Aquatic Ecology Module C1-4 PDF

Chapter I. Introduction Definitions of Ecology and its related sciences ECOLOGY Derived from the Greek word oikos, meaning “house” or place to live. (the study of organisms “at home” The totality or pattern of relations between organisms and their environment. The bod...

Chapter I. Introduction Definitions of Ecology and its related sciences ECOLOGY Derived from the Greek word oikos, meaning “house” or place to live. (the study of organisms “at home” The totality or pattern of relations between organisms and their environment. The body of knowledge concerning the economy of Nature – the investigation of the total relations of the animal to its inorganic and organic environment (Haeckle, 1870). Scientific natural history (Elton, 1927) The scientific study of the distribution and abundance of organisms (Andrewartha, 1961). The structure and function of Nature (Odum, 1963): the scientific study of the processes regulating the distribution and abundance of organisms and the interactions among them, and the study of how these organisms in turn mediate the transport and transformation of energy and matter in the biosphere (i.e., the study of the design of ecosystem structure and function) The word ecology was first proposed by Ernst Haeckel in 1869. Anton van Leeuwenhoek pioneered the study of food chains and population regulation. Burdon- Sanderson (1890s) elevated Ecology to one of the three natural divisions of Biology: Physiology - Morphology – Ecology. BEYOND FUNDAMENTAL ECOLOGY Terms Explanation and definition Applied Ecology Using ecological principles to maintain conditions necessary for the continuation of present-day life on earth. Industrial Ecology The design of the industrial infrastructure such that it consists of a series of interlocking “technological ecosystems” interfacing with global natural ecosystems. Industrial ecology takes the pattern and processes of natural ecosystems as a design for sustainability. It represents a shift in paradigm from conquering nature to becoming nature. Ecological Engineering Unlike industrial ecology, the focus of Ecological Engineering is on the manipulation of natural ecosystems by humans for our purposes, using small  amounts of supplemental energy to control systems in which the main energy drives are still coming from non-human sources. It is the design of new ecosystems for human purposes, using the self-organizing principles of natural ecosystems. Ecological Economics Integrating ecology and economics in such a way that economic and environmental policies are reinforcing rather than mutually destructive. Urban ecology For ecologists, urban ecology is the study of ecology in urban areas, specifically the relationships, interactions, types and numbers of species found in urban habitats. Also, the design of sustainable cities, urban design programs that incorporate political, infrastructure and economic considerations. Conservation Biology The application of diverse fields and disciplines to the conservation of biological diversity. Restoration Biology Application of ecosystem ecology to the restoration of deteriorated landscapes in an attempt to bring it back to its original state as much as possible. Example, prarie grass. Landscape Ecology Concerned with spatial patterns in the landscape and how they develop, with an emphasis on the role of disturbance, including human impacts‖ (Smith and Smith). It is a relatively new branch of ecology that employs Global Information Systems. The goal is to predict the responses of different organisms to changes in landscape, to ultimately facilitate ecosystem management. Fig. 1-1. The Biology “Layer Cake” illustrating “basic” (horizontal) and “Taxonomic” (vertical) divisions of Ecology in Odum (1971). SUBDIVISIONS OF ECOLOGY Aside from the basic subdivisions of ecology illustrated in fig.1-1. Ecology is subdivided into: Autecology – deals with the study of the individual organism or an individual species. Life histories and behavior as a means of adaptation to the environment are usually emphasized. Synecology – deals with the study of groups of organisms which are associated together as a unit. Subdivisions of ecology: in Subdivisions of ecology: in Subdivisions of ecology: in terms of environment or terms of application terms of taxonomic lines habitat 1. Freshwater ecology 1. Natural resources 1. Plant ecology 2. Marine ecology 2. Pollution 2. Insect ecology 3. Terrestrial ecology 3. Space travel 3. Microbial ecology 4. Applied human ecology 4. Vertebrate ecology  Ecological levels of organization hierarchy; seven transcending processes or functions are depicted as vertical components of eleven integrative levels of organization (after Barre.t et.al. 1997) Population groups of individuals of any kind of organism. Community Includes all of the populations occupying a given area. Ecosystem the community and the nonliving thing environment function together. Biome Large scale areas with similar vegetation and climatic characteristics. Biosphere The largest and most nearly self-sufficient biological system which includes all of the earth ‘s living organisms interacting with the physical environment as a whole so as to maintain a steady state system intermediate in the flow of energy between the high energy input of the sun and the thermal sink of space. 1.1 AUTOTROPHIC AND HETEROTROPHIC COMPONENTS Ecosystem (Ecological System) - any unit that includes all of the organisms in a given area interacting with the physical environment so that a flow of energy leads to clearly defined trophic structure, biotic diversity and material cycles within the system. The ecosystem is considered as the basic functional unit in ecology. Trophic (fr. Trophe = nourishment) Components Base on Nourishment Autotrophic component - Self nourishing where fixation of light energy, use of simple inorganic substances, and build-up of complex substance predominate. Heterotrophic component - Nourishment taken from other organism where utilization, rearrangement, and decomposition of complex materials predominate. Heterotrophs were subdivided into Biophages and saprophages. Biophages – organisms consuming other living organisms Saprophages – organisms feeding on dead organisms/feeding on dead organic matter Descriptive Components Comprising the Ecosystem: Inorganic substances – (C, N, CO2, H2O, etc) involved in nutrient cycles; Organic compounds – (proteins, carbohydrates, lipids, humic substances, etc.) that link biotic and abiotic; Climate regime – Temperature and other physical factors Producers – Autotrophic organisms, largely green plants, which are able to manufacture food from simple inorganic substances; Macroconsumers or phagotrophs – (Phago = to eat), heterotrophic organisms, chiefly animals, which ingest other organisms or particulate organic matter; Microconsumers, saprotrophs – (Sapro= to decompose), or osmotrophs (osmo=to pass through a membrane), heterotrophic organisms, chiefly bacteria and fungi. Interaction of the autotrophs and heterotrophs Green belt - stratum in which light energy is available Brown belt - stratum wherein in contains the most intense heterotrophic metabolism takes place. Energy circuits - Grazing circuit (where grazing refers to the direct consumption of living plants or plant parts. -Organic detritus (where energy come from the accumulation and decomposition of dead materials. 