Chapter 8 Aquatic Biodiversity
1.
2.
What Is the General Nature of Aquatic Systems?
a.
Most of the Earth Is Covered with Water
i. Saltwater covers about 71% of the earth’s surface.
ii. Freshwater occupies roughly another 2.2%.
iii. The aquatic equivalents of biomes are called aquatic life zones.
1. The distribution of many aquatic organisms is determined largely by the water’s salinity.
2. Aquatic life zones are classified into two major types:
a.
Saltwater or marine (oceans and their accompanying estuaries, coastal wetlands, shorelines, coral reefs, and
mangrove forests).
b. Freshwater (lakes, rivers, streams, and inland wetlands).
b. Most Aquatic Species Live in Top, Middle, or Bottom Layers of Water
i. Saltwater and freshwater life zones contain several major types of organisms.
1. Weakly, swimming, free-floating plankton.
a.
Phytoplankton (drifting plants) includes many types of algae. They and various rooted plants near shorelines
are primary producers that support most aquatic food webs.
b. Zooplankton (drifting animals) consist of primary consumers (herbivores) that feed on phytoplankton and
secondary consumers that feed on other zooplankton. They range from single-celled protozoa to large
invertebrates such as jellyfish.
c.
Ultraplankton are extremely small photosynthetic bacteria that may be responsible for 70% of the primary
productivity near the ocean surface.
2. Nekton are strongly swimming consumers such as fish, turtles, and whales.
3. Benthos consists of bottom dwellers such as oysters, which anchor themselves to one spot; clams and worms, which
burrow into the sand or mud; and lobsters and crabs, which walk about on the sea floor.
4. Decomposers (mostly bacteria) break down organic compounds in the dead bodies and wastes of aquatic organisms into
nutrients that can be used by aquatic primary producers.
ii. Most forms of aquatic life are found in the surface, middle, and bottom layers of saltwater and freshwater systems.
iii. In most aquatic systems, the key factors determining the types and numbers of organisms found in these layers are temperature,
dissolved oxygen content, availability of food, and availability of light and nutrients required for photosynthesis.
iv. In deep aquatic systems, photosynthesis is largely confined to the upper layer (the euphotic or photic zone) through which sunlight
can penetrate.
v. The depth of the euphotic zone in oceans and deep lakes can be reduced when the water is clouded (turbidity) by excessive algal
growth (algal blooms) resulting from nutrient overloads.
vi. In shallow systems such as small open streams, lake edges, and ocean shorelines, ample supplies of nutrients for primary producers
are usually available.
vii. By contrast, in most areas of the open ocean, nutrients are often in short supply, and this limits net primary productivity (NPP).
Why Are Marine Aquatic Systems Important?
a.
Oceans Provide Important Ecological and Economic Resources
i. Ecological Services
1. Climate moderation
2. CO2 absorption
3. Nutrient cycling
4. Waste treatment
5. Reduced storm impact (mangroves, barrier islands, coastal wetlands)
6. Habitats and nursery areas
7. Genetic resources and biodiversity
8. Scientific information
ii. Economic Services
1. Food
2. Animal and pet food
3. Pharmaceuticals
4. Harbors and transportation routes
5. Coastal habitats for humans
6. Recreation
7. Employment
8. Oil and natural gas
9. Minerals
10. Building materials
iii. Marine life is found in three major life zones:
1. The coastal zone is the warm, nutrient –rich, shallow water that extends from the high-tide mark on land to the gently
sloping, shallow edge of the continental shelf.
a.
It makes up less than 10% of the world’s oceans area but contains 90% of all marine species and is the site of
most large commercial marine fisheries.
b.
3.
Most coastal zone aquatic systems (estuaries, coastal wetlands, mangrove forests, and coral reefs) have a high
NPP per unit of area. This is the result of the zone’s ample supplies of sunlight and plant nutrients that flow
from land and are distributed by wind and ocean currents.
b. Estuaries and Coastal Wetlands Are Highly Productive
i. Estuaries are where rivers meet the sea. They are partially enclosed bodies of water where seawater mixes with freshwater as well
as nutrients and pollutants from streams, rivers, and runoff from the land.
ii. Estuaries and their associated coastal wetlands (coastal land areas covered with water all or part of the year) include river mouths,
inlets, bays, sounds, and salt marshes in temperate zones, and mangrove forests in tropical zones.
iii. They are some of the earth’s most productive ecosystems because of high nutrient inputs from rivers and nearby land, rapid
circulation of nutrients by tidal flows, and ample sunlight penetrating the shallow waters.
iv. Seagrass beds are another component of coastal marine biodiversity.
v. Life must adapt to significant daily and seasonal changes in tidal and river flows, water temperatures and salinity, and runoff of
eroded soil sediment and other pollutants from the land.
vi. Mangrove forests have extensive root systems that often extend above the water, were they can obtain oxygen and support the
trees during periods of changing water levels.
