Aquatic Systems

Six Factors of Importance:

In any aquatic system there are 6 quantitative factors which help us clue into to the organisms that will flourish there...

Since in one class we can't cover all the different oceanic systems, we will concentrate just on the coral reefs

Coral Reefs...

The reefs support about a million species of animals, including 4,000 fish species and 800 hard corals species. They are crucial to maintaining fishing stocks, as they serve as breeding grounds for fish and larvae that drift to nearby fisheries, scientists said.

Note: the following page is not of my making: this comes from the excellent site : http://www.sprl.umich.edu/GCL/paper_to_html/coral.html#THREATS

I recopied it to make sure you can get to it.....

1. Introduction - Some basic facts and definitions

A coral reef is the largest and most spectacular structure made by living things. The individual building blocks are tiny, and depend upon a partnership between a coral polyp and a photosynthetic "alga" (actually a dinoflagellate - a solitary, plantlike flagellate - the order Dinoflagellata includes luminescent forms, forms important in marine food chains, and forms causing red tide) of the kingdom Protista (a group of unicellular or acellular organisms comprising bacteria, protozoans, various algae and fungi, and sometimes viruses). The relationship is an example of endosymbiosis (symbiosis - the intimate living together of two dissimilar organisms in a mutually beneficial relationship; endo - within) (see Figure 1).

 

Coral reefs are home to spectacular biological diversity, but the corals themselves are not exceptionally diverse. Only one type of coral (hermatypic) builds reefs. About 500 such species are found in the Indo-Pacific, and fewer are in the Atlantic, so in total there are not very many species.

 

 

 

 

 

 

The areal extent of coral reefs is small compared to the entire ocean, but large compared to shallow water areas: Cover reefs cover 600 thousand km2, which is about 0.17% of the ocean surface and 15% of shallow (0-30 m depth) sea areas (see Figure 2).

 

2. The Making of a Coral Reef

From the film "City of Coral," shown in lecture, you should take note of the following:

1. A coral is a colony of many individual polyps.

2. Each polyp is a coelenterate (a phylum of basically radially symmetrical invertebrate animals including the corals, sea anemones, jellyfishes, and hydroids) and contains dinoflagellates capable of photosynthesis (zooxanthellae). A polyp looks like a tiny sea anemone with tentacles and stinging cells to capture animal prey.

3. By night, a polyp captures plankton with its tentacles. By day, the zooxanthellae photosynthesize. The polyp benefits from the photosynthate (product of photosynthesis), and the alga benefits from the nitrogenous wastes of the polyp.

4. Without the zooxanthellae, the polyps cannot grow fast enough to build reefs.

5. Coral can be either soft or hard (calcareous skeleton). Hard corals build reefs, creating rock out of sunlight, sea water, and minute animal prey (8 tons mile-2 day-1)

 

6. More than 65 species have been found in one reef: there are both shallow, fast-growing forms and deep, massive, slow-growing forms (living at a maximum depth of about 100 feet). The Great Barrier Reef has 350 named coral species.

7. There is intense competition for space. Corals have a rigid pecking order. When more aggressive species recognize less aggressive forms, they send out nasty filaments that wound living coral encroaching on their space (see Figure 4).


 

3. Types of coral reefs.

Reefs are constructions of calcium carbonate, made literally from sunlight and sea water, by hard corals. Some algae, called coralline algae, also secrete calcium carbonate. This is especially important in "gluing" loose sediments and reinforcing coral against wave damage (see Figure 3 above). Some encrusting invertebrates, such as sponges, complete the reef structure.

Reef-building corals require water temperatures of at least 20 degrees C, and ample light. This means clear water and shallow depths. Coral reefs are found on continental shelves, around islands, or on top of seamounts.

* Fringing reefs are the simplest and most common kind of reef. They develop near the shore throughout the tropics. Some hard substrate is necessary for the polyp to establish, but after that the corals can make their own hard bottom. Note in Figure 5 the different aspects, including the inner reef flat and outer reef slope.

