Different ecosystems vary considerably in their net

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Different ecosystems vary considerably in their net primary production and in their contribution to the total net primary production of the Earth: Limitations of primary production in marine and freshwater ecosystems: o Light is a key variable – as the light is moving down in the photic zone of the ocean or a lake most of it is absorbed by the water and does not reach into the deeper areas of water (aphotic zone) o Nutrients can also limit primary production. The most common elements that are limiting factors are nitrogen, phosphorous and iron. However, if there is too much nitrogen or phosphorous in the water it results in eutrophication (algal bloom of cyanobacteria) that substantially decreases the oxygen concentration in the water and kills most fish species. Limitations of primary production in terrestrial and wetland ecosystems: o Large scale limiting factors are temperature and moisture that control primary production. These factors and their effect on the environment can be measured by actual evapotranspiration – the annual amount of water transpired by plants and evaporated from the landscape measured in millimeters. o On the local scale the limiting factor can be the mineral nutrient content of the soil. The limiting minerals are usually nitrogen and phosphorous.
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III. ENERGY TRANSFER BETWEEN TROPHIC LEVELS Secondary production the amount of chemical energy in consumers’ food that is converted into their own biomass during a given time period – Much of the primary production in not used by consumers. We can measure the efficiency of animals as energy transformers by using the following equation: production efficiency = net secondary production / assimilation of primary production Where assimilation consists of the energy that is used for growth, reproduction and cellular respiration. Production efficiency is only a fraction of the energy stored in food and not used for respiration. Birds and mammals have the lowest production efficiency because of the upkeep of constant body temperature – 1 – 3 %, fish that are ectotherms have 10 %, insects have a 40 % production efficiency.
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