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Depth of Water (meters) Dissolved Oxygen (mg O2 per liter) Amount of Algae/sq. meter (kilogram) 1 9.2 550 5 9.0 535 10 8.5 404 15 7.0 155 20 5.5 45 25 4.2 12 Consider the data regarding the growth of algae in a large lake, like the one illustrated above. Plants release oxygen as a by-product of photosynthesis. Draw a conclusion about other life forms in the lake, based on this data. A) All of the life forms in the lake can be found within the top 10 feet of water. B) As you increase in depth of water, plant density decreases while fish density increases. C) Organisms in the lake rely on dissolved oxygen to survive. The lower the dissolved oxygen, the lower the survival rate. D) Plant survival is related to the amount of light (depth) but animal survival is related to the amount of oxygen available. The lower the amount of dissolved oxygen, the less animal life present in the lake.

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marianaegarciaperedo

Algae growth produces a high concentration of oxygen on the surface, which decreases with depth. The less oxygen available in deeper strata, the fewer surviving forms of life. The correct option is C.

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Available data:

                                                                                                                       

Depth of Water                Dissolved Oxygen         Amount of Algae/sq. meter

   (meters)                          (mg O₂ per liter)                           (kilogram)            

         1                                           9.2                                             550

         5                                          9.0                                             535

        10                                          8.5                                            404

        15                                          7.0                                             155

        20                                         5.5                                              45

        25                                         4.2                                               12                                                                                                                                  

According to this table,

  • On the surface, there is a higher amount of algae, and hence, more dissolved oxygen.
  • As it gets deeper, algae amount decreases as well as dissolved oxygen.

If oxygen decreases with depth, it suggests that plants -oxygen producers- lack, probably due to the lack of sunlight reaching the bottom.

Other forms of life

  • Algae growth on the surface blocks the pass of light to the deeper strata.

  • The lack of light in deeper strata affects plants' growth.

  • If there are no plants in deeper areas, there is no dissolved oxygen.

  • Since animals need oxygen, they probably can not survive in deep regions.

  • With no oxygen, the bottom turns into an anaerobic environment, in which anaerobic bacteria can survive.

These conditions can be related to eutrophication.

Eutrophication is a biological process produced in water by an oversupply of nutrients or organic matter.

This oversupply, mainly of nitrogen or phosphorous, favors phytoplankton multiplication, especially cyanobacteria.

Cyanobacteria overgrowth leads to an enhance in primary productivity and reduces the pass of radiation to deeper strata. This fact affects biota in deeper zones.  

Gradually sedimentation begins, caused by the death of some vegetable organisms that sink in the bottom.

These sediments suffer decomposition in the absence of oxygen. Anaerobic bacteria act and produce fermentation.

Phytoplankton overgrowth on the surface and fermentation process in the bottom, originate toxic compounds that can damage the animals in the water body.

Animals die, creating more sediments in the bottom where there are anaerobic conditions.

The continued increase in decomposition and fermentation produces a bad smell and toxicity.  

The result of eutrophication is the stratification of the water mass:

  • First superficial layer, the overgrowth of blue-green algae, produces toxins and interrupts the pass of light to deeper areas. The surface then becomes warmer.

  • The second layer might have oxygen available for the animals.

  • In the third layer, there is no oxygen, so life is impossible for animals or plants.

  • In the fourth layer, there are animals and plant remains in the process of sedimentation.

  • At the bottom, there are organic matter and anaerobic bacteria in charge of decomposition and fermentation, through which they produce toxic gases and smell.

Options:  

A) All of the life forms in the lake can be found within the top 10 feet of water. FALSE. Depends on the conditions and the species.

B) As you increase in depth of water, plant density decreases while fish density increases. FALSE. Plant and animals density decrease, as available oxygen decrease.

C) Organisms in the lake rely on dissolved oxygen to survive. The lower the dissolved oxygen, the lower the survival rate. TRUE.

D) Plant survival is related to the amount of light (depth) but animal survival is related to the amount of oxygen available. The lower the amount of dissolved oxygen, the less animal life is present in the lake. FALSE. Light does not reach the bottom, so there are no plants there. Plants also depend on oxygen, not only animals.

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