Final Paper Outline
CHAPTER 24
Smith, T. M., & Smith, R. L. (2015). Elements of Ecology (9th ed.). Boston, MA: Pearson.
24.1 Lakes Have Many Origins
Lakes and ponds are inland depressions containing standing water (Figure 24.2). They vary in depth from 1 meter (m) to more than 2000 m and they range in size from small ponds of less than a hectare (ha) to large lakes covering thousands of square kilometers. Ponds are small bodies of water so shallow that rooted plants can grow over much of the bottom. Some lakes are so large that they mimic marine environments. Most ponds and lakes have outlet streams, and both may be more or less temporary features on the landscape, geologically speaking (see Section 18.8, Figure 18.23).
Some lakes have formed through glacial erosion and deposition. Abrading slopes in high mountain valleys, glaciers carved basins that filled with water from rain and melting snow to form tarns. Retreating valley glaciers left behind crescent-shaped ridges of rock debris, called moraines, which dammed up water behind them. Many shallow kettle lakes and potholes were left behind by the glaciers that covered much of northern North America and northern Eurasia.
Lakes also form when silt, driftwood, and other debris deposited in beds of slow-moving streams dam up water behind them. Loops of streams that meander over flat valleys and floodplains often become cut off by sediments, forming crescent-shaped oxbow lakes.
Shifts in Earth’s crust, uplifting mountains or displacing rock strata, sometimes develop water-filled depressions. Craters of some extinct volcanoes have also become lakes. Landslides may block streams and valleys to form new lakes and ponds.
Many lakes and ponds form through nongeological activity. Beavers dam streams to make shallow but often extensive ponds (see Figure 19.5a). Humans create huge lakes by damming rivers and streams for power, irrigation, or water storage and construct smaller ponds and marshes for recreation, fishing, and wildlife. Quarries and surface mines can also form ponds.
24.2 Lakes Have Well-Defined Physical Characteristics
All lentic ecosystems share certain characteristics. Life in still-water ecosystems depends on light. The amount of light penetrating the water is influenced by natural attenuation, by silt and other material carried into the lake, and by the growth of phytoplankton (see Chapter 4,Quantifying Ecology 4.1 and Figure 20.8). Temperatures vary seasonally and with depth (Figure 3.9). Oxygen can be limiting, especially in summer, because only a small proportion of the water is in direct contact with air, and respiration by decomposers on the bottom consumes large quantities (see Figure 3.11). Thus, variation in oxygen, temperature, and light strongly influences the distribution and adaptations of life in lakes and ponds (see Chapter 3 for more detailed discussion).
Ponds and lakes may be divided into both vertical and horizontal strata based on penetration of light and photosynthetic activity (Figure 24.3). The horizontal zones are obvious to the eye; the vertical ones, influenced by depth of light penetration, are not. Surrounding most lakes and ponds and engulfing some ponds completely is the littoral zone, or shallow-water zone, in which light reaches the bottom, stimulating the growth of rooted plants. Beyond the littoral is open water, the limnetic zone, which extends to the depth of light penetration. Inhabiting this zone are microscopic phytoplankton (autotrophs) and zooplankton (heterotrophs) as well as nekton, free-swimming organisms such as fish. Beyond the depth of effective light penetration is the profundal zone. Its beginning is marked by the compensation depth of light, the point at which respiration balances photosynthesis (see Figure 20.8). The profundal zone depends on a rain of organic material from the limnetic zone for energy. Common to both the littoral and profundal zones is the third vertical stratum—the benthic zone, or bottom region, which is the primary place of decomposition. Although these zones are named and often described separately, all are closely dependent on one another in the dynamics of lake ecosystems.
24.3 The Nature of Life Varies in the Different Zones
Aquatic life is richest and most abundant in the shallow water about the edges of lakes and ponds as well as in other places where sediments have accumulated on the bottom and decreased the water depth (Figure 24.4). Dominating these areas is emergent vegetation such as cattails (Typha spp.) and sedges (Cyperaceae), that is, plants whose roots are anchored in the bottom mud, lower stems are immersed in water, and upper stems and leaves stand above water. Beyond the emergents and occupying even deeper water is a zone of floating plants such as pondweed (Potamogeton) and pond lily (Nuphar spp.). In depths too great for floating plants live submerged plants, such as species of pondweed with their finely dissected or ribbonlike leaves.
Associated with the emergents and floating plants is a rich community of organisms, among them hydras, snails, protozoans, dragonflies and diving insects, pickerel (Esox spp.), sunfish (Lepomis spp.), herons (Ardeidae), and blackbirds (Agelaius spp. and Xanthocephalus xanthocephalus). Many species of pond fish have compressed bodies, permitting them to move easily through the masses of aquatic plants. The littoral zone contributes heavily to the large input of organic matter into the system.
The main forms of life in the limnetic zone are phytoplankton and zooplankton (Figure 24.5). Phytoplankton, including desmids, diatoms, and filamentous algae, are the primary producers in open-water ecosystems and form the base on which the rest of life in open water depends. Also suspended in the water column are small grazing animals, mostly tiny crustaceans that feed on the phytoplankton. These animals form an important link in energy flow in the limnetic zone.
During the spring and fall turnovers, plankton are carried downward, but at the same time nutrients released by decomposition on the bottom are carried upward to the impoverished surface layers (see Section 21.10 and Figure 21.24). In spring when surface waters warm and stratification develops, phytoplankton have access to both nutrients and light. A spring bloom develops, followed by rapid depletion of nutrients and a reduction in planktonic populations, especially in shallow water.
Fish make up most of the nekton in the limnetic zone. Their distribution is influenced primarily by food supply, oxygen, and temperature. During the summer, large predatory fish such as largemouth bass (Micropterus salmoides), pike (Esox lucius), and muskellunge (Esox masquinongy), inhabit the warmer epilimnion waters, where food is abundant. In winter, these species retreat to deeper water. In contrast, lake trout (Salvelinus namaycush) require colder water temperatures and move to greater depths as summer advances. During the spring and fall turnover, when oxygen and temperature are fairly uniform throughout, both warm-water and cold-water species occupy all levels.