1.2 BIOTIC AND ABIOTIC COMMUNITIES A biotic community is any assemblage of populations living in a prescribed area or physical habitat; it is an organized unit to the extent that it has characteristics additional to its individual and population components. ❖ Major communities are those which are of sufficient size and completeness of organization that they are relatively independent; that is, they need only to receive sun energy from the outside and relatively independent of inputs and outputs from adjacent communities. ❖ Minor communities are those more or less dependent on neighboring aggregations. Whittaker’s five-kingdom tree. This system contains five kingdoms based on three levels of organization: prokaryotic (Kingdom Monera), eukaryotic unicellular (Kingdom Protista), and eukaryotic multicellular and multinucleate (Kingdoms Fungi, Animalia, and Plantae). The three kingdoms at the top of the figure are distinguished mainly be differences in nutrition. Abiotic components are the non-living part of an ecosystem that shapes its environments. Common abiotic factors include atmosphere, chemical element, sunlight/temperature, wind and water. ❖ Vital elements include Carbon, Hydrogen, Oxygen, Nitrogen, and Phosphorus (CHONP) ❖ Organic compounds - carbohydrates, proteins and lipids BASIC UNITS IN A POND Abiotic substances – basic inorganic and organic compounds, such as water, carbon dioxide, oxygen, calcium, nitrogen and phosphorus salts, amino and humic acids. Producers organisms – rooted plants and minute plants (phytoplankton) Macroconsumer organisms - animals such as insect larvae, crustacea and fishes. - Zooplankton (minute animals) - Benthos - Detritivore Saprotrophic organisms – aquatic bacteria, flagellates and fungi. 1.3 HOMEOSTASIS OF THE ECOSYSTEM The ecosystem is capable of self-maintenance and self-regulations as are their component populations and organisms. Thus, cybernetics (fr. kybernetes = pilot or governor), the service of controls, has important application in ecology especially since man increasingly tends to disrupt natural controls or attempts to substitute artificial mechanisms for natural ones. Homeostasis (homeo=same; stasis=standing) is the term generally applied to the tendency for biological systems to resist change and to remain in a state of equilibrium. Homeostasis is maintained through negative feedbacks. As changes in the environment push a system property from its equilibrium, negative feedbacks counteract the change in environment. Homeostasis is the ability of ecological systems to maintain stable system properties despite perturbations. Properties of systems reflect the system as a whole and are not solely determined by the identity of the species in the system. Homeostasis is a common trait of complex systems. Negative feedbacks in these complex systems counteract the effect of perturbations that would otherwise cause the system to change. Resource constraints are a strong mechanism for inducing negative feedbacks. As a resource is overutilized in an ecological system, processes such as increased death rates and decreased birth rates dampen population increases, resulting in homeostasis. Resource constraints are not necessarily affected by changes in the environment and therefore may still operate even when other abiotic conditions change. However, without compensatory dynamics among species – the ability for some species to do well while others are declining – resource constraints would provide weak or nonexistent control over ecological systems. There are also limits to the homeostatic abilities of ecological systems, which are not well understood but are probably related to biodiversity, immigration rates, and the range of niche characteristics contained within the ecological system. Cellular homeostasis involves the orchestration of complex biochemical events that ensure survival and preservation of differentiated functions. Toxic injury often alters cellular programming in ways that disrupt cellular signaling pathways and give way to altered states that may or may not be consistent with cellular functions. For simplicity, it has been organized to cover the major components of signal transduction: the specific signals, the sensors, and their corresponding signaling pathways.  Summary: Ecology is a branch of a scientific study (science) that deals with organisms and its natural history in relation to biotic and abiotic components of an ecosystem. The ecosystem refers to a specific self-sustaining environment that possesses abiotic (non-living) factors in support to its biotic (living) communities. Biotic communities compose of autotrophic and the heterotrophic components. Autotrophs are the food-bearing organism mostly plants and other entities such as photosynthetic and chemosynthetic bacteria. Heterotrophs are those living creatures that depend to other organisms for their nourishments. Maintaining homeostatic conditions in an ecosystem is vital to support processes in the bioeconosis. Chapter II. Energy and Ecological Systems THE LAWS OF THERMODYNAMICS The first law, also known as Law of Conservation of Energy, states that energy cannot be created or destroyed in an isolated system. The second law of thermodynamics states that the entropy of any isolated system always increases. The third law of thermodynamics states that the entropy of a system approaches a constant value as the temperature approaches absolute zero. System or Surroundings In order to avoid confusion, scientists discuss thermodynamic values in reference to a system and its surroundings. Everything that is not a part of the system constitutes its surroundings. The system and surroundings are separated by a boundary. For example, if the system is one mole of a gas in a container, then the boundary is simply the inner wall of the container itself. Everything outside of the boundary is considered the surroundings, which would include the container itself. The boundary must be clearly defined, so one can clearly say whether a given part of the world is in the system or in the surroundings. If matter is not able to pass across the boundary, then the system is said to be closed; otherwise, it is open. A closed system may still exchange energy with the surroundings unless the system is an isolated one, in which case neither matter nor energy can pass across the boundary. The First Law of Thermodynamics The first law of thermodynamics, also known as Law of Conservation of Energy, states that energy can neither be created nor destroyed; energy can only be transferred or changed from one form to another. For example, turning on a light would seem to produce energy; however, it is electrical energy that is converted.  A way of expressing the first law of thermodynamics is that any change in the internal energy (ΔE) of a system is given by the sum of the heat (q) that flows across its boundaries and the work (w) done on the system by the surroundings: [latex]\Delta E = q + w[/latex] This law says that there are two kinds of processes, heat and work, that can lead to a change in the internal energy of a system. Since both heat and work can be measured and quantified, this is the same as saying that any change in the energy of a system must result in a corresponding change in the energy of the surroundings outside the system. In other words, energy cannot be created or destroyed. If heat flows into a system or the surroundings do work on it, the internal energy increases and the sign of q and w are positive. Conversely, heat flow out of the system or work done by the system (on the surroundings) will be at the expense of the internal energy, and q and w will therefore be negative. According to Odum (1971), heat energy is composed of the vibrations and motions of the molecules that make up the object. The absorption of the sun ‘s rays by land and water results in hot and cold areas, ultimately leading to the flow of air which may drive windmills and perform work such as the pumping of water against the force of gravity. Thus, in this case, light energy passes to heat energy of the land to kinetic energy of moving air which accomplishes work of raising water. The energy is not