1. Help to maintain water quality in tropical coastal zones by filtering toxic pollutants, excess plant nutrients, and sediments,
and by absorbing other pollutants.
2. Provide food, habitats, and nursery sites for a variety of aquatic and terrestrial species.
3. Reduce storm damage and coastal erosion by absorbing waves and storing excess water produced by storms and
tsunamis.
c.
Rocky and Sandy Shores Host Different Types of Organisms
i. The area of shoreline between low and high tides is called the intertidal zone.
ii. Organisms living in this zone must be able to avoid being swept away or crushed by waves, and must deal with being immersed
during high tides and left high and dry (and much hotter) at low tides. They must also survive changing levels of salinity when heavy
rains dilute saltwater. To deal with such stresses, most intertidal organisms hold on to something, dig in, or hide in protective shells.
iii. Steep rocky shores are pounded by waves. The numerous pools and other habitats in their intertidal zones contain a great variety of
species that occupy different niches in response to daily and seasonal changes in environmental conditions such as temperature,
water flows, and salinity.
iv. Barrier beaches, or sandy shores are gently sloping. Most organisms keep hidden from view and survive by burrowing, digging, and
tunneling in the sand. These sandy beaches and their adjoining coastal wetlands are also home to a variety of shorebirds that feed
in specialized niches on crustaceans, insects, and other organisms.
v. Barrier islands are low, narrow, sandy islands that form offshore, parallel to some coastline. They have one or more rows of natural
sand dunes in which the sand is held in place by plant roots. These dunes are the first line of defense against the ravages of the sea.
d. Coral Reefs Are Amazing Centers of Biodiversity
i. Among the world’s oldest, most diverse, and productive ecosystems.
e. The Open Sea and Ocean Floor Host a Variety of Species
i. The sharp increase in water depth at the edge of the continental shelf separates the coastal zone from the vast volume of the ocean
called the open sea.
ii. Primarily on the basis of the penetration of sunlight, it is divided into three vertical zones.
1. The euphotic zone is the brightly lit upper zone.
a.
Drifting phytoplankton carry out about 40% of the world’s photosynthetic activity.
b. Nutrient levels are low (except around upwellings) and levels of dissolved oxygen are high.
c.
Large, fast-swimming predatory fishes such as swordfish, sharks, and Bluefin tuna populate this zone.
2. The bathyal zone is the dimly lit middle zone.
a.
Does not contain photosynthesizing producers.
b. Zooplankton and smaller fishes, many of which migrate to feed on the surface at night, populate this zone.
3. The deepest zone, called the abyssal zone, is dark and very cold.
a.
Little dissolved oxygen.
b. Teeming with life because it contains enough nutrients to support a large number of species.
c.
Most organisms of the deep waters and ocean floor get their food from showers of dead and decaying
organisms (marine snow) drifting down from upper lighted levels of the ocean.
i. Deposit feeders (worms) take mud into their guts and extract nutrients from it.
ii. Filter feeders (oysters, clams, and sponges) pass water through or over their bodies and extract
nutrients from it.
iii. Average primary productivity and NPP per unit of area are quite low in the open sea. However, because open sea covers so much of
the earth’s surface, it makes the largest contribution to the earth’s overall NPP.
How Have Human Activities Affected Marine Ecosystems?
a.
Human Activities Are Disrupting and Degrading Marine Systems
i. Coastal development, which destroys and pollutes coastal habitats.
ii. Overfishing which depletes populations of commercial fish species.
iii. Runoff of nonpoint source pollution such as fertilizers, pesticides, and livestock wastes from the land.
iv. Point source pollution such as sewage from passenger cruise ships and spills from oil tankers.
v. Habitat destruction from coastal development and trawler fishing boats that drag weighted nets across the ocean bottom.
vi. Invasive species, introduced by humans, that can deplete populations of native aquatic species and cause economic damage.
vii.
4.
Climate change, enhanced by human activities, that could cause a rise in sea levels, which could destroy coral reefs and flood coastal
marshes and coastal cities.
viii. Climate change from burning fossil fuels, which is also threatening marine ecosystems by warming the oceans and making them
more acidic.
ix. Pollution and degradation of coastal wetlands and estuaries.
Why Are Freshwater Ecosystems Important?
a.
Water Stands in Some Freshwater Systems and Flows in Others
i. Freshwater life zones include standing (lentic) bodies of freshwater, such as lakes, ponds, and inland wetlands, and flowing (lotic)
systems, such as streams and rivers.
ii. Ecological Services
1. Climate moderation
2. Nutrient cycling
3. Waste treatment
4. Flood control
5. Groundwater recharge
6. Habitats for many species
7. Genetic resources and biodiversity
8. Scientific information
iii. Economic Services
1. Food
2. Drinking water
3. Irrigation water
4. Hydroelectricity
5. Transportation corridors
6. Recreation
7. Employment
iv. Lakes are large natural bodies of standing freshwater formed when precipitation, runoff, or groundwater seepage fills depressions in
the earth’s surface caused by glaciations, crustal displacement, and volcanic activity.
v. Lakes are supplied with water from rainfall, melting snow, and streams that drain their surrounding watershed.
vi. Deep lakes normally consist of four distinct zones that are defined by their depth and distance from shore.