Life in the profundal zone depends not only on the supply of energy and nutrients from the limnetic zone above but also on the temperature and availability of oxygen. In highly productive waters, oxygen may be limiting because the decomposer organisms so deplete it that little aerobic life can survive. Only during spring and fall turnovers, when organisms from the upper layers enter this zone, is life abundant in profundal waters.
Easily decomposed substances drifting down through the profundal zone are partly mineralized while sinking. The remaining organic debris—dead bodies of plants and animals of the open water, and decomposing plant matter from shallow-water areas—settles on the bottom. Together with quantities of material washed in, these substances make up the bottom sediments, the habitat of benthic organisms.
The bottom ooze is a region of great biological activity—so great, in fact, that the oxygen curves for lakes and ponds show a sharp drop in the profundal water just above the bottom. Because the organic muck is so low in oxygen, the dominant organisms there are anaerobic bacteria. Under anaerobic conditions, however, decomposition cannot proceed to create inorganic end products. When the amounts of organic matter reaching the bottom are greater than can be used by bottom fauna, they form a muck that is rich in hydrogen sulfide and methane.
As the water becomes shallower, the benthos changes. The action of water, plant growth, and recent organic deposits modifies the bottom material, typically consisting of stones, rubble, gravel, marl, and clay. Increased oxygen, light, and food encourage a richness of life not found on the bottom of the profundal zone.
Closely associated with the benthic community are organisms collectively called periphyton or aufwuchs. They are attached to or move on a submerged substrate but do not penetrate it. Small aufwuchs communities colonize the leaves of submerged aquatic plants, sticks, rocks, and debris. Periphyton—mostly algae and diatoms living on plants—are fast growing and lightly attached. Aufwuchs on stones, wood, and debris form a more crustlike growth of cyanobacteria, diatoms, water moss, and sponges.
24.4 The Character of a Lake Reflects Its Surrounding Landscape
Because of the close relationship between land and water ecosystems, lakes reflect the character of the landscape in which they occur. Water that falls on land flows over the surface or moves through the soil to enter springs, streams, and lakes. The water transports with it silt and nutrients in solution. Human activities including road construction, logging, mining, construction, and agriculture add another heavy load of silt and nutrients—especially nitrogen, phosphorus, and organic matter. These inputs enrich aquatic systems by a process called eutrophication. The term eutrophy (from the Greek eutrophos, “well nourished”) means a condition of being rich in nutrients.
A typical eutrophic lake (Figure 24.6a) has a high surface-to-volume ratio; that is, the surface area is large relative to depth. Nutrient-rich deciduous forest and farmland often surround it. An abundance of nutrients, especially nitrogen and phosphorus, flowing into the lake stimulates a heavy growth of algae and other aquatic plants. Increased photosynthetic production leads to increased recycling of nutrients and organic compounds, stimulating even further growth.
Phytoplankton concentrate in the warm upper layer of the water, giving it a murky green cast. Algae, inflowing organic debris and sediment, and remains of rooted plants drift to the bottom, where bacteria feed on this dead organic matter. Their activities deplete the oxygen supply of the bottom sediments and deep water until this region of the lake cannot support aerobic life. The number of bottom species declines, although the biomass and numbers of organisms remain high. In extreme cases, oxygen depletion (anoxic conditions) can result in the die-off of invertebrate and fish populations (see this chapter, 24.12).
In contrast to eutrophic lakes and ponds are oligotrophic bodies of water (Figure 24.6b). Oligotrophy is the condition of being poor in nutrients. Oligotrophic lakes have a low surface-to-volume ratio. The water is clear and appears blue to blue-green in the sunlight. The nutrient content of the water is low; nitrogen may be abundant, but phosphorus is highly limited. A low input of nutrients from surrounding terrestrial ecosystems and other external sources is mostly responsible for this condition. Low availability of nutrients causes low production of organic matter that leaves little for decomposers, so oxygen concentration remains high in the hypolimnion. The bottom sediments are largely inorganic. Although the numbers of organisms in oligotrophic lakes and ponds may be low, species diversity is often high.
Lakes that receive large amounts of organic matter from surrounding land, particularly in the form of humic materials that stain the water brown, are called dystrophic (from dystrophos, “ill-nourished”). These bodies of water occur generally on peaty substrates, or in contact with peaty substrates in bogs or heathlands that are usually highly acidic (see Section 25.6). Dystrophic lakes generally have highly productive littoral zones. This littoral vegetation dominates the lake’s metabolism, providing a source of both dissolved and particulate organic matter.
24.5 Flowing-Water Ecosystems Vary in Structure and Types of Habitats
Even the largest rivers begin somewhere back in the hinterlands as springs or seepage areas that become headwater streams, or they arise as outlets of ponds or lakes. A few rivers emerge fully formed from glaciers. As a stream drains away from its source, it flows in a direction and manner dictated by the lay of the land and the underlying rock formations. Joining the new stream are other small streams, spring seeps, and surface water.
Just below its source, the stream may be small, straight, and swift, with waterfalls and rapids. Farther downstream, where the gradient is less steep, velocity decreases and the stream begins to meander, depositing its load of sediment as silt, sand, or mud. At flood time, a stream drops its load of sediment on surrounding level land, over which floodwaters spread to form floodplain deposits.
Where a stream flows into a lake or a river into the sea, the velocity of water is suddenly checked. The river then is forced to deposit its load of sediment in a fan-shaped area about its mouth to form a delta (Figure 24.7). Here, its course is carved into several channels, which are blocked or opened with subsequent deposits. As a result, the delta becomes an area of small lakes, swamps, and marshy islands. Material the river fails to deposit in the delta is carried out to open water and deposited on the bottom.
Because streams become larger on their course to rivers and are joined along the way by many others, we can classify them according to order (Figure 24.8). A small headwater stream with no tributaries is a first-order stream. Where two streams of the same order join, the stream becomes one of higher order. If two first-order streams unite, the resulting stream becomes a second-order one; and when two second-order streams unite, the stream becomes a third-order one. The order of a stream increases only when a stream of the same order joins it. It does not increase with the entry of a lower-order stream. In general, headwater streams are orders first to third; medium-sized streams, fourth to sixth; and rivers, greater than sixth.