* Barrier reefs are similar to fringe reefs, but they lie farther from shore and contain a relatively deep lagoon. Some reefs are as far as 100 km from shore, and 2,000 km in length. Figure 6 shows the structure.

* Atolls are rings of reef, with steep outer slopes, that surround a central lagoon (see Figure 7). They are found far from land, and mainly in the west Pacific. How they are formed was a major puzzle. Charles Darwin, famous for the theory of evolution, solved this puzzle by postulating that atolls formed around subsiding islands. If the upward growth of the coral keeps pace with the downward subsidence of the original volcanic island, an atoll is formed.

4. Reef Productivity

Coral reefs are like tropical rain forests in at least two ways. First, they are the most species-rich ecosystems of the sea. Second, they are oases of abundant life in generally nutrient-poor water. The symbiotic relationship between polyp and zooxanthellae makes for especially efficient nutrient cycling, and this contributes to the high productivity. Fish and other consumers that graze on algae and the corals themselves also excrete nutrients back into the water. In addition, blue-green algae and other specialized bacteria are capable of nitrogen-fixation, meaning they can convert nitrogen gas into NH3, which then is then available for plant growth.

5. Species Interactions

A great deal of ecological study is concerned with how species live together and interact with one another. Because species interactions are so apparent within coral reef communities, it is useful to explore some of these ideas here.

Symbiosis simply means "living together," and refers to any close and intimate association of two species. Subdivisions of symbiosis include: mutualism, commensalism, and parasitism. In mutualisms , both partners benefit (+/+). Coral reefs probably have more mutualistic relationships than any other system. In addition to the polyp-dinoflagellate symbiosis, another famous example is the clown fish-sea anemone. Mutualisms range from "casual" to "obligate." Neither of these examples is obligate, although the coral comes close.

Commensalism is +/0, meaning one individual benefits and the other is neither helped nor harmed. A shark and a remora are good examples. Parasitism is a +/-, where one partner wins and another loses. There are many examples of small crustaceans adapted to live on the gills of fish, and obtain their nourishment from the blood that is close to the gill surface.

The video showed some very impressive time lapse photos of competition between corals. In the rich and crowded world of coral reefs, space is at a premium. All sessile organisms compete for space; in addition, corals and seaweeds compete for access to sunlight (see Figure 8). You might expect that corals might compete with one another for light in the same, mundane way that most terrestrial plants do, using rapid growth to overshadow competitors and steal the sunlight. This they do, but some also engage in aggressive interactions using modified tentacles loaded with stinging cells, as you saw in the video. Large, slow-growing corals tend to be the most aggressive. Fast-growing, upright branching forms are less aggressive, while massive, slow-growing forms must compete aggressively to hold their position for decades to centuries.

Seaweeds and filamentous algae are capable of even faster growth, and have the potential to smother coral reefs. Hungry grazers, and some degree of nutrient limitation, generally prevent this from happening.

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What's happening to our Reefs?

THE WORLDWATCH REPORT: CORAL DEATH -- DISASTER IN THE MAKING

Coral reefs are perhaps the greatest collective enterprise in nature. Reefs are the massed calcareous skeletons of millions of coral -- small, sedentary, worm-like animals that live on the reef surface, filtering the water for edible debris. Reefs form in shallow tropical and subtropical waters, and host huge numbers of plants and animals. The reef biome is small in terms of area -- less than one percent of the earth's surface -- but it's the richest type of ecosystem in the oceans and the second richest on earth, after tropical forests.

One-quarter of all ocean species thus far identified are reef-dwellers, including at least 65 percent of marine fish species. Coral is extremely vulnerable to heat stress and the unusually high sea surface temperatures (SST) of the past two decades may have damaged this biome. Much of the ocean warming is related to El Nino.

El Ninos appear to be growing more frequent and more intense; many climate scientists suspect that this trend is connected with climate change. It's very difficult to sort out the patterns, but there is probably also a general SST warming trend in the background, behind the El Ninos. That too is a likely manifestation of climate change.