1. The top layer, called the littoral zone, is near the shore and consists of the shallow sunlit waters to the depth at which
rooted plants stop growing. It has a high biological diversity because of ample sunlight and inputs of nutrients from the
surrounding land. Species living in the littoral zone include many rooted plants and animals such as turtles, frogs, crayfish,
and many fishes such as bass, perch, and carp.
2. The limnetic zone is the open sunlit surface layer away from the shore that extends to the depth penetrated by sunlight.
The main photosynthetic body of the lake, this zone produces the food and oxygen that support most of the lake’s
consumers. Its most abundant organisms are microscopic phytoplankton and zooplankton. Some large fishes spend
most of their time in this zone, with occasional visits to the littoral zone to feed and reproduce.
3. The profundal zone is the deep, open water where it is too dark for photosynthesis to occur. Without sunlight and plants,
oxygen levels are often low here.
4. The bottom of the lake contains the benthic zone inhabited mostly by decomposers, detritus feeders, and some fishes.
b. Some Lakes Have More Nutrients Than Others
i. Lakes that have a small supply of plant nutrients are called oligotrophic (poorly nourished) lakes.
ii. A lake with a large supply of nutrients needed by producers is called a eutrophic (well-nourished) lake.
iii. Human inputs of nutrients from the atmosphere and from nearby urban and agricultural areas can accelerate the eutrophication of
lakes, a process called cultural eutrophication.
1. This process often puts excessive nutrients into lakes, which are then described as hypereutrophic.
iv. Many lakes fall somewhere between the two extremes of nutrient enrichment and are called mesotrophic lakes.
c.
Freshwater Streams and Rivers Carry Water from the Mountains to the Oceans
i. Precipitation that does not sink into the ground or evaporates becomes surface water.
ii. It becomes runoff when it flows into streams.
iii. A watershed, or drainage basin, is the land area that delivers runoff, sediment, and dissolved substances to a stream.
iv. The downward flow of surface water and groundwater from mountain highlands to the sea typically takes place in three aquatic life
zones characterized by different environmental conditions:
1. In the first, narrow source zone, headwaters, or mountain highland streams are usually shallow, cold, clear, and swiftly
flowing. As this turbulent water flows and tumbles downward over waterfalls and rapids, it dissolves large amounts of
oxygen from the air. Most of these streams are not very productive because of a lack of nutrients and primary producers.
Their nutrients come primarily from organic matter that falls into the stream from nearby land.
2. In the transition zone, headwater streams merge to form wider, deeper, and warmer streams that flow down gentler
slopes with fewer obstacles.
3. In these floodplain zones, streams join into wider and deeper rivers that flow across broad, flat valleys.
v. Coastal deltas and wetlands and inland wetland and floodplains absorb and slow the velocity of floodwaters from coastal storms,
hurricanes, and tsunamis.
d. Freshwater Inland Wetlands Are Vital Sponges
i.
5.
Inland wetlands are lands covered with freshwater all or part of the time and located away from coastal areas. They include
marshes, swamps, prairie potholes, floodplains, and arctic tundra. Seasonal wetlands remain under water for only a short time each
year.
ii. Ecological and Economic Services
1. Filtering and degrading toxic wastes and pollutants.
2. Reducing flooding and erosion by absorbing storm water and releasing it slowly and by absorbing overflows from streams
and lakes.
3. Helping to replenish stream flows during dry periods.
4. Helping to recharge groundwater aquifers.
5. Helping to maintain biodiversity by providing habitats for a variety of species.
6. Supplying valuable products such as fishes and shellfish, blueberries, cranberries, wild rice, and timber.
7. Providing recreation for birdwatchers, nature photographers, boaters, anglers, and waterfowl hunters.
How Have Human Activities Affected Freshwater Ecosystems?
a.
Human Activities Are Disrupting and Degrading Freshwater Systems
i. Dams and canals fragment about 40% of the world’s 237 large rivers. They alter and destroy terrestrial and aquatic wildlife habitats
along rivers and in coastal deltas and estuaries by reducing water flow and increasing damage from coastal storms.
ii. Flood control levees and dikes built along rivers disconnect the rivers from their floodplains, destroy aquatic habitats, and alter or
reduce the functions of nearby wetlands.
iii. Cities and farms add pollutants and excess plant nutrients to nearby streams, rivers, and lakes.
iv. Many inland wetlands have been drained or filled to grow crops or have been covered with concrete, asphalt, and buildings.