The velocity of a current molds the character and structure of a stream (see Quantifying Ecology 24.1). The shape and steepness of the stream channel, its width, depth, and roughness of the bottom, and the intensity of rainfall and rapidity of snowmelt all affect velocity. Fast streams are those whose velocity of flow is 50 centimeters (cm)/second (s) or higher. At this velocity, the current removes all particles less than 5 millimeters (mm) in diameter and leaves behind a stony bottom. High water increases the velocity; it moves bottom stones and rubble, scours the streambed, and cuts new banks and channels. As the gradient decreases and the width, depth, and volume of water increase, silt and decaying organic matter accumulate on the bottom. The character of the stream changes from fast water to slow (Figure 24.9), with an associated change in species composition.
Quantifying Ecology 24.1 Streamflow
The ecology of a stream ecosystem is determined largely by its streamflow, which is the water discharge occurring within the natural streambed or channel. The rate at which water flows through the stream channel influences the water temperature, oxygen content, rate of nutrient spiraling, physical structure of the benthic environment, and subsequently the types of organisms inhabiting the stream. As such, streamflow is an important parameter used by ecologists to characterize the stream environment.
Flow is defined simply as the volume of water moving past a given point in the stream per unit time. As such, it can be estimated from the cross-sectional area of the stream channel and the velocity of the flow as follows:
For example, in the simple representation of a stream channel depicted in Figure 1, let us assume that the stream depth (d) is 1.2 m and the stream width (w) is 7 m. The cross-sectional area (A) of the stream is then 8.4 m2 (1.2 m × 7 m = 8.4 m2). If the measured velocity (v) is 0.5 m/s, then the streamflow (Q) is 4.2 m3/s (8.4 m2 × 0.5 m/s = 4.2 m3/s). In reality, the profile of the stream channel is never as simple as the rectangular profile presented in Figure 1, and the water velocity varies as a function of depth and position relative to the stream bank. For this reason, multiple measurements of depth and velocity are taken across the stream profile. For example, the stream profile in Figure 3 is 6 m wide; however, the depth varies across the width of the stream channel. A simple approach to estimating flow for this stream would be to sample water depth and velocity at several locations along the width of the stream, using the average values of water depth and velocity to calculate the value of streamflow. In most cases, however, stream ecologists will use a much more elaborate sampling scheme, estimating water depth at regular intervals along the stream profile and water velocity at several depths at each interval.
The cross-sectional area and velocity of a stream varies through time based on the amount of water being discharged from the surrounding watershed. In turn, the amount of water discharged reflects the input of water to the surrounding watershed from precipitation. As a result, an accurate picture of streamflow requires a systematic sampling of the stream morphology (width and depth) and velocity through time.
Flowing-water ecosystems often alternate between two different but related habitats: the turbulent riffle and the quiet pool (Figure 24.10). Processes occurring in the rapids influence the waters of the pool below; in turn, the waters of the rapids are influenced by events in the pools upstream.
Riffles are the sites of primary production in the stream. Here the periphyton or aufwuchs, organisms that are attached to or move on submerged rocks and logs, assume dominance. Periphyton, which occupy a position of the same importance as phytoplankton in lakes and ponds, consist chiefly of diatoms, cyanobacteria, and water moss.
Above and below the riffles are the pools. Here, the environment differs in chemistry, intensity of current, and depth. Just as the riffles are the sites of organic production, so the pools are the sites of decomposition. Here, the velocity of the current slows enough for organic matter to settle. Pools, the major sites of carbon dioxide production during the summer and fall, are necessary for maintaining a constant supply of bicarbonate in solution (see Section 3.7). Without pools, photosynthesis in the riffles would deplete the bicarbonates and result in smaller and smaller quantities of available carbon dioxide downstream.
24.6 Life Is Highly Adapted to Flowing Water
Living in moving water, inhabitants of streams and rivers face the challenge of remaining in place without being swept downstream. Unique adaptations have evolved among these organisms that help them deal with life in the current (Figure 24.11a). A streamlined form, which offers less resistance to water current, is typical of many animals found in fast water. Larval forms of many insect species have extremely flattened bodies and broad, flat limbs that enable them to cling to the undersurfaces of stones where the current is weak. The larvae of certain species of caddisflies (Trichoptera) construct protective cases of sand or small pebbles and cement them to the bottoms of stones. Sticky undersurfaces help snails and planarians cling tightly and move about on stones and rubble in the current. Among the plants, water moss (Fontinalis spp.) and heavily branched, filamentous algae cling to rocks by strong holdfasts. Other algae grow in cushionlike colonies or form sheets—covered with a slippery, gelatinous coating—that lie flat against the surfaces of stones and rocks.
All animal inhabitants of fast-water streams require high, near-saturation concentrations of oxygen and moving water to keep their absorbing and respiratory surfaces in continuous contact with oxygenated water. Otherwise, a closely adhering film of liquid, impoverished of oxygen, forms a cloak about their bodies.
In slow-flowing streams where current is at a minimum, streamlined forms of fish give way to fish species such as smallmouth bass (Micropterus dolomieu), whose compressed bodies enable them to move through beds of aquatic vegetation. Pulmonate snails (Lymnaeacea) and burrowing mayflies (Ephemeroptera) replace rubble-dwelling insect larvae. Bottom-feeding fish, such as catfish (Akysidae), feed on life on the silty bottom, and back-swimmers and water striders inhabit sluggish stretches and still backwaters (Figure 24.11b).
Invertebrate inhabitants are classified into four major groups based on their feeding habits (Figure 24.12). Shredders, such as caddisflies (Trichoptera) and stoneflies (Plecoptera), make up a large group of insect larvae. They feed on coarse particulate organic matter (CPOM) that is > 1 mm in diameter—mostly leaves that fall into the stream. The shredders break down the CPOM, feeding on the material not so much for the energy it contains as for the bacteria and fungi growing on it. Shredders assimilate about 40 percent of the material they ingest and pass off 60 percent as feces.
When broken up by the shredders and partially decomposed by microbes, the leaves, along with invertebrate feces, become part of the fine particulate organic matter (FPOM), that is < 1 mm but > 0.45 micrometers (μm) in diameter. Drifting downstream and settling on the stream bottom, FPOM is picked up by another feeding group of stream invertebrates, the filtering collectors and gathering collectors. The filtering collectors include, among others, the larvae of black flies (Simuliidae) with filtering fans and the net-spinning caddisflies. Gathering collectors, such as the larvae of midges, pick up particles from stream-bottom sediments. Collectors obtain much of their nutrition from bacteria associated with the fine detrital particles.
