STRUCTURE OF THE MARINE ENVIRONMENT
As mentioned previously, the vertical component of the marine environment is a critical factor in the distribution of marine organisms, as is proximity to the landmasses. Two basic biogeographical realms of the oceans can be recognized – the neritic realm, which consists of shallow seas associated with continental shelves, and the pelagic, or open ocean, realm. fte ocean floor has much different ecological communities than those existing up in the water column. Organisms associated with the ocean floor are referred to as benthic, whereas those that live up in the water column are known as pelagic. Pelagic organisms can be nektonic (active swimmers) or planktonic (drifting organisms). In general, planktonic organisms are tiny or even microscopic, whereas nektonic organisms mostly include vertebrates and large invertebrates.
fte ocean floor varies tremendously in depth. fte continental shelf region, which is basically a submerged extension of the continents, slopes down to about 200 m in depth. At the shelf break, the rate of descent increases dramatically along the continental slope until reaching the abyssal plain, which ranges from about 4,000 to 6,000 m depth. But there are regions of the ocean floor that are even deeper. fte deep ocean trenches are subduction zones where old ocean floor disappears into the Earth. ftese trenches can be over 10,000 m in depth (Fig. 6.6). fte deepest of these, the Mariana Trench of the western Pacific Ocean, reaches depths of about 11,000 m.
Figure 6.6: Layers of the pelagic realm. (Wikipedia ‘Pelagic zone;’ Attribution: DieBuche, Finlay McWalter, & TomCatX)
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Environmental conditions vary depending on depth and proximity to landmasses, and abundance and diversity of benthic and pelagic organisms vary with these conditions as well. Light penetrates the ocean waters to only a limited extent. In the neritic realm, the waters are well-lit, and high photosynthetic rates occur here. Oxygen is generally abundant, and, except for the intertidal zone, conditions are relatively stable. fte photosynthetic organisms, as well as nutrients originating from the continents, provide abundant sources of nutrients and energy for the marine life there. Compared to the pelagic realm, the neritic realm has greater abundance and diversity of life per unit volume. fte irregular shapes of the continents can form isolated communities along the coastlines, and neritic communities of different landmasses are also isolated from each other, resulting in greater chances of genetic isolation and speciation among neritic communities than in the pelagic realm. For example, the neritic faunas of islands often exhibit a high degree of endemism, as does the Red Sea, a semi-isolated inlet of the Indian Ocean which is connected to the Ocean via the narrow Bab el Mandeb strait and the Gulf of Aden. Because of the high evaporation rate and little input of fresh water, the Red Sea is highly saline, providing a unique environment for it biota. fte Red Sea fauna exhibits a high degree of endemism – about 17% of Red Sea fish species and 50% of cephalopod species are found nowhere else.
Within the pelagic realm (Fig. 6.6), the ocean layer in which enough light is available for photosynthesis is known as the epipelagic zone. Because this is the zone in which photosynthesis can take place, it contains the highest concentration of marine life in the pelagic realm. Photosynthetic phytoplankton are abundant in this zone, as well as the zooplankton that feed on them. ftese tiny organisms serve as the base of food chains that include larger invertebrates such as jellyfish as well as a diversity of fish species. fte depth of this zone varies, but generally reaches around 200 m. Below this is a zone of twilight conditions, where little light penetrates – the mesopelagic zone (Fig. 6.6). Within the mesopelagic zone, there is insufficient light for photosynthesis to occur. ftis zone extends down to roughly 1,000 m depth, and is associated with a region of rapid water density change called the pycnocline. Within the pycnocline, water temperatures decrease dramatically with depth. Since cold water is denser than warm water, this rapid decrease in water temperature results in a change in water density from lower density warm waters to higher density cold waters. fte pycnocline acts as a barrier, inhibiting vertical water circulation and affecting the vertical movement and distribution of chemicals and marine animals. Animals that frequent the mesopelagic zone include detritivores that feed on dead organisms and wastes descending from the epipelagic zone, and predators that in turn feed on these detritivores. ftese mesopelagic animals include various species of squid and cuttlefish, as well as fishes such as lancetfish, daggertooths, and lanternfish. Mesopelagic fish often have large eyes and binocular vision, particularly visual predators that must detect prey in the low light conditions. Many mesopelagic fish undertake vertical migrations, moving up to the epipelagic zone at night to feed on zooplankton and then back to the mesopelagic zone, where there is more safety from predators, during the day.
Below the mesopelagic zone lies a realm of almost total darkness. fte bathypelagic zone (Fig. 6.6) extends down to about 6,000 m. No photosynthesis takes place here, and, per unit volume, there is much less abundance and diversity of organisms than is found in the epipelagic zone. Bathypelagic food chains are supported by the relatively small amounts of detritus, or “marine snow,” that make it down to this zone. Waters are cold; temperatures average about 4°C, with little variation. fte characteristics of species inhabiting this zone reflect the selective pressures applied by these conditions. Metabolic rates and activity levels of animals in the bathypelagic zone are generally low, an adaptation for energy efficiency in an environment where food is not very abundant, and temperatures and dissolved oxygen levels are low. fte skeleton (if present) and muscle tissues of bathypelagic animals are generally weak with high water content, which allows these animals to survive in the crushing water pressures at these depths. Fishes that inhabit the mesopelagic zone depend primarily on detection of sound and water pressure changes to sense their environment, and chemical detection can be important in locating mates or prey. Most animals in this zone have poorly developed vision, or no eyes at all. However, some can detect light, and some predators take advantage of this by employing bioluminescence to attract prey. Some anglerfish (Order Lophiiformes) have a bioluminescent lure that serves this function (Fig. 6.7). When the prey approaches close to the lure, the anglerfish eats it whole with its huge, tooth-filled jaws. Despite their ferocious appearance, most anglerfish are only a few centimeters long, with the male usually much smaller than the female. In some species, such as the humpback anglerfish, Melanocetus johnsonii (Fig. 6.7), mating occurs via the tiny male (barely 3 cm long) attaching to the body wall of the much larger female (about 18 cm long). fte male becomes a symbiont of the female, deriving nutrition from her and basically becoming a sperm provider. In general, there are relatively few fishes inhabiting the bathypelagic zone. ftis zone is dominated by various species of whales and invertebrates such as cephalopods, brachiopods, sponges, and echinoderms.