destroyed by lifting of the water, but becomes potential energy, because the latent energy inherent in having the water at an elevation can be turned back into some other type of energy by allowing the water to fall back down the well. Kinetic energy (Energy in motion) - energy that a body possesses by virtue of being in motion. Potential energy (Energy at rest) - the energy possessed by a body by virtue of its position relative to others, stresses within itself, electric charge, and other factors. The Second Law of Thermodynamics The second law of thermodynamics says that the entropy of any isolated system always increases. Isolated systems spontaneously evolve towards thermal equilibrium—the state of maximum entropy of the system. More simply put: the entropy of the universe (the ultimate isolated system) only increases and never decreases. A simple way to think of the second law of thermodynamics is that a room, if not cleaned and tidied, will invariably become messier and more disorderly with time – regardless of how careful one is to keep it clean. When the room is cleaned, its entropy decreases, but the effort to clean it has resulted in an increase in entropy outside the room that exceeds the entropy lost. Odum (1971) states that the transfer of energy toward an ever less available and more dispersed state. It deals with the dispersal of energy relative to the stability principle. The Third Law of Thermodynamics The third law of thermodynamics states that the entropy of a system approaches a constant value as the temperature approaches absolute zero. The entropy of a system at absolute zero is typically zero, and in all cases is determined only by the number of different ground states it has. Specifically, the entropy of a pure crystalline substance (perfect order) at absolute zero temperature is zero. This statement holds true if the perfect crystal has only one state with minimum energy. 2.2 CONCEPT OF PRODUCTIVITY Primary productivity is the rate at which radiant energy is stored by photosynthetic and chemosynthetic activity of producer organisms in the form of organic substances which can be used as food materials. Steps in production process: Gross primary productivity (total photosynthesis or total assimilation) – the total rate of photosynthesis, including the organic matter used up in respiration during the measurement period. Net primary productivity (apparent photosynthesis or net assimilation) – the rate of storage of organic matter in plant tissues in excess of the respiratory utilization by the plants during the period of measurement. Net community productivity - the rate of storage of organic matter not used by heterotrophs (that is, net primary production minus heterotrophic consumption) during the period of consideration, usually the growing season or a year Secondary productivities - the rates of energy storage at a consumer level. METHODS FOR MEASURING PRIMARY PRODUCTIVITY 1. THE HARVEST METHOD - The common and at the same time oldest method of measuring primary production is the Harvest method. In this method the plants grown on a particular field are clipped at ground level and their weight is taken. This is done at periodic intervals. 2. OXYGEN MEASUREMENT – Since there is a definite equivalence between oxygen and food produced, oxygen production can be a basis for determining productivity. However, in most situations’ animals and bacteria (as well as plants themselves) are rapidly using up the oxygen; and often there is gas exchange with other environments. The sum of the oxygen produced in the light bottle and oxygen used in the dark bottle is the total oxygen production, thus providing an estimate of primary production with appropriate conversion to calories. 3. CARBON DIOXIDE METHODS – Techniques for measuring rates of primary productivity that monitors changing carbon dioxide (CO2) concentrations as a means of assessing the rate of carbon dioxide uptake in photosynthesis and release in respiration. 4. THE pH METHOD – In aquatic ecosystems the pH of the water is a function of the dissolved carbon dioxide content, which, in turn, is decreased by photosynthesis and increased by respiration. 5. DISAPPEARANCE OF RAW MATERIALS – This method measures the net production of the whole community. This technique refers to the usage of raw materials minerals by organism in an ecosystem. 6. PRODUCTIVITY DETERMINATIONS WITH RADIOACTIVE MATERIALS – One of the most sensitive and widely used methods for measuring aquatic plant  production is done in bottles with radioactive carbon (14C) added as carbonate. After a short period of time, the plankton or other plants are filtered from the water, dried, and placed in a counting device. With suitable calculations and a correction for ―dark up- take‖ (14C adsorption in a dark bottle) the amount of carbon dioxide fixed in photosynthesis can be determined from the radioactive counts made. 7. THE CHLOROPHYLL METHOD - The method was first to use to estimate primary productivity in large water bodies such as sea but later applied to terrestrial ecosystem as well as. This method involves the determination of chlorophyll contents of phytoplankton in a given volume of water. PRODUCTION AND DECOMPOSITION IN NATURE PHOTOSYNTHESIS – the way where organisms (plants) manufacture their own food. Chlorophyll bearing plants manufacture carbohydrates, proteins, fats and other complex materials. PRODUCTION AND DECOMPOSITION IN NATURE 1. Photosynthetic bacteria largely aquatic (marine and freshwater) and in most situations play a minor role in the production of organic matter Play a role in the cycling of certain minerals in aquatic sediments Obligate anaerobes Facultative anaerobes Green and purple sulfur bacteria Non-sulfur photosynthetic bacteria (important in sulfur cycle) Able to function with or without Able to function only in the absence oxygen of oxygen Can also act as heterotrophs in the Occur in the boundary layer between absence of light oxidative and reduced zones in sediments or water where there is light of low intensity. 2. Chemosynthetic Bacteria Considered to be “producers” Intermediate between autotrophic and heterotrophic Manufacture food by chemical oxidation of simple inorganic compounds. (Ammonia to nitrite, nitrite to nitrate, sulfide to nitrate and ferrous to ferric iron. Types of respiration ❖ Aerobic respiration – gaseous oxygen is the hydrogen acceptor (Oxidant). ❖ Anaerobic respiration – gaseous oxygen not involved. An inorganic compound other than oxygen is the electron acceptor. ❖ Fermentation – also anaerobic but an organic compound is the electron acceptor. 2.3. FOOD CHAINS Food chain - the transfer of food energy from the source in plants through a series of organisms with repeated eating and being eaten. (80 to 90% of energy is lost at each transfer). TWO BASIC TYPES OF FOOD CHAIN 1. Grazing food chain – starting from green plant base, goes to grazing herbivores and on to carnivores. Predator food chain, where the sequence of organisms is generally from small too big. b. Parasitic food chain, where organisms tend to decrease in size as one goes higher up the food chain. Detritus food chain – start from dead organic matter into microorganisms (saprotrophs) and then to detritus feeding organisms (detritivores) and to their predators. Food chains are not isolated sequences but are interconnected with one another. The interlocking/interconnected pattern is called food web. Consumers have different sources of food in an ecosystem and do not only rely on only one species for their food. If we put all the food chains within an ecosystem together, then we end up with many interconnected food chains. This is called a food web. A food web is very useful to show the many different feeding relationships between different species