While shredders and collectors feed on detrital material, another group, the grazers, feeds on the algal coating of stones and rubble. This group includes the beetle larvae, water penny (Psephenus spp.), and a number of mobile caddisfly larvae. Much of the material they scrape loose enters the drift as FPOM. Another group, associated with woody debris, is composed of the gougers, which are invertebrates that burrow into waterlogged limbs and trunks of fallen trees.
Feeding on the detrital feeders and grazers are predaceous insect larvae and fish such as the sculpin (Cottus) and trout (Salmoninae). Even these predators do not depend solely on aquatic insects; they also feed heavily on terrestrial invertebrates that fall or wash into the stream.
Because of the current, quantities of CPOM, FPOM, and invertebrates tend to drift downstream to form a traveling benthos. This drift is a normal process in streams, even in the absence of high water and abnormally fast currents. Drift is so characteristic of streams that a mean rate of drift can serve as an index of a stream’s production rate.
24.7 The Flowing-Water Ecosystem Is a Continuum of Changing Environments
From its headwaters to its mouth, the flowing-water ecosystem is a continuum of changing environmental conditions (Figure 24.13). Headwater streams (orders first to third) are usually swift, cold, and in shaded forested regions. Primary productivity in these streams is typically low, and they depend heavily on the input of detritus from terrestrial streamside vegetation, which contributes more than 90 percent of the organic input. Even when headwater streams are exposed to sunlight and autotrophic production exceeds inputs from adjacent terrestrial ecosystems, organic matter produced invariably enters the detrital food chain. Dominant organisms are shredders, processing large-sized litter and feeding on CPOM, and collectors, processors of FPOM. Populations of grazers are minimal, reflecting the small amount of autotrophic production. Predators are mostly small fish—sculpins, darters, and trout. Headwater streams, then, are accumulators, processors, and transporters of particulate organic matter of terrestrial origin. As a result, the ratio of gross primary production to respiration is less than 1.
As streams increase in width to medium-sized creeks and rivers (orders fourth to sixth), the importance of riparian vegetation and its detrital input decreases. With more surface water exposed to the sun, water temperature increases, and as the elevation gradient declines, the current slows. These changes bring about a shift from dependence on terrestrial input of particulate organic matter to primary production by algae and rooted aquatic plants. Gross primary production now exceeds community respiration. Because of the lack of CPOM, shredders disappear. Collectors, feeding on FPOM transported downstream, and grazers, feeding on autotrophic production, become the dominant consumers. Predators show little increase in biomass but shift from cold-water species to warm-water species, including bottom-feeding fish such as suckers (Catostomidae) and catfish.
As the stream order increases from sixth to tenth and higher, riverine conditions develop. The channel is wider and deeper. The flow volume increases, and the current becomes slower. Sediments accumulate on the bottom. Both riparian and autotrophic production decrease. A basic energy source is FPOM, used by bottom-dwelling collectors that are now the dominant consumers. However, slow, deep-water and dissolved organic matter (DOM), which is < 0.45 μm in diameter, support a minimal phytoplankton and associated zooplankton population.
Throughout the downstream continuum, the community capitalizes on upstream feeding inefficiency. Downstream adjustments in production and the physical environment are reflected in changes in consumer groups.
24.8 Rivers Flow into the Sea, Forming Estuaries
Waters of most streams and rivers eventually drain into the sea. The place where freshwater joins saltwater is called an estuary. Estuaries are semi-enclosed parts of the coastal ocean where seawater is diluted and partially mixed with freshwater coming from the land (Figure 24.14). Here, the one-way flow of freshwater streams and rivers into an estuary meets the inflowing and outflowing saltwater tides. This meeting sets up a complex of currents that vary with the structure of the estuary (size, shape, and volume), season, tidal oscillations, and winds. Mixing waters of different salinities and temperatures create a counterflow that works as a nutrient trap (see Figure 21.26). Inflowing river waters most often impoverish rather than fertilize the estuary, with the possible exception of phosphorous. Instead, nutrients and oxygen are carried into the estuary by the tides. If vertical mixing occurs, these nutrients are not swept back out to sea but circulate up and down among organisms, water, and bottom sediments (see Figure 21.26).
Organisms inhabiting the estuary face two problems: maintaining their position and adjusting to changing salinity. Most estuarine organisms are benthic. They attach to the bottom, bury themselves in the mud, or occupy crevices and crannies. Mobile inhabitants are mostly crustaceans and fish, largely the young of species that spawn offshore in high-salinity water. Planktonic organisms are wholly at the mercy of the currents. Seaward movements of streamflow and ebb tide transport plankton out to sea, and the rate of water movement determines the size of the plankton population.
Salinity dictates the distribution of life in the estuary. The vast majority of the organisms inhabiting an estuary are marine, able to withstand full seawater. Some estuarine inhabitants cannot withstand lowered salinities, and these species decline along a salinity gradient from the ocean to the river’s mouth. Sessile and slightly motile organisms have an optimum salinity range within which they grow best. When salinities exceed this range in either direction, populations decline.
Anadromous fish are those that live most of their lives in saltwater and return to freshwater to spawn. These fish are highly specialized to endure the changes in salinity. Some species of fish, such as the striped bass (Morone saxatilis), spawn near the interface of fresh and low-salinity water. The larvae and young fish move downstream to more saline waters as they mature. Thus, for the striped bass, an estuary serves as both a nursery and as a feeding ground for the young. Anadromous species such as the shad (Alosa spp.) spawn in freshwater, but the young fish spend their first summer in an estuary and then move out to the open sea. Species such as the croaker (Sciaenidae) spawn at the mouth of the estuary, but the larvae are transported upstream to feed in plankton-rich, low-salinity areas.
The oyster bed and oyster reef are the outstanding communities of the estuary (Figure 24.15). The oyster (Ostreidae) is the dominant organism about which life revolves. Oysters may be attached to every hard object in the intertidal zone, or they may form reefs—areas where clusters of living organisms grow cemented to the almost buried shells of past generations. Oyster reefs usually lie at right angles to tidal currents, which bring planktonic food, carry away wastes, and sweep the oysters clean of sediment and debris. Closely associated with oysters are encrusting organisms such as sponges, barnacles, and bryozoans, which attach themselves to oyster shells and depend on the oysters or algae for food.
