[cheryl j (OP)]

I live in in the Great Lakes area of Canada and when ever I go snorkeling, I always wonder why fish and other creatures in big lakes aren't more as colorful like those ocean fish in National Geographic. Is it the temperature, fresh water, or size? Or why is camouflage more important in lakes?

[CliffordK]

Part of the answer is that lakes lack the coral, and multitude of semi-fixed animal species that are found in the oceans.  The corals use methods other than camouflage to protect themselves such as armor, and perhaps toxins. 

Fish camouflage would be based in part on the background colors of the environment.

The lakes lack the corals.  There is still plant life in the lakes, but generally using photosynthesis, and thus found in shades of green.  Thus the background colors are far more drab, leading to more plain colored fish.

So, what is it in lakes that discourages the same coral growth that one finds in the sea?  Perhaps not high enough levels of free calcium in the water.

[cheryl j (OP)]

Where we are it's all limestone. There are some shells (mussels, little snails, zebra mussels) but not as many as in the ocean.

[alancalverd]

There's a balance to be made between camouflage from predators and recognition for shoaling and mating. I suspect the latter is more important among ocean reef fish. Not sure about poisonous lake fish - I can't think of any, but some reef fish advertise their unpalatability with bright stripes, and others may mimic them, like hoverflies mimicking wasps.

It's also worth considering "dazzle" camouflage  http://en.wikipedia.org/wiki/File:USS_West_Mahomet_(ID-3681)_cropped.jpg doesn't look much like a warship to the human eye, and it's possible that a striped or spotted fish may not look much like dinner to a shark in a hurry.

[cheryl j (OP)]

Is there a relationship between the size of aquatic mammals and the size of the body of water they depend on? We have otters and beavers.

[CliffordK]

There may be somewhat of a relationship. 

Certainly you don't have any blue whales living in the great lakes. 

Thinking of just Aquatic mammals, one's list might include:

Fully Aquatic:

• Whales,
• Dolphins
• River Dolphins
• Manatees (some fresh water, some ocean going, and some near the coast).
• Walruses & Seals (mostly in the sea).
  They will hunt salmon migrations in some rivers

Semi Aquatic:

• Polar Bears
• Hippos
• Otters
• Beavers
• Platypus

Also note, some animals such as black bears and grizzly bears will hunt fish.

There are probably quite a few more that should be added to the list.  The whales, of course, are generally sea animals, although they will occasionally venture into rivers. 

The Amazon river, of course, is smaller than the Great Lakes, but does support several fully aquatic mammals.

Beavers like smaller streams. 

Anyway, certainly the size of a water body would influence the size of animals living off of it to some extent.  However, another determining factor may be the water temperature.

[Lmnre]

So, what is it in lakes that discourages the same coral growth that one finds in the sea?  Perhaps not high enough levels of free calcium in the water.

Does calcium readily leach out of soil? Yes, it would, wouldn't it, because it's an alkaline ion. So it accumulates in the oceans where it's readily used to make coral.

[evan_au]

as colorful like those ocean fish in National Geographic

I'm sure that the fish in National Geographic are not a random sample!