within an ecosystem. TROPHIC STRUCTURE AND ECOLOGICAL PYRAMIDS The interaction of the food chain phenomena (energy loss at each transfer) and the size metabolism relationship results in communities having a definite trophic structure, which is often characteristic of a particular type of ecosystem (lake, forest, coral reef, pasture, etc.). Trophic structure maybe measured and described either in terms the standing crop per unit area or in terms of the energy fixed per unit area per unit of time at successive trophic levels. Trophic level - A level of the energy pyramid. Organisms whose food is obtained by plants by the same number of steps are said to belong to the same trophic level. - is the position an organism occupies in a food chain. It refers to food or feeding. - Apex predator – top level predators with few or no predators of their own. In complex natural communities, organisms whose food is obtained from plants by the same number of steps are said to belong to the same trophic level. Thus, green plants (the producer level, plant-eaters the second level (primary consumer level), carnivores, which eat the herbivores, the third level (the secondary consumer level), and secondary carnivores the fourth level (the tertiary consumer level). Important notes The energy flow from one trophic level to the other is known as a food chain Producers are at the first TROPHIC LEVEL Primary Consumers are the SECOND TROPHIC LEVEL Secondary consumers are at the THIRD TROPHIC LEVEL The energy flow through a trophic level equals the total assimilation (A) at that level, which, in turn, equals the production (P) of biomass plus respiration (R). A=P+R Trophic structure and also trophic function may be shown maybe shown graphically by means of ecological pyramids in which the first for producer level forms the base and successive levels the tiers which make up the apex. Ecological pyramids may be of three general types: The pyramids of numbers in which the number of individual organisms is depicted. The pyramid of biomass based on the total dry weight, caloric value, or other measure of the total amount of living material. The pyramid of energy in which the rate of energy flow and/or “productivity” at successive trophic levels is shown. 2.4 ENERGY BUDGETS An energy budget is the specification of the uptake of energy from the environment by an organism (feeding and digestion) and of the use of this energy for the various purposes: maintenance, development, growth and reproduction. No process involving an energy transformation will spontaneously occur unless there is a degradation of the energy from a concentrated form into a dispersed form. Because some energy is always dispersed into unavailable heat energy, no spontaneous transformation of energy (light, for example) into potential energy (i.e. protoplasm) is 100% efficient. THE ENERGY ENVIRONMENT Solar radiation is used by green plants during the process of photosynthesis to supply energy to the biotic components of the ecosystem. Extraterrestrial sunlight reaches the biosphere at a rate of 2gcal per cm2 per min, but it is attenuated exponentially as it passes through the atmosphere. At most 67% (1.34gcal per cm2 per min) reach the earth surface at noon on a clear summer day. Solar radiation is further attenuated, and the spectral distribution of its energy greatly altered as it passes through cloud cover, water and vegetation. Solar irradiance, cloud light, sky light and shortwave radiation beneath vegetative canopy (from Gates 1980) Radiant energy reaching the surface of the earth on a clear day is about ▪ 10% Ultraviolet (UV) ▪ 45% Visible light ▪ 45% Infrared Blue and red light is most useful in photosynthesis An energy budget describes the ways in which energy is transformed from one state to another within some defined system, including an analysis of inputs, outputs, and changes in the quantities stored. Ecological energy budgets focus on the use and transformations of energy in the biosphere or its components. Solar electromagnetic radiation is the major input of energy to Earth. This external source of energy helps to heat the planet, evaporate water, circulate the atmosphere and oceans, and sustain ecological processes. Ultimately, all of the solar energy absorbed by Earth is re-radiated back to space, as electromagnetic radiation of a longer wavelength than what was originally absorbed. Earth maintains a virtually perfect energetic balance between inputs and outputs of electromagnetic energy. Earth's ecosystems depend on solar radiation as an external source of diffuse energy that can be utilized by photosynthetic autotrophs, such as green plants, to synthesize simple organic molecules such as sugars from inorganic molecules such as carbon dioxide and water. Plants use the fixed energy of these simple organic compounds, plus inorganic nutrients, to synthesize an enormous diversity of biochemicals through various metabolic reactions. Plants utilize these biochemicals and the energy they contain to accomplish their growth and reproduction. Moreover, plant biomass is directly or indirectly utilized as food by the enormous numbers of heterotrophic organisms that are incapable of fixing their own energy. These organisms include herbivores that eat plants, carnivores that eat animals, and detritivores that feed on dead biomass. Worldwide, the use of solar energy for this ecological purpose is relatively small, accounting for much less than 1% of the amount received at Earth's surface. Although this is a quantitatively trivial part of Earth's energy budget, it is clearly very important qualitatively, because this is the absorbed and biologically fixed energy that subsidizes all ecological processes. Summary: The laws of thermodynamics states knowledge on the properties of energy in the earth ‘s biosphere. Energy is responsible in ecosystems productivity. Primary productivity of an ecosystem can be measured in several methods and techniques depending on its suitability – harvest method, oxygen measurement, carbon dioxide methods, the pH method, disappearance of raw materials, using radioactive materials and the chlorophyll method. Energy is being transferred from the environment to various space and organisms as illustrated on food chains/web, ecological pyramids. The process of photosynthesis by producers marks the start of energy utilization that passes to consumers (herbivores to carnivores until the top predators). Chapter 3. Biogeochemical cycles 3.1 Patterns and types of biochemical cycle The chemical elements, including all essential elements of protoplasm, tent to circulate in the biosphere in characteristic paths from environment to organisms and back to the environment known as biochemical cycle. It is a pathway by which a chemical element or molecule moves through both biotic (biosphere) and abiotic (lithosphere, atmosphere, and hydrosphere) compartments of Earth. Specifically, it involves the circulation of chemical nutrients like carbon, oxygen, nitrogen, phosphorus, calcium, and water etc. through the biological and physical world. A cycle is a series of change which comes back to the starting point and which can be repeated. In effect, the element is recycled, although in some cycles there may be places (called reservoirs) where the element is accumulated or held for a long period of time (such as an ocean or lake for water). Water, for example, is always recycled through the water cycle, as shown in the diagram. The water undergoes evaporation, condensation, and precipitation, falling back to Earth clean and fresh. Elements, chemical compounds, and other forms of matter are passed from one organism to another and from one part of the biosphere to another through biogeochemical cycles. Nutrient cycling – the movement of elements