In shallow estuarine waters, rooted aquatics such as the sea grasses widgeongrass (Ruppia maritima) and eelgrass (Zostera marina) assume major importance (Figure 24.16; also see Figure 18.8). These aquatic plants represent complex systems supporting many epiphytic organisms. Such communities are important to certain vertebrate grazers, such as geese, swans, and sea turtles, and they provide a nursery ground for shrimp and bay scallops.
24.9 Oceans Exhibit Zonation and Stratification
The marine environment is marked by several differences compared to the freshwater world. It is large, occupying 70 percent of Earth’s surface, and it is deep, in places more than 10 km. The surface area lit by the sun is small compared to the total volume of water. This small volume of sunlit water and the dilute solution of nutrients limit primary production. All of the seas are interconnected by currents, influenced by wave actions and tides, and characterized by salinity (see Chapter 3).
Just as lakes exhibit stratification and zonation, so do the seas. The ocean itself has two main divisions: the pelagic, or whole body of water, and the benthic zone, or bottom region. The pelagic is further divided into two provinces: the neritic province, which is water that overlies the continental shelf, and the oceanic province. Because conditions change with depth, the pelagic is divided into several distinct vertical layers or zones (Figure 24.17). From the surface to about 200 m is the epipelagic zone, or photic zone, in which there are sharp gradients in illumination, temperature, and salinity. From 200 to 1000 m is the mesopelagic zone, where little light penetrates and the temperature gradient is more even and gradual, without much seasonal variation. This zone contains an oxygen-minimum layer and often the maximum concentration of nutrients (nitrate and phosphate). Below the mesopelagic is the bathypelagic zone, where darkness is virtually complete, except for bioluminescent organisms, temperature is low, and water pressure is great. The abyssopelagic zone (Greek meaning “no bottom”) extends from about 4000 m to the sea floor. The only zone deeper than this is the hadalpelagic zone, which includes areas found in deep-sea trenches and canyons.
24.10 Pelagic Communities Vary among the Vertical Zones
When viewed from the deck of a ship or from an airplane, the open sea appears to be monotonously the same. Nowhere can you detect as strong a pattern of life or well-defined communities, as you can over land. The reason is that pelagic ecosystems lack the supporting structures and framework provided by large, dominant plant life. The dominant autotrophs are phytoplankton, and their major herbivores are tiny zooplankton.
There is a reason for the smallness of phytoplankton. Surrounded by a chemical medium that contains the nutrients necessary for life in varying quantities, they absorb nutrients directly from the water. The smaller the organism, the greater is the surface-to-volume ratio (see Section 7.1). More surface area is exposed for the absorption of nutrients and solar energy. Seawater is so dense that there is little need for supporting structures (see Section 3.2).
Because they require light, autotrophs are restricted to the upper surface waters where light penetration varies from tens to hundreds of meters (see Section 20.4, Figure 20.8). In shallow coastal waters, the dominant marine autotrophs are attached algae—restricted by light requirements to a maximum depth of about 120 m. The brown algae (Phaeophyceae) are the most abundant, associated with the rocky shoreline. Included in this group are the large kelps—such as species of Macrocystis, which grows to a length of 50 m and forms dense subtidal forests in the tropical and subtropical regions (see Figure 4.1). The red algae (Rhodophyceae) are the most widely distributed of the larger marine plants. They occur most abundantly in the tropical oceans, where some species grow to depths of 120 m.
The dominant autotrophs of the open water are phytoplankton (see Figure 24.5a). Each ocean or region within an ocean appears to have its own dominant forms. Littoral and neritic waters and regions of upwelling are richer in plankton than are the mid-oceans. In regions of downwelling, the dinoflagellates—a large, diverse group characterized by two whiplike flagella—concentrate near the surface in areas of low turbulence. These organisms attain greatest abundance in warmer waters. In summer, they may concentrate in the surface waters in such numbers that they color it red or brown. Often toxic to vertebrates, such concentrations of dinoflagellates are responsible for “red tides.” In regions of upwelling, the dominant forms of phytoplankton are diatoms. Enclosed in a silica case, diatoms are particularly abundant in Arctic waters.
The nanoplankton, which are smaller than diatoms, make up the largest biomass in temperate and tropical waters. Most abundant are the tiny cyanobacteria. The haptophytes—a group of primarily unicellular, photosynthetic algae that includes more than 500 species—are distributed in all waters except the polar seas. The most important members of this group, the coccolithophores, are a major source of primary production in the oceans. Coccolithophores have an armored appearance because of the calcium carbonate platelets, called coccoliths, covering the exterior of the cell (Figure 24.18).
Converting primary production into animal tissue is the task of herbivorous zooplankton, the most important of which are the copepods (see Figure 24.5b). To feed on the minute phytoplankton, most of the grazing herbivores must also be small—between 0.5 and 5 mm. Most grazing herbivores in the ocean are copepods, which are probably the most abundant animals in the world. In the Antarctic, the shrimplike euphausiids, or krill (Figure 24.19), fed on by baleen whales and penguins, are the dominant herbivores. Feeding on the herbivorous zooplankton are the carnivorous zooplankton, which include such organisms as the larval forms of comb jellies (Ctenophora) and arrow worms (Chaetognatha).
However, part of the food chain begins not with the phytoplankton, but with organisms even smaller. Bacteria and protists—both heterotrophic and photosynthetic—make up one-half of the biomass of the sea and are responsible for the largest part of energy flow in pelagic systems. Photosynthetic nanoflagellates (2–20 m) and cyanobacteria (1–2 m), responsible for a large part of photosynthesis in the sea, excrete a substantial fraction of their photosynthate in the form of dissolved organic material that heterotrophic bacteria use. In turn, heterotrophic nanoflagellates consume heterotrophic bacteria. This interaction introduces a feeding loop, the microbial loop (Figure 24.20), and adds several trophic levels to the plankton food chain.