When SSTs reach the 28- to 30-degree Celsius range, the coral polyp may expel the algae that live within its tissues. This action is known as "bleaching" because it turns the coral white. Coral usually recovers from a brief bout of bleaching, but if the syndrome persists it is generally fatal because the coral depends on the algae to help feed it through photosynthesis. Published records of bleaching date back to 1870, but show nothing comparable to what began in the early 1980s, when unusually warm water caused extensive bleaching throughout the Pacific. Coral bleached over thousands of square kilometers. By the end of the decade, mass bleaching was occurring in every coral reef region in the world. The full spectrum of coral species was affected in these events -- a phenomenon that had never been observed before.

In the second half of this decade, SSTs set new records over much of the coral's range and the bleaching has become even more intense.

Last year saw the most extensive bleaching to date. Over a vast tract of the Indian Ocean, from the African coast to southern India, 70 percent of the coral appears to have died. Some authorities think that a shift from episodic events to chronic levels of bleaching is now under way. The bleaching has triggered outbreaks of the crown-of-thorns starfish, a coral predator that is chewing its way through reefs in the Red Sea, off South Africa, the Maldives, Indonesia Australia and throughout much of the Pacific. The starfishare normally kept at bay by antler-like "branching corals," which have stinging cells and host various aggressive crustaceans. But as the branching corals bleach and die, the more palatable "massive corals" growing among them become ever more vulnerable to starfish attack. Over the course of a year, a single adult crown-of-thorns can consume 13 square meters of coral.

Overfishing is also promoting these outbreaks, by removing the fish that eat starfish. Overfishing also helps another enemy of the reefs: various types of algae that compete with coral. Floating algae can starve corals for light; macro-algae -- "seaweeds" -- can colonize the reefs themselves and displace the coral directly.

Because reefs are shallow-water communities, they generally occur in coastal zones, where they are likely to be exposed to nitrogen-rich agricultural runoff and sewage. Nitrogen pollution is as toxic to reefs as it is to temperate-zone forests, because nitrogen fertilizes algae. Remove the algae-eating fish under these conditions, and you might as well have poisoned the coral directly.

In the Caribbean, overfishing seems to have played a role in yet another complication for the reefs: the population collapse of an algae-eating sea urchin, Diadema antillarum. This urchin appears to have been the last line of defense against the algae after the progressive elimination of other algae-eating creatures. The first to go may have been the green sea turtle. Now endangered, the turtle once apparently roamed the Caribbean in immense herds, like bison on the Great Plains.

The removal of its competitors must have given the urchin a great deal of room, and for most of this century it was one of the reefs' most common denizens. But its abundance seems to have set it up for the epidemic that struck during the El Nino of the early 1980s. In roughly a year, a mysterious pathogen virtually eliminated D. antillarum from the Caribbean; some 98 percent of the species disappeared over an area of more than 2.5 million square kilometers.

 Contemporary history offers no precedent for a die-off of that magnitude in a marine animal. The urchin is reportedly back in evidence, at least in some areas of its former range, but until its relationship with the pathogen is better understood, it won't be possible to define its long term appetite for algae.

 With the algae, the pollution, and the warming waters, the Caribbean is becoming an increasingly hostile environment for the organism that has shaped so much of its biological character. And now the coral itself is sickening; the Caribbean has become a caldron of epidemic coral diseases. The first such epidemic, called black-band disease, was detected in 1973 in Belizean waters. Black band is caused by a three-layer complex of "blue-green algae" (actually, cyanobacteria), each layer consisting of a different species. The bottom layer secretes highly toxic sulfides which kill the coral.

 The complex creeps very slowly over a head of coral in a narrow band, leaving behind only the bare white skeleton. Black band has since been joined by a whole menagerie of other diseases: white-band, yellow-band, red-band, patchy necrosis, white pox, white plague type I and II, rapid-wasting syndrome, dark spot. For most of these diseases, a pathogen has yet to be identified; it's not even clear whether each of those names really refers to a distinct syndrome. But it's not likely that the diseases are "new" in the sense of being caused by pathogens that have recently evolved. It's much more likely that the coral's vulnerability to them is new.