and inorganic compounds that are essential to life. Compartments/pools of nutrient cycle ❖ Reservoir pool, the large, slow-moving, generally non-biological component. ❖ Exchange or cycling pool, a smaller but more active portion that is exchanging (i.e., moving back and forth) rapidly between organisms and their immediate environment. Basic groups of biogeochemical cycles ❖ Gaseous types, in which the reservoir is in the atmosphere or hydrosphere (ocean) ❖ Sedimentary types, in which the reservoir is in the earth‘s crust. The most well-known and important biogeochemical cycles in the aquatic ecosystem, for example, include; ❖ Water cycle (hydrogen cycle) ❖ Carbon dioxide cycle (carbon cycle), ❖ Nitrogen cycle, ❖ Phosphorus cycle, ❖ Sulfur cycle, ❖ Oxygen cycle, and ❖ Rock cycle/sedimentary cycle 3.2 Water cycle and hydrogen cycle Water cycle The water cycle, also known as the hydrologic cycle or the H2O cycle, describes the continuous movement of water on, above and below the surface of the Earth. The mass water on Earth remains fairly constant over time but the partitioning of the water into the major reservoirs of ice, fresh water, saline water and atmospheric water is variable depending on a wide range of climatic variables. The water moves from one reservoir to another, such as from river to ocean, or from the ocean to the atmosphere, by the physical processes of evaporation, condensation, precipitation, infiltration, runoff, and subsurface flow. The water goes through different phases: liquid, solid (ice), and gas (vapor). The water cycle involves the exchange of energy, which leads to temperature changes. For instance, when water evaporates, it takes up energy from its surroundings and cools the environment. When it condenses, it releases energy and warms the environment. These heat exchanges influence climate. The evaporative phase of the cycle purifies water which then replenishes the land with freshwater. The flow of liquid water and ice transports minerals across the globe. It is also involved in reshaping the geological features of the Earth, through processes including erosion and sedimentation. The water cycle is also essential for the maintenance of most life and ecosystems on the planet.  Precipitation Condensed water vapor that falls to the Earth's surface. Most precipitation occurs as rain, but also includes snow, hail, fog drip, graupel, and sleet. Approximately 505,000 km3 (121,000 cu mi) of water falls as precipitation each year, 398,000 km3 (95,000 cu mi) of it over the oceans. The rain on land contains 107,000 km3 (26,000 cu mi) of water per year and a snowing only 1,000 km3 (240 cu mi). Canopy interception the precipitation that is intercepted by plant foliage, eventually evaporates back to the atmosphere rather than falling to the ground. Snowmelt The runoff produced by melting snow. Runoff The variety of ways by which water moves across the land. This includes both surface runoff and channel runoff. As it flows, the water may seep into the ground, evaporate into the air, become stored in lakes or reservoirs, or be extracted for agricultural or other human uses. Infiltration The flow of water from the ground surface into the ground. Once infiltrated, the water becomes soil moisture or groundwater. Subsurface flow the flow of water underground, in the vadose zone and aquifers. Subsurface water may return to the surface (e.g. as a spring or by being pumped) or eventually seep into the oceans. Water returns to the land surface at lower elevation than where it infiltrated, under the force of gravity or gravity induced pressures. Groundwater tends to move slowly, and is replenished slowly, so it can remain in aquifers for thousands of years. Evaporation The transformation of water from liquid to gas phases as it moves from the ground or bodies of water into the overlying atmosphere. The source of energy for evaporation is primarily solar radiation. Evaporation often implicitly includes transpiration from plants, though together they are specifically referred to as evapotranspiration. Total annual evapotranspiration amounts to approximately 505,000 km3 (121,000 cu mi) of water, 434,000 km3 (104,000 cu mi) of which evaporates from the oceans. Sublimation The state changes directly from solid water (snow or ice) to water vapor. Deposition This refers to changing of water vapor directly to ice. Advection The movement of water — in solid, liquid, or vapor states — through the atmosphere. Without advection, water that evaporated over the oceans could not precipitate over land. Condensation The transformation of water vapor to liquid water droplets in the air, creating clouds and fog. Transpiration The release of water vapor from plants and soil into the air. Water vapor is a gas that cannot be seen. Percolation Water flows horizontally through the soil and rocks under the influence of gravity. The Carbon Cycle Initially discovered by Joseph Priestley and Antoine Lavoisier, and popularized by Humphry Davy A cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth. Comprises a sequence of events that are key to making the Earth capable of sustaining life; it describes the movement of carbon as it is recycled and reused throughout the biosphere.  The global carbon budget is the balance of the exchanges (incomes and losses) of carbon between the carbon reservoirs or between one specific loop (e.g., atmosphere ↔ biosphere) of the carbon cycle. An examination of the carbon budget of a pool or reservoir can provide information about whether the pool or reservoir is functioning as a source or sink for carbon dioxide. Main components The global carbon cycle is now usually divided into the following major reservoirs of carbon interconnected by pathways of exchange: ❖ The atmosphere ❖ The terrestrial biosphere ❖ The oceans, including dissolved inorganic carbon and living and non-living marine biota ❖ The sediments, including fossil fuels, fresh water systems and non-living organic material, such as soil carbon ❖ The Earth's interior, carbon from the Earth's mantle and crust. These carbon stores interact with the other components through geological processes The carbon exchanges between reservoirs occur as the result of various chemical, physical, geological, and biological processes. The ocean contains the largest active pool of carbon near the surface of the Earth. The natural flow of carbon between the atmosphere, ocean, and sediments is fairly balanced, so that carbon levels would be roughly stable without human influence. Carbon Pools in the Major Reservoirs on Earth Quantity Quantity Pool Pool (gigatons) (gigatons) Atmosphere 720 Terrestrial biosphere (total) 2,000 Oceans (total) 38,400 Living biomass 600-1,000 Total inorganic 37,400 Dead biomass 1,200 Total Organic 1,000 Aquatic biosphere 1-2 Surface Layer 670 Fossil fuels (total) 4,130 Deep Layer 36,730 Coal 3,510 Lithosphere Oil 230 Sedimentary carbonates >60,000,000 Gas 140 Kerogens 15,000,000 Other (peat) 250 This diagram of the fast carbon cycle shows the movement of carbon between land, atmosphere, and oceans in billions of tons of carbon per year. Yellow numbers are natural fluxes; red are human contributions in billions of tons of carbon per year. White numbers indicate stored carbon. 