Like phytoplankton, zooplankton live mainly at the mercy of the currents, but many forms of zooplankton have enough swimming power to exercise some control. Some species migrate vertically each day to arrive at a preferred level of light intensity. As darkness falls, zooplankton rise rapidly to the surface to feed on phytoplankton. At dawn, they move back down to preferred depths.
Feeding on zooplankton and passing energy along to higher trophic levels are the nekton, which are swimming organisms that can move at will in the water column. They range in size from small fish to large predatory sharks and whales, seals, and marine birds such as penguins. Some predatory fish, such as tuna, are more or less restricted to the photic zone. Others are found in deeper mesopelagic and bathypelagic zones or move between them, as the sperm whale does. Although the ratio in size of predator to prey falls within limits, some of the largest nekton organisms in the sea—the baleen whales (Mysticeti)—feed on disproportionately small prey, euphausiids, or krill (see Figure 24.19). By contrast, the sperm whale attacks very large prey such as the giant squid.
Residents of the deep have special adaptations for securing food. Darkly pigmented and weak bodied, many deep-sea fish depend on luminescent lures, mimicry of prey, extensible jaws, and expandable abdomens (enabling them to consume large items of food). In the mesopelagic region, bioluminescence reaches its greatest development—two-thirds of the species produce light. Fish have rows of luminous organs along their sides and lighted lures that enable them to bait prey and recognize other individuals of the same species. Bioluminescence is not restricted to fish. Squid and euphausiid shrimp possess searchlight-like structures complete with lens and iris, and some discharge luminous clouds to escape predators.
24.11 Benthos Is a World of Its Own
The term benthic refers to the floor of the sea, and benthos refers to plants and animals that live there. In a world of darkness, no photosynthesis takes place, so the bottom community is strictly heterotrophic (except in vent areas), depending entirely on the rain of organic matter drifting to the bottom. Patches of dead phytoplankton as well as the bodies of dead whales, seals, birds, fish, and invertebrates all provide an array of foods for different feeding groups and species. Despite the darkness and depth, benthic communities support a high diversity of species. In shallow benthic regions, the polychaete worms may be represented by more than 250 species, and the pericarid crustaceans by well more than 100. But the deep-sea benthos supports a surprisingly greater diversity. The number of species collected in more than 500 samples—of which the total surface area sampled was only 50 m2—was 707 species of polychaetes and 426 species of pericarid crustaceans.
Important organisms in the benthic food chain are the bacteria of the sediments. Commonly found where large quantities of organic matter are present, bacteria may reach several tenths of a gram (g) per square meter in the topmost layer of silt. Bacteria synthesize protein from dissolved nutrients and in turn become a source of protein, fat, and oils for other organisms.
In 1977, oceanographers first discovered high-temperature, deep-sea hydrothermal vents along volcanic ridges in the ocean floor of the Pacific near the Galápagos Islands. These vents spew jets of superheated fluids that heat the surrounding water to 8 to 16°C, considerably higher than the 2°C ambient water. Since then, oceanographers have discovered similar vents on other volcanic ridges along fast-spreading centers of the ocean floor, particularly in the mid-Atlantic and eastern Pacific.
Vents form when cold seawater flows down through fissures and cracks in the basaltic lava floor deep into the underlying crust. The waters react chemically with the hot basalt, giving up some minerals but becoming enriched with others such as copper, iron, sulfur, and zinc. The water, heated to a high temperature, reemerges through mineralized chimneys rising up to 13 m above the sea floor. Among the chimneys are white smokers and black smokers (Figure 24.21). White-smoker chimneys rich in zinc sulfides issue a milky fluid with a temperature of less than 300°C. Black smokers, narrower chimneys rich in copper sulfides, issue jets of clear water from 300°C to more than 450°C that are soon blackened by precipitation of fine-grained sulfur–mineral particles.
Associated with these vents is a rich diversity of unique deep-sea life, confined to within a few meters of the vent system. The primary producers are chemosynthetic bacteria that oxidize reduced sulfur compounds such as hydrogen sulfide (H2S) to release energy used to form organic matter from carbon dioxide. Primary consumers include giant clams, mussels, and polychaete worms that filter bacteria from water and graze on the bacterial film on rocks (Figure 24.22).
24.12 Coral Reefs Are Complex Ecosystems Built by Colonies of Coral Animals
Lying in the warm, shallow waters about tropical islands and continental landmasses are coral reefs—colorful, rich oases within the nutrient-poor seas (Figure 24.23). They are a unique accumulation of dead skeletal material built up by carbonate-secreting organisms, mostly living coral (Cnidaria, Anthozoa) but also coralline red algae (Rhodophyta, Corallinaceae), green calcerous algae (Halimeda), foraminifera, and mollusks. Although various types of corals can be found from the water’s surface to depths of 6000 m, reef-building corals are generally found at depths of less than 45 m. Because reef-building corals have a symbiotic relationship with algal cells, their distribution is limited to depths where sufficient solar radiation (photosynthetically active radiation) is available to support photosynthesis (zooxanthellae; see Section 15.10 and Figure 15.9). Precipitation of calcium from the water is necessary to form the coral skeleton. This precipitation occurs when water temperature and salinity are high and carbon dioxide concentrations are low. These requirements limit the distribution of reef-building corals to the shallow, warm tropical waters (20–28°C).
Coral reefs are of three basic types: (1) Fringing reefs grow seaward from the rocky shores of islands and continents. (2) Barrier reefs parallel shorelines of continents and islands and are separated from land by shallow lagoons. (3) Atolls are rings of coral reefs and islands surrounding a lagoon, formed when a volcanic mountain subsides beneath the surface. Such lagoons are about 40 m deep, usually connect to the open sea by breaks in the reef, and may have small islands of patch reefs. Reefs build up to sea level.
Coral reefs are complex ecosystems that begin with the complexity of the corals themselves. Corals are modular animals, anemone-like cylindrical polyps, with prey-capturing tentacles surrounding the opening or mouth. Most corals form sessile colonies supported on the tops of dead ancestors and cease growth when they reach the surface of the water. In the tissues of the gastrodermal layer live zooxanthellae, which are symbiotic, photosynthetically active, endozoic dinoflagellate algae that corals depend on for most efficient growth (see Chapter 15). On the calcareous skeletons live still other kinds of algae—the encrusting red and green coralline species and the filamentous species, including turf algae—and a large bacterial population. Also associated with coral growth are mollusks, such as giant clams (Tridacna, Hippopus), echinoderms, crustaceans, polychaete worms, sponges, and a diverse array of fishes, both herbivorous and predatory.
