01/09/08 

Tour of Ocean Habitats - Open Ocean and Coral Reefs

  

We'll start our tour with the open ocean.  Here's a picture of it:

 

 

Not much to look at, even if you toss in some icebergs:

Now, both those pictures are from the North Atlantic, but you get the overall idea.  The surface waters of an ocean aren't much to look at, and there is a good reason for this.  All life in the open ocean depends on the photosynthesis carried out by algae and cyanobacteria.  These phytoplankton, in turn, depend on the presence of nutrients such as nitrogen and phosphorous in the surface waters.  They take the nutrients out of the water and use them to build up their own bodies.  However, when they die, these same phytoplankton - and the zooplankton that consume them - sink to the bottom, which is far below where light levels capable of sustaining photosynthesis reach.  They carry their nutr4ients with them, and the nutrients stay there, locked away from potential use in photosynthesis on the surface.  

Thus, surface waters in the oceans are often almost devoid of life for want of these nutrients.  For this reason, oceans are sometimes called biological deserts - not because they lack water, obviously, but because life itself is so sparse.

Some areas of the oceans are productive, however.  These areas are areas where there is a ample source of nutrients.  Shallow waters, where the nutrients on the bottom can be stirred up by storms, often are very productive.  These areas are often found on the continental shelves, which have the added advantage of being close to shore.  Rivers draining continental areas also bring in a lot of nutrients (often too many, see below), and this contributes to production in the nearshore ocean environment.  Also, areas where upwellings occur are productive since the water coming up from the ocean floor in these areas is rich in nutrients.  Upwellings occur where strong persistent winds drive water away from a coast; the water is replaced by water from deeper areas.  One of the best known upwellings occur off the western coast of Peru (as opposed to the eastern coast?).  The rich ocean waters here support a sardine fishery, except in the years when El Nińo event cuts off the upwelling. 

Despite the lack of nutrients, the open oceans do support a lot of life including the largest animals on the planet, whales.  Many of these large organisms are quite nomadic; ranging over the oceans in order to take advantage of local abundance of food.

Relatives of corals, anemones and jellyfish, these beautiful ctenophores (above) are creatures of the open oceans.  Gliding through the water on currents produced by "comb plates" of iridescent cilia, they capture small prey.  Below, a jellyfish. Photos taken at Sea World, Orlando.

 

Coral Reefs:

 

http://www.nos.noaa.gov/education/kits/corals/media/supp_coral05a.html

For more on corals:  http://www.nos.noaa.gov/education/kits/corals/welcome.html

Let's turn our attention now to coral reefs.  Coral reefs form in tropical shallow waters where there is plenty of sunshine and the water is clear. There are several reasons for this.  First, warm water is necessary because the coral animals depend on a particular chemical reaction to remove calcium carbonate from the water to convert it into the hard coral rock that serves as the skeleton of the coral polyps, and this reaction proceeds best in warm waters.  Second, the coral animals harbor algal endosymbionts (zooxanthellae).  This means that tiny algae actually live within the coral animal and provide it with sugars that the algae produce through photosynthesis (the animals also capture tiny prey).  Obviously, in order to photosynthesize, the zooxanthellae need a lot of sunlight, and as we learned about what happens to light in water, this is only available in the shallows.  Finally, if the water is not clear, sunlight will be blocked and debris settling on the coral polyps will prevent them from capturing prey.

As you can see from the map to the left, most coral reefs are found in the tropics and their environs - roughly from 30 degrees S latitude to 30 degrees N latitude.  We often distinguish between Caribbean and Indo-Pacific reefs as the fish and invertebrate fauna differ greatly between these regions, with the Indo-Pacific reefs generally considered to be the more diverse. We will look at reefs in the Caribbean off the coasts of Florida and Jamaica.

Fringing reefs form when corals grow directly seaward from rocks extending onto land.  Barrier reefs form some distance offshore, with a lagoon separating the shoreline from the reef.  A special type of barrier reef is called an atoll; an atoll forms when the island that the reef originally surrounded sinks back into the ocean (such islands are often volcanic in nature; the reefs don't submerge because they keep growing towards the surface.  Pictured below is a barrier reef in Jamaica.  The area between the beach and the reef (marked here by the surf well offshore) is the lagoon:

The lagoon is largely covered by vegetation on its sandy bottom, however in places storms or boats have removed the vegetation and a bare patch of sand can be recognized.  In this particular lagoon, you could walk almost all the way to the reef in places as the water was not that deep.