Figure 6.7: Humpback anglerfish, Melanocetus johnsonii. (Wikipedia ‘Humpback anglerfish;’ Attribution: August Brauer, 1906)
Believe it or not, there are even more extreme ocean depths. fte hadopelagic zone (Fig. 6.6) is found in the deep ocean trenches, at depths down to 11,000 m. Conditions at these depths are generally similar to those of the bathypelagic zone, only more extreme in terms of pressure and low nutrient levels, which again are dependent on the relatively little “marine snow” that filters down from above. At these depths, animal life is poorly known, and invertebrates such as annelid tube worms, jellyfish, and sea anemones dominate. A high proportion of species in the hadopelagic zone are endemics that are confined to isolated locations along the trenches.
At the other end of the “ocean floor conveyor belt” are the mid-ocean ridges. ftese ridges are underwater mountain systems with a valley or rift along the spine that functions as a spreading center where new seafloor is being produced. Mid-ocean ridges are volcanically active, and at some locations along the rift, fissures in the rock allow cold water to penetrate. When the water contacts the upwelling magma, water temperatures may exceed 400°C. ftese locations where geothermally heated water exits are known as hydrothermal vents (Fig. 6.8). Previously we pointed out that photosynthesis doesn’t occur below the epipelagic zone because of lack of sunlight. But these hydrothermal vents are home to unique ecological communities that are far more diverse than could be supported by the sparse marine rain they receive. How do these communities persist without a base of photosynthesizing organisms? fte dissolved minerals contained in the water support chemosynthetic bacteria. ftese amazing bacteria are the primary producers of the vent community. Rather than using light to produce energy, these organisms use chemicals, such as hydrogen sulfide produced by the hydrothermal vents, to produce energy, and are fed upon by invertebrates such as copepods and amphipods. In turn, these tiny invertebrates support food chains of larger animals, resulting in vibrant ocean floor communities that include tubeworms, snails, crabs, a variety of other invertebrates, and various fish species.
Figure 6.8: Diagram of hydrothermal vent biogeochemical cycle. (Wikipedia ‘Hydrothermal vent;’ Attribution: U.S. National Oceanic and Atmospheric Administration)
fte biological diversity of hydrothermal vent communities is impressive, but pales in comparison to that of the “rainforests of the oceans,” the coral reefs (Fig. 6.9). As the name would suggest, coral reefs are formed by reef-building stony corals, tiny, colonial members of the Phylum Cnidaria, which also includes jellyfish and sea anemones. ftese corals secrete a calcium carbonate skeleton that forms a hard outer shell covering the colony. It is this skeleton, along with other organisms such as coralline algae and shelled molluscs, that provides the reef structure. Coral reefs are widespread, but are most abundant in shallow, clear tropical waters. fte greatest concentration of coral reefs is in the Australian/Indonesian region, with the Indian Ocean and the Caribbean Sea also containing substantial reefs (Fig. 6.10). Coral reefs comprise less than 0.1% of the total ocean surface, but contain over 25% of marine species.
Figure 6.9: A blue starfish (Linckia laevigata) resting on hard acropora coral. Lighthouse, Ribbon Reefs, Great Barrier Reef. (Wikipedia ‘Great Barrier Reef;’ Attribution: Copyright
(c) 2004 Richard Ling; www.rling.com)
Figure 6.10: Locations of coral reefs. (Wikipedia ‘Coral reef;’ Attribution: U.S. National Aeronautics and Space Administration)
Paradoxically, coral reefs thrive in nutrient-poor waters. Corals can absorb some nutrients directly from the water, and they also catch zooplankton with their tentacles. But endosymbiotic dinoflagellates, mostly in the genus Symbiodinium, are a major source of energy for the corals. ftese single-celled, photosynthetic protists reside in the tissues of tropical corals, and provide a significant amount of energy to the corals. fte corals, in return, provide their endosymbiotic mutualists with nutrients, CO2, and an advantageous location for receiving sunshine. ftese endosymbionts are sensitive to environmental conditions, and changes in water chemistry, temperature, salinity, and other factors can trigger the loss of these zooxanthellae, a process
known as “coral bleaching.” fte corals can sometimes be recolonized by zooxanthellae if conditions improve, but long-term or repeated bleaching can result in death of the corals. Because coral reefs provide so much of our marine biodiversity, coral bleaching is a serious problem associated with pollution, other anthropogenic changes in water quality, and climate change.
In the last couple of chapters, we have focused primarily on biogeographic patterns from an ecological perspective. In the next chapter, we will consider longer term historical factors that have played important roles in shaping the distribution of Earth’s biodiversity.