Because the coralline community acts as a nutrient trap, offshore coral reefs are oases of productivity (1500 to 5000 g C/m2/yr) within the relatively nutrient-poor, lower-productivity sea (15 to 50 g C/m2/yr; see Figure 20.10). This productivity and the varied habitats within the reef support a great diversity of life—thousands of species of invertebrates (such as sea urchins, which feed on coral and algae). Many kinds of herbivorous fish graze on algae, including zooxanthellae within the coral tissues; hundreds of predatory species feed on both invertebrate and vertebrate prey. Some of these predators, such as puffers (Tetraodontidae) and filefish (Monacanthidae), are corallivores that feed on coral polyps. Other predators lie in ambush for prey in coralline caverns. In addition, there is a wide array of symbionts, such as cleaning fish and crustaceans, that pick parasites and detritus from larger fish and invertebrates.
24.13 Productivity of the Oceans Is Governed by Light and Nutrients
Primary productivity in marine environments is limited to regions where the availability of light and nutrients can support photosynthesis (see Chapters 20 and 21). The vertical attenuation of light in water limits productivity to the shallower waters of the photic zone. The presence of a thermocline, however, limits the movement of nutrients from the deeper to the surface waters where light is adequate to support photosynthesis—especially in the tropics, where the thermocline is permanent (see Section 21.10). The rate at which nutrients are returned to the surface, and therefore productivity, is controlled by two processes: (1) the seasonal breakdown of the thermocline and subsequent turnover, and (2) the upwelling of deeper nutrient-rich waters to the surface (see Sections 21.10 and 21.13). As a result, the highest primary productivity is found in coastal regions (see Figure 20.10). There, the shallower waters of the continental shelf allow for turbulence and seasonal turnover (where it occurs) to increase vertical mixing, and coastal upwelling brings deeper, colder, nutrient-rich water to the surface (Figure 24.24; see also Figure 3.16).
In open waters, productivity is low in most tropical oceans because the permanent nature of the thermocline slows the upward diffusion of nutrients. In these regions, phytoplankton growth is essentially controlled by the cycling of nutrients within the photic zone. Production rates remain more or less constant throughout the year (Figure 24.25a). The highest production in the open waters of the tropical oceans occurs where nutrient-rich water is brought to the surface in the equatorial regions, where upwelling occurs as surface currents diverge (see Figures 3.16 and 24.24).
Productivity is also low in the Arctic, mainly because of light limitations. A considerable amount of light energy is lost through reflection because of the low sun angle, or it is absorbed by the snow-covered sea ice that covers as much as 60 percent of the Arctic Ocean during the summer.
In contrast, the waters of the Antarctic are noted for their high productivity as a result of the continuous upwelling of nutrient-rich water around the continent (Figure 24.25b). The growing season is limited by the short summer period. Primary productivity in temperate oceans (Figure 24.25c) is strongly related to seasonal variation in nutrient supply, driven by the seasonal dynamics of the thermocline (see Section 21.10).
Ecological Issues & Applications Inputs of Nutrients to Coastal Waters Result in the Development of “Dead Zones”
Human inputs of pollutants from urban, agricultural, and industrial activities have negative impacts on water quality in both freshwater and marine ecosystems (see Chapter 22), and although it may at first seem counterintuitive, one of the major classes of pollutants are the essential mineral nutrients that support plant growth and net primary productivity—nitrogen and phosphorus. As we discussed in Chapters 20 and 21, and previously in Section 24.13, the primary constraint on net primary productivity in aquatic ecosystems is nutrient availability, so the input of nutrients to aquatic ecosystems from human activities lead to eutrophication (Section 24.4), which functions to enhance primary productivity. At first this might appear to be a good thing, however, the exceedingly high levels of net primary productivity that result from eutrophication can result in anoxia (depletion of oxygen) and the development of “dead zones.”
The development of dead zones begins with the input of “unnaturally” high levels of nitrogen and phosphorus to an aquatic ecosystem such as a lake or coastal waters. The abundance of nutrients results in increased net primary productivity of phytoplankton and other aquatic autotrophs. As the autotrophs senesce they sink from the surface waters (photic zone) to the benthic zone, where bacteria feed on the dead organic matter. The increase in respiration by the decomposer community functions to decrease dissolved oxygen levels in the water. The decline in dissolved oxygen begins in the benthic zone, and the presence of the thermocline limits the vertical transport of oxygen from the surface waters. As winds move surface waters away from the coast, however, the oxygenated surface waters are replaced by the oxygen-depleted deeper waters (upwelling, see Section 3.8, Figure 3.16). This leaves no oxygen in any layer of the water. Often aquatic life moves toward the shoreline in an attempt to find oxygenated waters. Under extreme conditions, dissolved oxygen levels drop to such low levels that anoxia results in the death of marine organisms, hence the term dead zone (Figure 24.26).
Dead zones can be found worldwide (Figure 24.27). Marine dead zones can be found in the Baltic Sea, Black Sea, off the coast of Oregon, and in the Chesapeake Bay. Dead zones may also be found in lakes, such as Lake Erie. One of the largest dead zones in the world is in the Gulf of Mexico. The Gulf of Mexico dead zone is an area of hypoxic (less than 2 ppm dissolved oxygen) waters at the mouth of the Mississippi River. Its area varies in size, but cover an area of up to 7000 square miles. The zone occurs between the inner and mid-continental shelf region of the northern Gulf of Mexico, beginning at the Mississippi River delta and extending westward to the upper Texas coast (Figure 24.28).