Inhabitants of the Jamaican Lagoon

(Clockwise from upper left)

Turtle Grass is perhaps the dominant vegetation in the lagoon, covering most of the bottom and playing host to a variety of animals and even epiphytic algae.  Turtle Grass is replaced in areas with freshwater intrusion (the mouth of a stream, for instance) by Manatee Grass, above.  The Sea Hare (Aplysia sp.) (upper right) is a gastropod - a mollusc which in this case lacks a shell.  It is a herbovore which feeds on and among the manatee grass and which can expel a purple ink to distract predators.  It's giant nerve axons have made it a favorite research model for neurobiologists; much of what we know about how nerves work comes from experiments done on these animals.  The male Blue-headed Wrasse (center left) can be found here in the lagoon or out on the reef; the lagoon is an important feeding ground/nursery for the juveniles of many reef fish.  Sea Urchins (center right) may try to camoflauge themselves with pices of manatee grass held in place by the tube feet of the urchin.  Urchins are herbivores.  The Sun Anenome (bottom left) is unusual in that rather than stining it sticks to its prey.  The tnetacles are sticky to the touch, even underwater.  The specimens here were attached to a pice of coral thrown back from the reef by a storm. 

The Coral Reef Proper:

Moving past the lagoon we come to the reef, which, as you saw, may actually break the surface when there are waves.  Many of the reefs in Jamaica and in the Florida Keys are of a Spur-and-Groove form, where the barrier reef parallel to shore puts off intermittent ridges which grow away from shore.  These ridges, or spurs, alternate with grooves, where the sandy bottom lies in view 30 to 60 feet (10-20 meters) below:

In the image above you can see two grooves and two spurs; starting at the left is the sandy bottom of one groove, then a coral spur, then a narrow groove, and on the right side of the picture an even taller groove.  This pattern of growth stabilizes the reef; the grooves allow water that has been pushed up onto the reef by waves to drain back to the depths.

Corals have two basic growth strategies.  They can grow "Up and Out" or they can lie flat.  Both forms have advantages and disadvantages.

This Elkhorn Coral is growing "up and out".  By doing so, it moves its zooxanthellae closer to the sun, it shades out rival corals, and the current-washed branches shed debris that would otherwise smother the coral polyps.

These two sea urchins are grazing on low-growing corals.  By lying flat, these corals may be shaded out by competitors like the Elkhorn coral, but they won't end up like this:

 

Brain Coral:  One of my favorite types of coral - perhaps because it is so easy to recognize - is brain coral.  I have see this coral ranging from young colonies about the size of a fist to monsters over 12 feet tall that have been growing for about 10,000 years!  Remember that the only living part of the coral is the thin outer layer where the coral polyps live.  The rock underneath is the skeletons of millions of coral polyps of preceding generations; an individual polyp may be only a few millimeters across.

In the photos above and to the left you can see Christmas Tree Worms (as obviously named as the brain coral itself).  These worms are over a foot long (0.3m); only the respiratory organs protrude from the burrow the worm excavates in the coral.  These organs obtain oxygen and filter out food; they have eyes and can be withdrawn into the tube in a heartbeat if a predator should appear.  Because the worms' burrows penetrate into the non-living rocky core of the coral, they don't do as much damage as you might expect. In the photos below you can see a young brain coral just starting out, as well as an old geezer.  The barracuda is about 5 feet (1.5m) long, just for comparison.

Going diving with Clive and Nally in Jamaica.

Sea Turtles are often found near coral reefs.

 

Proceed to Coral Reefs Proceed to Mangrove Swamps
Proceed to Coral Reef Fish Proceed to Sandy Shores
Proceed to Coral Reef Invertebrates Proceed to Rocky Shores

More on Coral Reefs from Cahuita, Costa Rica!