3.4 The Nitrogen Cycle ❖ The process by which nitrogen is converted between its various chemical forms. ❖ This transformation can be carried out through both biological and physical processes. ❖ Important processes in the nitrogen cycle include fixation, ammonification, nitrification, and denitrification. The majority of Earth's atmosphere (78%) is nitrogen, making it the largest pool of nitrogen. ❖ However, atmospheric nitrogen has limited availability for biological use, leading to a scarcity of usable nitrogen in many types of ecosystems. ❖ The nitrogen cycle is of particular interest to ecologists because nitrogen availability can affect the rate of key ecosystem processes, including primary production and decomposition. ❖ Human activities such as fossil fuel combustion, use of artificial nitrogen fertilizers, and release of nitrogen in wastewater have dramatically altered the global nitrogen cycle. Nitrogen occurs in numerous dissolved and particulate forms. The particulate forms include organic nitrogen incorporated in living plankton, organic nitrogen in dead organic matter, and ammonia adsorbed to inorganic particles and colloids. The dissolved forms include dissolved organic nitrogen, ammonia, nitrite, nitrate, and dissolved molecular nitrogen gas (N2). The organic forms of nitrogen include many compounds such as amino acids, ammines, nucleotides, proteins, and humic compounds (Wetzel, 2001). (https://www.researchgate.net/profile/Sujoy_Roy3/publication/319990389/figure/fig6/AS:631643878 457382@1527607044829/2-Nitrogen-cycle-in-aquatic ecosystems.png, February 2021)  A simple and complete diagram of the nitrogen cycle. The blue boxes represent stores of nitrogen, the green writing is for processes that occur to move the nitrogen from one place to another and the red writing are all the bacteria involved. (https://upload.wikimedia.org/wikipedia/en/thumb/e/e3/ The_Nitrogen_Cycle.png/260pxThe_Nitrogen_Cycle.pn g, February 2021) Schematic representation of the flow of nitrogen through the ecosystem. The importance of bacteria in the cycle is immediately recognized as being a key element in the cycle, providing different forms of nitrogen compounds able to be assimilated by higher organisms (https://upload.wikimedia.org/wikipedia/commons/thu mb/b/bd/Nitrogen_Cycle_2.svg/300pxNitrogen_Cycle_ 2.svg.png, February 2021) 3.5 Phosphorous cycle Phosphorus cycle in aquatic ecosystems (https://www.researchgate.net/profile/Sujoy_Roy3/publication/319990389/figure/fig5/AS:6316438784 69667@1527607044772/1-Phosphorus-cycle-in-aquatic-ecosystems.png, February 2021)  The phosphorus cycle is the biogeochemical cycle that describes the movement of phosphorus through the lithosphere, hydrosphere, and biosphere. Unlike many other biogeochemical cycles, the atmosphere does not play a significant role in the movement of phosphorus, because phosphorus and phosphorus-based compounds are usually solids at the typical ranges of temperature and pressure found on Earth. On the land, phosphorus (chemical symbol, P) gradually becomes less available to plants over thousands of years, because it is slowly lost in runoff. Low concentration of P in soils reduces plant growth, and slows soil microbial growth - as shown in studies of soil microbial biomass. Soil microorganisms act as both sinks and sources of available P in the biogeochemical cycle. Locally, transformations of P are chemical, biological and microbiological: the major long-term transfers in the global cycle, however, are driven by tectonic movements in geologic time. Humans have caused major changes to the global P cycle through shipping of P minerals, and use of P fertilizer, and also the shipping of food from farms to cities, where it is lost as effluent. Phosphorus is the key variable most commonly used to characterize the trophic status of lakes and reservoirs. Phosphorus is present in both dissolved and particulate forms. The particulate forms include organic phosphorus incorporated in living plankton, organic phosphorus in dead organic matter, inorganic mineral phosphorus in suspended sediments, phosphate adsorbed to inorganic particles and colloids such as clays and precipitated carbonates and hydroxides, phosphate adsorbed to organic particles and colloids, and phosphate co-precipitated with chemicals such as iron and calcium. The dissolved forms include dissolved organic phosphorus (DOP), orthophosphate, and polyphosphates. The organic forms of phosphorus can be separated into two functional fractions. The labile fraction cycles rapidly, with particulate organic phosphorus quickly being converted to soluble low-molecular-weight compounds. The refractory fraction of the colloidal and dissolved organic phosphorus cycles more slowly, regenerating orthophosphate at a much lower rate. 3.6 The Sulfur Cycle The Sulfur cycle – in general (https://upload.wikimedia.org/wikipedia/commons/thumb/5/5c/Sulfur_cycle_-_English.jpg/800px- Sulfur_cycle_-_English.jpg, February 2021) Sulfur dioxide from the atmosphere becomes available to terrestrial and marine ecosystems when it is dissolved in precipitation as weak sulfuric acid or when it falls directly to the Earth as fallout. Weathering of rocks also makes sulfates available to terrestrial ecosystems. Decomposition of living organisms returns sulfates to the ocean, soil and atmosphere. (credit: modification of work by John M. Evans and Howard Perlman, USGS) The sulfur cycle is the collection of processes by which sulfur moves to and from minerals (including the waterways) and living systems. Such biogeochemical cycles are important in geology because they affect many minerals. Biogeochemical cycles are also important for life because sulfur is an essential element, being a constituent of many proteins and cofactors. Steps of the Sulphur cycle are: 1. Mineralization of organic sulfur into inorganic forms, such as hydrogen sulfide (H2S), elemental sulfur, as well as sulfide minerals. 2. Oxidation of hydrogen sulfide, sulfide, and elemental sulfur (S) to sulfate (SO42–). 3. Reduction of sulfate to sulfide. 4. Incorporation of sulfide into organic compounds (including metal-containing derivatives). These are often termed as follows: Assimilative sulfate reduction in which sulfate (SO42−) is reduced by plants, fungi and various prokaryotes. The oxidation states of sulfur are +6 in sulfate and –2 in R–SH. Desulfurization in which organic molecules containing sulfur can be desulfurized, producing hydrogen sulfide gas (H2S, oxidation state = –2). An analogous process for organic nitrogen compounds is deamination. Oxidation of hydrogen sulfide produces elemental sulfur (S8), oxidation state = 0. This reaction occurs in the photosynthetic green and purple sulfur bacteria and some chemolithotrophs. Often the elemental sulfur is stored as polysulfides. Oxidation in elemental sulfur by sulfur oxidizers produces sulfate. Dissimilative sulfur reduction in which elemental sulfur can be reduced to hydrogen sulfide.  Dissimilative sulfate reduction in which sulfate reducers generate hydrogen sulfide from sulfate. Sulfur, an essential element for the macromolecules of living things, is released into the atmosphere by the burning of fossil fuels, such as coal. As a part of the amino acid cysteine, it is involved in the formation of disulfide bonds within proteins, which help to determine their 3-D folding patterns, and hence their functions. As shown in Figure below, sulfur cycles between the oceans, land, and atmosphere. Atmospheric sulfur is found in the form of sulfur dioxide (SO2) and enters the atmosphere in three ways: from the decomposition of organic molecules, from volcanic activity and geothermal vents, and from the burning of fossil fuels by humans. On land, sulfur is deposited in four major ways: precipitation, direct fallout from the atmosphere, rock weathering, and geothermal vents (Figure 2). Atmospheric sulfur is found in the form of sulfur dioxide (SO2), and as rain falls through the atmosphere, sulfur is dissolved in the form of weak sulfuric acid (H2SO4). Sulfur can also fall directly from the atmosphere in a process called fallout. Also, the weathering of sulfur-containing rocks releases sulfur into the soil. These rocks originate from ocean sediments that are moved to land by the geologic uplifting of ocean sediments. Terrestrial ecosystems can