 Take, for example, the disease that's killing sea-fan coral around the Caribbean. In this case, the pathogen is known: it's Aspergillus sydowii, a member of a very common genus of terrestrial fungi. The last time you threw something out of your refrigerator because it was moldy -- there's a good chance you were looking at an Aspergillus species. In a very bizarre form of invasion, A. sydowii breached the land-sea barrier, and found a second home in the ocean. But it evidently took the plunge decades ago and has only been killing sea-fansfor some 15 years or so. Why? Part of the answer is probably the higher SSTs: A. sydowii likes warmer water. Other coral diseases appear to do especially well in nutrient-laden waters.

 Disease lies at one end of the spectrum of threat. Pathogens create a kind of microscopic pressure, but there are macroscopic pressures too: the ecosystems allied in one way or another with the reef biome are also deteriorating. The stretch of shallow, protected water between a reef and the coast often nurtures beds of seagrass. These beds filter out sediment and effluent that would injure the reefs, and the seagrass provides crucial cover for young fish. Seagrass is the major nursery for many fish species that spend their adult lives out on the reefs. Perhaps 70 percent of all commercially important fish spend at least part of their lives in the seagrass. But the tropical seagrass beds are silting up under tons of sediment from development, logging, mining and the construction of shrimp farms.

 They are suffocating under algal blooms in nitrogen-polluted waters; they are being poisoned by herbicide runoff. If you follow the seagrass-choking sediment back the way it came, you're increasingly likely to find a shoreline denuded of mangroves.

 In the warmer regions of the world, mangroves knit the land and sea together. These stilt-rooted trees trap sediment that would otherwise leak out to sea and they stabilize coastlines against incoming storms. Like the seagrass beds and the reefs, the mangrove ecosystem is incredibly productive -- in the mangroves' case, with both terrestrial and aquatic organisms. (Mangrove roots are important fish nurseries too.)

 The mangroves' importance as a sediment filter is perhaps greatest in the center of reef diversity, the Indonesian archipelago and adjoining areas. About 450 coral species are known to grow in the Australasian region; the Caribbean, by comparison, contains just 67 species. Australasia is correspondingly rich in fish too: a quarter of the world's fish species inhabit these waters. It is estimated that half of all the sediments received by oceanic waters are washed from the Indonesian archipelago alone.

 Nearby areas of Southeast Asia are also major contributors of sediment. But throughout the region, logging and shrimp farming are obliterating the mangroves that once filtered this tremendous burden of silt. Southeast Asia has lost half its mangrove stands over the past half century. A third of the mangrove cover is gone from Indonesian coasts, three-quarters from the Philippines.

 About 10 percent of the world's coral reefs may already have been degraded beyond recovery. If we can't find a way to ease the reefs' afflictions, nearly three-quarters of the ocean's richest biome may have disappeared 50 years from now. Such a prospect gives new meaning to the term "natural disaster," but it's also a social disaster in the making. Reef fish make up perhaps 10 percent of the global fish catch; one estimate puts their contribution to the catch of developing countries at 20 to 25 percent.

 And there's much more at stake here than just fisheries.The death of the coral would also jeopardize the reef structures-- leaving them unable to repair storm damage. If the reefs give way, wave erosion of the coasts behind them will increase. The coasts are already facing some unavoidable degree of damage from climate change, as sea levels rise. (Warming water expands; that physical effect will combine with runoff from melting glaciers to push sea levels up.) Rising seas, like the crumbling reefs, will allow storm surges to reach farther inland.

About one-sixth of the world's coasts are shielded by reefs, and some of these coasts, like the ones in South and Southeast Asia, support some of the densest human populations in the world. The disintegration of the reefs would leave a large portion of humanity hungrier, poorer, and far more vulnerable to the vagaries of a changing climate. (Chris Bright is senior editor of World Watch, the magazine of Worldwatch Institute.),,,,,,,,,Source: Worldwatch Institute

The United States has 10,540 square miles of coral reefs, about 85 percent of which are in the Pacific Ocean near Hawaii, American Samoa and the Mariana Islands.