B6 Life in Water

1. Plentiful supply of water. No danger of water shortage or dehydration, unless drought.

2. Less varitation in temperature, doesn't cool up/down as quickly as air, no sudden temperature changes.

3. Water provides support for plants/animals with no skeletal system e.g. jellyfish.

4. Easier waste disposal, excretion easily dispersed, loss of water doesn't matter because so much water.

• Water more resistant to movement than air so animals in water use more energy to move
• Aquatic animals have to be able to control amount of water in body (water regulation) because water in surroundings have different concentration of solutes from animal's cells. If animal couldn't regulate water, water molecules would enter or leave animal cells through osmosis to even up solute concentration causing damage to cells
• Animal in salt water, cells have lower solute concentration than surroundings. If no regulation, water molecules would leave cell by osmosis causing them to shrivel and die
• If animal lived in freshwater, cells would have higher solute concentration than surroundings. If no regulation, water molecules would enter cell by osmosis causing swelling and bursting

Regulate water content using contractile vacuoles. Most single-celled organisms only have one cell membranes between them and water. 

Contractile vacuole collects water that diffuses in through osmosis. Vacuole then moves to cell membrane and contracts to empty water outside cell.

Microscopic organisms that live in fresh and salt water. Two types:

• Phytoplankton - microscopic plants that photosynthesise and main producers of aquatic food webs, important in both fresh and saltwater ecosystems
• Zooplankton - microscopic animals that feeds on phytoplankton

Insects and amphibians spend part of life cycle in water and other half on land to exploit both habitats.

Two environments present different challenges so different parts of life cycle usually have different body forms e.g. tadpole and frog.

Population varies to season:

1. Photosynthesis affected by temperature, light intensity and availability of minerals. These factors vary at different depths and different seasons causing rate of photosynthesis to vary too.

• Winter months and deep water, low light intensity and temperature but high mineral concentration so light and temperature is limiting factor
• Summer, surface water higher temperature and light intensity but low mineral concentration so mineral concentration is limiting factor

2. Phytoplankton (a type of algae) populations usually increase between late spring and late summer, called algal bloom, making water all green and murky. Increase due to:

• More light for photosynthesis and energy for growth
• Increased temperature causing both photosynthesis and growth to increase
• Population of zooplankton also increases because more phytoplankton to feed on

Most food webs in oceans are grazing food webs where they begin with a living producer (produce own food).

Producers often phytoplankton but in deep water where light cannot penetrate, photosynthesis cannot take place, so grazing food webs supported by bacterial producers relying on sulphur from deep sea vents instead of sunlight.

In other deep sea food webs, animals feed on dead, decomposing material slowly fallen from surface - marine snow major source of nutrients for these food webs.

Water polluted by fertilisers and sewage causing eutrophication: fertilisers and sewage runs off into water adding nutrients, algae grows rapidly, algae dies and decay, bacteria feeds on dead algae, uses up oxygen in water, animals cannot respire and die.

Some organisms very sensitive to level of oxygen, species used to indicate water pollution (more oxygen less pollution) - indicator species. Species to pH can also be used.

• Clean - mayfly nymph
• Low - freshwater shrimp
• High - bloodworm
• Very high - rat-tailed maggot

Pesticides like DDT (used to kill lice and mosquitoes) and industrial chemicals like PCBs (used as coolants and electrical insulators) can pollute water.

Taken up by organisms at bottom of food chain, aren't broken down so when eaten, chemical passed on, concentration rises further up chain because each organism eats many of the organisms below it. Organisms at the top accumulate huge dose and die.