then make use of these soil sulfates (SO4−), and upon the death and decomposition of these organisms, release the sulfur back into the atmosphere as hydrogen sulfide (H2S) gas. Sulfur enters the ocean via runoff from land, from atmospheric fallout, and from underwater geothermal vents. Some ecosystems rely on chemoautotrophs using sulfur as a biological energy source. This sulfur then supports marine ecosystems in the form of sulfates. Human activities have played a major role in altering the balance of the global sulfur cycle. The burning of large quantities of fossil fuels, especially from coal, releases larger amounts of hydrogen sulfide gas into the atmosphere. As rain falls through this gas, it creates the phenomenon known as acid rain. Acid rain is corrosive rain caused by rainwater falling to the ground through sulfur dioxide gas, turning it into weak sulfuric acid, which causes damage to aquatic ecosystems. Acid rain damages the natural environment by lowering the pH of lakes, which kills many of the resident fauna; it also affects the man-made environment through the chemical degradation of buildings. For example, many marble monuments, such as the Lincoln Memorial in Washington, DC, have suffered significant damage from acid rain over the years. These examples show the wide-ranging effects of human activities on our environment and the challenges that remain for our future. 3.7 The oxygen cycles Oxygen cycle reservoirs and flux (https://cdn1.byjus.com/wp-content/uploads/2017/11/Oxygen-Cycle1.png, February 2021) The oxygen cycle is the biogeochemical cycle that describes the movement of oxygen within its three main reservoirs: the atmosphere (air), the total content of biological matter within the biosphere (the global sum of all ecosystems), and the lithosphere (Earth's crust). Failures in the oxygen cycle within the hydrosphere (the combined mass of water found on, under, and over the surface of a planet) can result in the development of hypoxic zones. The main driving factor of the oxygen cycle is photosynthesis, which is responsible for the modern Earth's atmosphere and life on earth. Oxygen Cycle Steps: 1. Atmosphere: Only a small percentage of the world ‘s oxygen is present in the atmosphere, only about 0.35 %. This exchange of gaseous oxygen happens through Photolysis. Photolysis: This is the process by which molecules like atmospheric water and nitrous oxide are broken down by the ultraviolet radiation coming from the sun and release free oxygen. 2. Biosphere: The exchange of oxygen between the living beings on the planet, between the animal kingdom and the plant kingdom. The exchange of oxygen in the biosphere is codependent on the Carbon cycle and hydrogen cycle as well. It mainly occurs through 2 processes. Photosynthesis: The process by which plants make energy by taking in carbon dioxide from the atmosphere and give out oxygen. Respiration: The process by which animals and humans take in oxygen from the atmosphere and use it to break down carbohydrates and give out carbon dioxide.  3. Lithosphere: The part of the planet containing most of the oxygen content through biomass, organic content and mineral deposits. These deposits are formed when free radical elements were exposed to free oxygen and over time, they form silicates and oxides. This trapped oxygen is released back due to several weathering processes. Also, animals and plants draw nutrient materials from the lithosphere and free some trapped oxygen. 4. Hydrosphere: Oxygen dissolved in water is responsible for the sustenance of the aquatic ecosystem present beneath the surface. The hydrosphere is 33% oxygen by volume present mainly as a component of water molecules with dissolved molecules including carbonic acids and free oxygen. 3.8 Rock cycle/sedimentary cycle ▪ The rock cycle is a fundamental concept in geology that describes the dynamic transitions through geologic time among the three main rock types: sedimentary, metamorphic, and igneous. ▪ An igneous rock such as basalt may break down and dissolve when exposed to the atmosphere, or melt as it is subducted under a continent. ▪ Due to the driving forces of the rock cycle, plate tectonics and the water cycle, rocks do not remain in equilibrium and are forced to change as they encounter new environments. The rock cycle is an illustration that explains how the three rock types are related to each other, and how processes change from one type to another over time. ▪ Most elements and compounds are more earthbound than nitrogen, oxygen, carbon dioxide, water and their cycles follow a basic sedimentary cycle pattern in that erosion, sedimentation, mountain building and volcanic activity as well as biological transport, are the primary agents affecting circulation. A diagram of the rock cycle. 1. Magma; 2. Crystallization (freezing of rock); 3. Igneous rocks; 4. Erosion; 5. Sedimentation; 6. Sediments & sedimentary rocks; 7. Tectonic burial and metamorphism; 8. Metamorphic rocks; 9. Melting. Each of the types of rocks are altered or destroyed when it is forced out of its equilibrium conditions. Summary: The different biogeochemical cycles are considered to fuel the earths ‘processes - Water cycle (hydrogen cycle), Carbon dioxide cycle (carbon cycle), Nitrogen cycle, Phosphorus cycle, Sulfur cycle, Oxygen cycle, and Rock cycle/sedimentary cycle. These cycles directly or indirectly support the presence of life in the biosphere. The cycles of nutrients and elements from the reserved pools to utilization back to reservoir is a continuous paradigm/pattern. Chapter 4. Concepts on Limiting Factors LIMITING FACTOR- a factor present in an environment that controls a process particularly the growth, abundance or distribution of a population of organism in an ecosystem. A limiting factor is anything that constrains a population's size and slows or stops it from growing. Some examples of limiting factors are biotic, like food, mates, and competition with other organisms for resources. Others are abiotic, like space, temperature, altitude, and amount of sunlight available in an environment. Limiting factors are usually expressed as a lack of a particular resource. For example, if there are not enough prey animals in a forest to feed a large population of predators, then food becomes a limiting factor. Likewise, if there is not enough space in a pond for a large number of fishes, then space becomes a limiting factor. There can be many different limiting factors at work in a single habitat, and the same limiting factors can affect the populations of both plant and animal species. Ultimately, limiting factors determine a habitat's carrying capacity, which is the maximum size of the population it can support. 4.1 Liebig’s “Law of Minimum” To occur and thrive in a given situation, an organism must have essential materials which are necessary for growth and reproduction. These basic requirements vary with the species and with the situation. Under a “SteadyState” conditions the essential material available in amounts most closely approaching the critical minimum needed will tend to be the limiting one. This ―Law o minimum is less applicable under “transient-state” conditions when the amounts, and hence the effects, of many constituents are rapidly changing. Growth of plant is dependent on the amount of food stuff (nutrients) which is presented to it in minimum quantity -This was first expressed by JUSTUS LIEBIG in 1840. Liebig’s Barrel - Showing essential nutrients needed in yielding plants good growth and overall health. (https://fifthseasongardening.com/liebigs- barrel-a-paradigm-for-thinking-about- nutrients, February 2021). Liebig illustrated the concept of limitation using a metaphorical barrel with each stave representing a different element. A nutrient stave that was shorter than the others would cause the liquid contained in the barrel to spill out at that level. Illustrated otherwise, a cooper that built his barrel ten feet high would have worked in vain if his last stave was only five feet long. With this visual aid, the notion of limiting factors is intuitive and seems almost obvious, but is easily overlooked when attempting to remediate problems or improve yields.  