Distribution of Organisms II Aquatic Biomes 1 1
The Distribution of Organisms desert Some of the major biomes on our planet. freshwater river coral reef tropical forest temperate broadleaf forest 2 As you learned in the previous lesson, the distribution of organisms is affected by both biotic and abiotic factors. When the distribution of organisms is viewed in a global context, it is possible to divide the biosphere into a relatively small number of consistent ecological associations, or biomes. In general, biomes are classified as one of several major types of aquatic or terrestrial ecological associations. Aquatic biomes include all of the aquatic regions of our planet, such as the various regions of the oceans, and freshwater lakes and rivers. Terrestrial biomes include all of the surface land areas of our planet, such as the deserts, tundra, grasslands, and forests. In this lesson, we ll investigate the major aquatic biomes. In the following lesson, you will be introduced to several of the major terrestrial biomes. 2
The Major Aquatic Biomes 3 As you can see, the aquatic regions on our planet are roughly divided into several major biomes, including freshwater lakes, rivers, estuaries, intertidal zones, coral reefs and abyssal zones. Of these biomes, by far the most extensive is the oceanic pelagic zone, or the open sea, which covers nearly 70% of the earth s surface. In contrast, some other biomes, in particular the coral reefs, occupy a much smaller portion of the earth s surface but hold a relatively much higher level of organismal diversity. Freshwater and marine biomes are generally distinguished from each other by their salt concentrations. Freshwater biomes typically have salt concentrations of less than one 1%, while marine biomes average around 3% salt concentration. Estuaries are found where fresh- and saltwater biomes meet, for example where a river flows into the ocean. 3
Stratification of Aquatic Biomes Stratification is common in aquatic habitats, particularly with regard to light and temperature 4 Aquatic biomes are usually stratified with respect to light availability due to the lightabsorbing properties of water. Photic zones, in which light penetrates sufficiently to support photosynthesis, exist in the upper levels of bodies of water. Aphotic zones, on the other hand, in which light is largely excluded, exist below the photic zones. For example, the oceanic photic zone extends from the surface down to about 200 meters. Within this zone, one finds much of the life which is normally associated with the oceans, including most fish and marine mammal species, most marine algae and plankton, and ecosystems such as coral reefs and kelp beds. Below 200 meters in the oceans, life is fairly sparse, and is often concentrated around thermal and/or chemical energy sources such as deep sea vents or dead, sunken organisms. Also as a result of light absorption in the upper layers of bodies of water, surface waters tend to be much warmer than deeper waters. Thus, most bodies of water are also stratified by temperature in addition to light availability. In many cases, a relatively sharp gradient exists that separates warmer surface waters from colder, deeper waters. This gradient is referred to as a thermocline. Now, let s take a more detailed look at some of the world s aquatic biomes, starting with some of the marine biomes. 4
Intertidal Zones Zone between high and low tide found globally 5 Intertidal zones are defined as ocean shoreline between high and low tide. Intertidal zones are typically submerged and exposed by fluctuating ocean waters twice daily. So, in a sense, this biome is both terrestrial and aquatic, depending on the time of day. The organisms inhabiting an intertidal zone face several challenges, including the daily changes in water availability, and the associated fluctuations in temperature and sunlight, as well as the pounding wave action characteristic of ocean shorelines. Substrate, which can vary from rock to sand to mud, also plays a significant role in determining which types of organisms can inhabit a particular intertidal zone. In general, it is common to find more diverse plant and animal communities in rocky intertidal zones, as compared to sandy or muddy intertidal zones. This is largely due to the fact that the rocky substrate is more secure and less apt to change, allowing various plant and animal species to become securely established. It is common to find various species of marine algae, such as sea grass and rockweed, various marine animals, such as crabs, sea anemones, mollusks, starfish, and some fish, as well as associated plankton in intertidal zones. 5
Coral Reefs Found in warm, shallow tropical waters biologically diverse 6 Coral reefs are often referred to as the tropical rain forests of the ocean, due to the astounding diversity of life associated with them. Corals form the foundation of this biome. In addition to coral, reefs are inhabited by a tremendous diversity of life forms. These life forms include, for instance, various algae and plankton, including symbiotic photosynthetic algae living within the cells of the coral, marine invertebrates, such as sea anemones, shrimp, and lobsters, and marine vertebrates, such as fish and eels. Coral reefs are strongly affected by light and oxygen availability, as well as temperature. Because of this, coral reefs are found only in certain shallow waters between 30 o north latitude and 30 o south latitude. In addition, the sensitivity of coral reef systems to their immediate environment makes these systems very delicate, and prone to serious disturbance by pollution, global warming, and mechanical disruption. 6
