are a major source of disturbance, and there is extensive illegal hunting and trapping, not only by fishermen and wood-cutters, but also by high officials of civil and defense services stationed in the area. The small population of Estuarine Crocodiles remains under pressure from illegal hunting for the skin trade. Populations of the Green Frog have declined rapidly in recent years as a result of commercial exploitation, despite a Government ban on the catching of frogs during the breeding season. At least six species of mammals have already become extinct in the Sundarbans, and the populations of several others are declining. As ecosystems, estuaries are under threat from human activities such as pollution and overfishing. Land run-off and industrial, agricultural, and domestic waste enter rivers and are discharged into estuaries. Contaminants can be introduced which do not disintegrate rapidly in the marine environment such as plastics, pesticides, furans, dioxins, phenols and heavy metals. Such toxins can accumulate in the tissues of many species of aquatic life in a process called bioaccumulation. They are also known to accumulate in benthic environments such as estuaries and bay muds. For example, Phenols, heavy metals and industrial pollution in Chinese and Russia the Amur River have devastated fish stocks and damaged its estuary soil. Estuaries tend to be naturally eutrophic because land runoff discharges nutrients into estuaries. With human activities, land run-off also now includes the many chemicals used as fertilizers in agriculture as well as waste from livestock and humans. Excess oxygen depleting chemicals in the water can lead to hypoxia and the creation of dead zones. It can detariorate water quality which result in reductions of fish and other animal populations. Over fishing also occurs. Chesapeake Bay, North America's largest estuary, once had a flourishing oyster population which has been almost wiped out by overfishing. Historically, the oysters filtered the estuary's entire water volume of excess nutrients every three or four days. Today that process takes almost a year and sediment, nutrients, and algae can cause problems


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in local waters. Oysters filter these pollutants and either eat them or shape them into small packets that are deposited on the bottom where they are harmless. So, at a glance, the major threats are:  excess silt flowing in from land clearance  pollution from sewage, industrial wastes and agricultural run-off  oil spills / oil spillage  invasion by introduced species  reclamation for marinas  extraction of sand and gravel  Construction of harbours and channels  Construction of embankments and roads  Solid wastes and garbage disposal  Liquid wastes disposal  Natural disasters (e.g. cyclone, earthquake)  Conversion to mining  Conversion to aquaculture  Conversion to agriculture  Conversion to salt pans  Conversion to urban development  Over exploitation by traditional users  Different pests and diseases  .Other hazardous chemicals  Lack of sustainable mangrove management etc These activities decrease the habitat available for estuarine plants and animals, spoil recreational activity in the area, and jeopardise the role estuaries play in maintaining the health of coastal fisheries and waters. Suggested sample questions to be understood and familiar with the typology of query.…. Q. Define estuary. Why estuarine ecosystems are unique? Describe the characteristics of an estuary. Q. Why are estuaries valuable or important? Explain or cite with examples in case of Bangladesh. Q. Classify estuary on the basis of geomorphology, water circulation and systems energetic and freshwater discharge along with brief explanation. Q. Why limiting factors and driving forces of estuaries are important? Narrate the major factors and driving forces of an estuarine ecosystem. Q. Why estuaries are called nursery ground for many different aquatic species? Briefly describe about different types of estuarine organisms or estuarine communities. Q. Why the natural productivity and food production potential of estuaries are so high? Write notes on estuarine nutrient cycling. Q. What are trophic level, food chain and food web? Explain the food web of an estuarine ecosystem. Q. Draw a map and point out major rivers and their estuaries in Bangladesh. Describe about the Sundarbans, Meghna and Karnafuli river estuary as representative.


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