Examples of limiting factors of a population growth A. Terrestrial Ecosystem B. Marine/aquatic Ecosystem 1. Temperature 1. Salinity 2. Water 2. Temperature 3. Moisture 3. Sunlight 4. Soil nutrients 4. Dissolved Oxygen 4.2 Shelford’s Law of Tolerance The presence and success of an organism depend upon the completeness of a complex of conditions. Absence or failure of an organism can be controlled by the quantitative deficiency or excess with respect to any one of several factors which may approach the limit of tolerance for that organism. The existence and the, abundance, and the distribution of a species in an ecosystem are determined by whether the levels of one or more physical or chemical factors fall within the range tolerated by that species. To express the relative degree of tolerance, a series of terms have come into general use in ecology that utilize the prefixes ―steno-― meaning narrow and ―eury-‖ meaning wide (see table below). Technical Terms on the Implication of Tolerances to Specific Conditions Narrow “steno” Wide “eury-” Referring to- Stenophyric Euryhydric Water Stenothermal Eurythermal Temperature Stenohaline Euryhaline Salinity Stenophagic Euryphagic Food stenoecious euryecious Habitat selection Zone of Tolerance (figure below) – an organism grows best in the Zone of tolerance, which is favorable for its development. This zone is subdivided into three zones: (https://1.bp.blogspot.com/-EhhsSNoo8gQ/WDJScjTNMJI/AAAAAAAAABQ/aCOmIJ3ibZ YodQeIJXWgrsz4sA1VUYsXgCLcB/s640/tolerance%2Bzone.jpg, February 2021)  Optimum zones: the most favorable zone in the range between two extreme limits thus supports maximum growth and development of organism. Critical minimum zone: the lowest limit of minimum below which the organism growth is inhibited Critical maximum zone: the maximum limit of tolerance zone above which the organism growth ceases. 4.2 Physical factors of importance as limiting factors in the aquatic ecosystem 1. SALINITY: Changing salinity is a master factor in the distribution of both marine and estuarine species and is limiting to freshwater organisms, hence salinity is fundamental with far-reaching effects in creating and modifying aquatic ecosystem assemblage structure and functioning. The effects of changing salinity on the ecology of different habitats is driven ultimately by the underlying physiology and tolerances of organisms and their ability to cope with salinity fluctuations on both long- and short-term scales. Estuarine species are often euryhaline, adapted to tolerate fluctuating salinity, whereas many marine species are stenohaline and limited by their narrow range of physiological tolerance. For estuarine species, lowered salinities may be a subsidy, i.e. benefitting the organisms by reducing competition, whereas for non-tolerant species they are a stressor. Salinities at the margins or outside the tolerance range of particular species will prevent their occurrence, change their behavior or limit reproduction and germination, reducing their fitness for survival in that environment. Salinity can act synergistically or antagonistically with other environmental stressors. 2. TEMPERATURE: Life can only exist within -200º to 100ºC. Most species are restricted to a narrower range of temperature. Aquatic organisms have narrower range of tolerance than equivalent land animals. Organisms which are subjected to temperature variations tend to be depressed, inhibited or slowed down by a constant temperature. 3. SUNLIGHT: The quality of light, the intensity of, and the duration are known to be the important factors of light. Both plant and animals respond to different quality of light. Individual plants as well as communities adapt to different light intensities by becoming “shade-adapted”. Or “sun adapted”. Light cannot penetrate to some considerable depth in the water column making it one of the greatest limiting factors in the aquatic environment. 4. DISSOLVE OXYGEN: Dissolved oxygen concentrations are constantly affected by diffusion and aeration, photosynthesis, respiration and decomposition. Dissolved oxygen is necessary to many forms of life including fish, invertebrates, bacteria and plants. These organisms use oxygen in respiration, similar to organisms on land. Fish and crustaceans obtain oxygen for respiration through their gills, while plant life and phytoplankton require dissolved oxygen for respiration when there is no light for photosynthesis. The amount of dissolved oxygen needed varies from creature to creature. Bottom feeders, crabs, oysters and worms need minimal amounts of oxygen (1-6 mg/L), while shallow water fish need higher levels (4-15 mg/L). Microbes such as bacteria and fungi also require dissolved oxygen. These organisms use DO to decompose organic material at the bottom of a body of water. Microbial decomposition is an important contributor to nutrient recycling. However, if there is an excess of decaying organic material (from dying algae and other organisms), in a body of water with infrequent or no turnover (also known as stratification), the oxygen at lower water levels will get used up quicker. Low-dissolved oxygen levels can limit the bacterial metabolism of certain organic compounds. 5. NUTRIENTS: A limiting nutrient is a relatively scarce naturally occurring element. Growth only occurs as long as the nutrient is available. Lakes and rivers are freshwater systems that depend on phosphorous and nitrogen to maintain the balance of plant and animal life in them. Generally speaking, phosphorous is the limiting nutrient in freshwater systems, meaning less phosphorous occurs naturally in rivers and lakes than nitrogen; this limits the amount of plant life that can grow in a body of water. When phosphorous quantities rise, plants grow to nuisance levels, choking rivers and making navigation difficult. In lakes, excess phosphorous fuels algal blooms that deplete water of oxygen and can lead to fish kills; this phenomenon is known as eutrophication. Excess phosphorous enters bodies of water from fertilizer runoff on lawns and sewage treatment plants. Nitrogen and phosphorous both occur naturally in the ocean, where they support the growth of aquatic plants that shellfish and other marine organisms feed on. Nitrogen is usually the limiting nutrient that keeps ocean ecosystems in balance. When it increases in quantity, phytoplankton blooms can result. The microscopic plant grows at an accelerated rate, forming a green scum on the water ‘s surface near land. Excess nitrogen enters ocean ecosystems through storm water runoff and burning fossil fuels. 6. CURRENTS AND PRESSURES: Currents in water not only influence the concentration of gases and nutrients, but act directly as limiting factors. Summary: Essential factors like nutrients and physical parameters can inhibit or exhibit processes in an aquatic environment. The law of minimum by Liebig illustrated that optimum condition must be maintained in order to promote healthy organisms in a certain ecosystem. The law of tolerance by Shelford explains the facts herein that organisms have their specific tolerances to physical limiting factors and other chemical components. In an aquatic environment (freshwater, marine or estuarine), several limiting factors that affect the life forms present. These include salinity, dissolve oxygen, sunlight, temperature, nutrients including water currents and pressure.