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Lakes & Ponds

Lakes & Ponds Still Waters brimming with life

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Lakes & Ponds Objectives

  • Classify lakes based on productivity and identify three different lake zones. 
  • Write the general process of photosynthesis and provide example of photosynthetic pond organisms.
  • Provide examples of pond consumers, both microscopic and macroscopic, and explain the cascade hypothesis using a lake food web.
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Lakes contain water that primarily stays within a basin.  There can be water movement on the surface or some water entering and leaving, but the bulk of water is not moving from one point to another (like in a stream).

Ponds are small lakes that can be strongly impacted by seasonal changes, sometimes completely freezing or drying out.

Lakes are classified based on productivity, the amount of photosynthesis occurring in lake producers.

Oligotrophic lakes have low levels of productivity, these are often clearer water.

Mesotrophic lakes have a middle level of productivity, enough photosynthesis to support a more complex food web.

Eutrophic lakes are high in productivity, so high they can be green in color.

lake prod

Crater Lake is an example of an oligotrophic lake.  Productivity is low and the water is clear to well over a meter below the water’s surface.

Within a single lake, there are different zones that are impacted by the amount of available light and nutrients.  Each zone can have its own organisms.

The Littoral lake zone is the area near shore that is shallow enough for light penetration.  Many amphibians and aquatic insects spend part of their life cycle here.

The Limnetic lake zone is the open water area that sunlight penetrates.  Many large fish swim here.

The Benthic lake zone is the bottom, below the sunlit limnetic zone.  Although light is low, nutrients are high and scavengers like crayfish and catfish can be found here.

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Some species utilize different zones in a lake during different life stages.  For example, fish eggs may be laid on plants in the near shore littoral zone, but later stages of larger fish may swim and eat in the open limnetic zone.
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Eutrophic lakes and small ponds are typically very productive, meaning there are many producers, including algae and plants.  Producers carry out the process of photosynthesis.
Photosynthesis is the combination of carbon dioxide and water in the presence of sunlight energy, producing oxygen and energy-rich sugars.  Only producers carry out this process.
 
Respiration is the opposite reaction: using sugars and oxygen and producing carbon dioxide, water, and energy.  All organisms carry out some form of this process.
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These are common microscopic producers that you are likely to find in a lake or pond.

Photosynthetic bacteria can also extract nitrogen from water.

Cyanobacteria

Photosynthetic bacteria can also extract nitrogen from water.
Single-celled and colonial (groups of single cells) protists.

Algae

Single-celled and colonial (groups of single cells) protists.
Single-celled protists that produce a symmetrical silica-rich covering that survives after the organism dies.

Diatoms

Single-celled protists that produce a symmetrical silica-rich covering that survives after the organism dies.
Single-celled photosynthetic protists that use a whip-like structure to move through the water.

Euglena

Single-celled photosynthetic protists that use a whip-like structure to move through the water.

These are producers; plants with different strategies for obtaining sunlight.

Some pond plants are completely submerged like this Elodea.

Elodea

Some pond plants are completely submerged like this Elodea.
Some plants float on the water surface.

Azolla

Some plants float on the water surface.
Some plants root into the pond sediment and produce leaves that reach the water surface.

Water Lily

Some plants root into the pond sediment and produce leaves that reach the water surface.
l duckweed

Duckweed

Duckweed and Azolla are not rooted into the soil like water lily plants; they float on top of the water.  From earlier this term, what organims live within the Azolla, providing nitrogen?

l azolla

Azolla

l water lily

Aquatic plants found in local ponds have strategies for continuing photosynthesis in conditions that could impact gas exchange or light availability.

The leaves of this water lily plant have stomata located on the top of the leaves instead of on the bottom as you observed in an earlier lab.  Why?

All three of the species below are submerged aquatic plants.  Water is abundant, but what two other components of productivity can be limited?

 

Answer: light & nutrients

l elodea

Elodea

l hornwort

Hornwort

l vallis

Vallisnaria

Here are a few examples of common microscopic lake and pond consumers.

Protists that use tiny hair-like cilia to move through the water, eating smaller protists.

Paramecium

Protists that use tiny hair-like cilia to move through the water, eating smaller protists.
Single-celled protist that wraps its membrane around its food and crawls along surfaces.

Amoeba

Single-celled protist that wraps its membrane around its food and crawls along surfaces.
Microscopic animal consumers that rotate through the water.

Rotifers

Microscopic animal consumers that rotate through the water.
Microscopic to macroscopic crustaceans with jointed legs and antennae.

Copepods

Microscopic to macroscopic crustaceans with jointed legs and antennae.

These are a few commonly observed macroscopic (but still small) consumers.

Small animals that are transparent so you can sometimes see internal organs, including what they have eaten.

Daphnia

Small animals that are transparent so you can sometimes see internal organs, including what they have eaten.
Distant relatives of jellyfish, these animals also catch their food with tentacles.

Hydra

Distant relatives of jellyfish, these animals also catch their food with tentacles.
Small worms that consume a variety of foods and can move in a whip-like manner.

Nematodes

Small worms that consume a variety of foods and can move in a whip-like manner.
Small flatworm scavengers that creep along surfaces.

Planaria

Small flatworm scavengers that creep along surfaces.
decomposer

A food web would not be complete without decomposers.  In aquatic ecosystems, bacteria are the primary decomposers.  

In the middle of this photo is a single photosynthetic diatom.  The smaller brownish material around it may be bacteria, they are more difficult to identify.

Sunlight energy continually enters a pond wood web, and is converted by photosynthetic producers into energy-rich sugars.  Organisms convert that sugar energy into heat that is lost from the system.

Decomposers enable nutrients from dead organisms to re-enter the food web’s producers, cycling repeatedly over time.

pond web

Within ecosystems, an impact on one trophic (feeding) level can cascade (move) through other trophic levels.  For example, if you add large fish to a lake, it can impact the amout of phytoplankton even if the large fish do not directly eat the phytoplankton.  Looking at the simplified food chain, can you explain why?

cascade hyp
Answer: an increase of large fish would decrease the small fish and insects they eat.  Without those small fish and insects, the organisms they eat (smaller consumers) would increase.  They eat phytoplankton, so an increase in the herbivores would decrease the amount of producers like algae.

Start this Guide’s first journal page here

Journal Page #9: Lake Food Web

Sketch and label a hypothetical lake food web.  Include the following in your labeled sketch:

  • three lake zones (littoral, limnetic, benthic)
  • examples of producers (microscopic and macroscopic)
  • examples of consumers (microscopic and macroscopic)
  • decomposers
  • arrows for energy flow (including sun and heat)
  • arrows for nutrient cycling
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You are turning in a journal page of your handmade (either on paper or digitally produced) lake food web that contains all of the required sketched and labeled components.

It may help to review food webs here.

The next section explores wetlands that are partially aquatic and partially terrestrial.
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Check your knowledge.  Can you:

  • classify lakes based on productivity and identify three different lake zones?
  • write the general process of photosynthesis and provide example of photosynthetic pond organisms?
  • provide examples of pond consumers, both microscopic and macroscopic, and explain the cascade hypothesis using a lake food web?

Go back to the Freshwater Guide Page

Go forward to the Wetlands Page

Freshwater Guide Contents

Complete all four of these sections before taking the quiz and submitting your two journal pages.

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