The Abyssal Zone Deep ocean floor many species concentrated around hydrothermal vents http://www.photolib.no aa.g ov/nurp /nur04506.htm http://www.photolib.no aa.g ov/nurp /nur04509.htm 7 The abyssal zone consists of the deepest regions of the ocean floor, well below the photic zone. As a result of the lack of light penetration and the depth of these regions, organisms living in the abyssal zone are adapted to life without light, at fairly cold temperatures (around 3 o C), and extremely high pressures. In addition, organisms largely depend on marine snow, the slow and consistent fall of organic material from surface waters, for carbon and energy. In certain areas of the abyssal ocean, super-heated, chemical-rich solutions are released from volcanic vents, providing energy to support small but diverse ecosystems. These fairly recently discovered deep-sea hydrothermal vents support some of the most interesting assemblages and interactions of life found on our planet. In general, the deep ocean floor is not particularly overflowing with life. In a sense, it is not totally different from terrestrial deserts large stretches of fairly lifeless surface, with the occasional oasis, such as the deep sea vents, supporting a higher concentration of organisms. A diversity of life, including bacteria that utilize chemical compounds for energy, tube-worms, marine arthropods, and various fish, are associated with the deep sea vents. Photosynthetic organisms, of course, are conspicuously absent from the abyssal zone communities, as there is no light to support them. 7
Temperate Freshwater Lakes Seasonal turnover affects temperature and oxygen distribution Crater Lake, an oligotrophic lake. 8 While aquatic biomes cover around 70% of the earth s surface, only about 3% of this area is found as freshwater. However, a great portion of life s diversity has evolved to utilize this relatively small percentage of available water. Freshwater biomes are typically classified as lakes, wetlands, and rivers and streams. Let s first take a look at freshwater lakes and some of their characteristics. As with the oceans, freshwater lakes are generally stratified with respect to sunlight availability and temperature. In temperate areas, most lakes experience a seasonal event called turnover. During turnover, a combination of temperature change and seasonal winds in the fall and spring cause the water of the lake to circulate, leading to mixing of the surface and deeper waters. As a result, temperature and oxygen availability become fairly uniform throughout the lake during these times of year. Seasonal turnover has significant impacts on the behavior and distribution patterns of organisms within freshwater lakes. Lakes and the organisms that live in them are also greatly affected by chemical factors, such as nutrient availability, ph, salinity, as well as oxygen availability. Lake may be classified according to their production of organic matter. Lakes that tend to be rich in nutrients and organic matter, and generally deficient in oxygen are referred to as eutrophic. On the other hand, deep, clear lakes that are nutrient-poor and oxygen-rich are referred to as oligotrophic. In any case, it is common to find a diversity of life associated both directly and indirectly with freshwater lakes, ranging from plankton, to diverse plant and algal species, to numerous terrestrial and aquatic invertebrate and vertebrate animals. 8
Wetlands Shallow, seasonally inundated with water important filtering systems 9 As opposed to lakes, wetlands are often inundated with water seasonally, rather than year-round, and are typically not as deep on average as are lakes. However, wetlands are still able to support aquatic plant life during part or all of the year. Included in the definition of wetlands are swamps, marshes, and bogs, which extend the range of wetlands to every continent except Antarctica. Wetlands also serve important ecological roles, such as acting as natural filtering systems for nutrients and even pollutants, and, of course, providing habitat for various types of organisms. As with lakes, diverse life is found directly or indirectly associated with wetlands, including plankton, plants, fungi, and invertebrate and vertebrate animals. 9
Rivers and Streams Characterized by continuous current More even distribution of temperature and oxygen 10 Finally, rivers and streams represent another important freshwater biome. Found on all continents and at all elevations, rivers and streams support many different types of life, but provide some different challenges and opportunities than do lakes and wetlands. Primarily, rivers and streams differ from lakes and wetlands because they have current. Current, in turn, results in regular changes in the physical structure of the underwater environment, and leads to more even distribution of oxygen, temperature, and nutrients. Rivers and streams are also quite variable in terms of their physical characteristics. Some, such as many high mountain streams, are fast-moving, with cold, clear water, rocky or sandy substrates, high oxygen levels and low nutrient content. Others, such as many lowland rivers, are sluggish, turbid, muddy-bottomed waters with an abundance of organic material and relatively low oxygen levels. 10
The Distribution of Organisms 11 Now that you ve gained a little familiarity with the earth s major aquatic biomes, in the next lesson you will learn about some of the major terrestrial biomes which characterize the land surfaces of our planet. 11