The zone of anoxic water in the Gulf of Mexico is caused by nutrient enrichment (eutrophication) from the Mississippi River. Watersheds within the Mississippi River Basin drain much of the United States, from Montana to Pennsylvania and extend southward along the Mississippi River. Most of the nutrient input comes from major farming states in the Mississippi River Valley, including Minnesota, Iowa, Illinois, Wisconsin, Missouri, Tennessee, Arkansas, Mississippi, and Louisiana (Figure 24.29a). Nitrogen and phosphorous enter the river through upstream runoff of fertilizers, soil erosion, animal wastes, and sewage. An estimated 66 percent of nitrogen entering the Gulf of Mexico is derived from crop fields (largely corn and soybean) within the Mississippi River Basin (Figure 24.29b). The anoxic zone develops each spring as the rains leach chemical fertilizers from farm fields (see Chapter 21, Ecological Issues & Applications), and the size of the input and resulting dead zone is heavily influenced by weather conditions. For example, drought conditions in 2012 resulted in the fourth-smallest dead zones on record, measuring only 2889 square miles, an area slightly larger than Delaware.
The dead zone that forms in the northern Gulf of Mexico each summer threatens valuable commercial and recreational Gulf fisheries. Ongoing research indicates that long-term changes in species diversity and the structure of food webs are occurring, and numerous areas of the Gulf experience large-scale fish kills on an annual basis. The Gulf of Mexico currently supplies 72 percent of U.S. harvested shrimp, 66 percent of harvested oysters, and 16 percent of commercially harvested fish.
The future of the northern Gulf will require a concerted effort at the national level because the Mississippi River Basin encompasses such a vast area of the continent (see Figure 24.29a). The key to minimizing the Gulf dead zone is to address it at the source. There is a need to manage nutrients more efficiently in farm fields by using fewer fertilizers and adjusting the timing of fertilizer applications to limit runoff of excess nutrients into adjacent aquatic ecosystems. In addition, the restoration of wetlands and riparian ecosystems (see Chapter 25) can help to capture nutrients and reduce runoff. The federal government is also funding efforts to restore wetlands along the Gulf coast to naturally filter the water before it enters the Gulf.
Summary
Lakes Defined 24.1
Lake and pond ecosystems are bodies of water that fill a depression in the landscape. They are formed by many processes, ranging from glacial and geological to human activities. Geologically speaking, lakes and ponds are successional features. In time, most of them fill, get smaller, and finally may be replaced by a terrestrial ecosystem.
Lake Stratification 24.2
As a nearly self-contained ecosystem, a lake exhibits both vertical and horizontal gradients. Seasonal stratification in light, temperature, and dissolved gases influences the distribution of life in the lake.
Zonation of Life in Lakes 24.3
The area where light penetrates to the bottom of the lake, called the littoral zone, is occupied by rooted plants. Beyond this is the open-water or limnetic zone inhabited by plant and animal plankton and fish. Below the depth of effective light penetration is the profundal region, where the diversity of life varies with temperature and oxygen supply. The bottom or benthic zone is a place of intense biological activity where decomposition of organic matter takes place. Anaerobic bacteria are dominant on the bottom beneath the profundal water, whereas benthic zone of the littoral is rich in decomposer organisms and detritus feeders.
Nutrient Input into Lakes 24.4
Lakes are strongly influenced by their surrounding landscape. They may be classified as eutrophic (nutrient rich), oligotrophic (nutrient poor), or dystrophic (acidic and rich in humic material). Most lakes are subject to cultural eutrophication, which is the rapid addition of nutrients—especially nitrogen and phosphorus—from sewage, industrial wastes, and agricultural runoff.
Flowing-Water Habitat 24.5
Currents and their dependence on detrital material from surrounding terrestrial ecosystems set flowing-water ecosystems apart from other aquatic systems. Currents shape the life in streams and rivers and carry nutrients and other materials downstream. Flowing-water ecosystems change longitudinally in flow and size from headwater streams to rivers. They may be fast or slow, characterized by a series of riffles and pools.
Adaptations to Flowing Water 24.6
Organisms inhabiting fast-water streams are well-adapted to living in the current. They may be streamlined in shape, flattened to conceal themselves in crevices and underneath rocks, or attached to rocks and other substrates. In slow-flowing streams where current is minimal, streamlined forms of fish tend to be replaced by those with compressed bodies that enable them to move through aquatic vegetation. Burrowing invertebrates inhabit the silty bottom. Stream invertebrates fall into four major groups that feed on detrital material: shredders, collectors, grazers, and gougers.
River Continuum 24.7
Life in flowing water reflects a continuum of changing environmental conditions from headwater streams to the river mouth. Headwater streams depend on inputs of detrital material. As stream size increases, algae and rooted plants become important energy sources as reflected in the changing species composition of fish and invertebrate life. Large rivers depend on fine particulate matter and dissolved organic matter as sources of energy and nutrients. River life is dominated by filter feeders and bottom-feeding fish.
Estuaries 24.8
Rivers eventually reach the sea. The place where the one-way flow of freshwater meets the incoming and outgoing tidal water is an estuary. The intermingling of freshwater and tides creates a nutrient trap exploited by estuarine life. Salinity determines the nature and distribution of estuarine life. As salinity declines from the estuary up through the river, so do marine species. An estuary serves as a nursery for many marine organisms, particularly some of the commercially important finfish and shellfish because here the young are protected from predators and competing species unable to tolerate lower salinity.
Open Ocean 24.9
The marine environment is characterized by salinity, waves, tides, depth, and vastness. Like lakes, oceans are characterized by both stratification of temperature (and other physical parameters) and stratification of the organisms that inhabit the differing vertical strata. The open sea can be divided into several vertical zones. The hadalpelagic zone includes areas found in the deep-sea trenches and canyons. The abyssopelagic zone extends from the sea floor to a depth of about 4000 m. Above is the bathypelagic zone, void of sunlight and inhabited by darkly pigmented, bioluminescent animals. Above that lies the dimly lit mesopelagic zone, inhabited by characteristic species, such as certain sharks and squid. The bathypelagic and mesopelagic zones depend on a rain of detrital material from the upper lighted zone, the epipelagic zone, for their energy.
Ocean Life 24.10
Phytoplankton dominate the surface waters. The littoral and neritic zones are richer in plankton than the open ocean. Tiny nanoplankton, which make up the largest biomass in temperate and tropical waters, are the major source of primary production. Feeding on phytoplankton are herbivorous zooplankters, especially copepods. They are preyed on by carnivorous zooplankton. The greatest diversity of zooplankton, including larval forms of fish, occurs in the water over coastal shelves and upwellings; the least diversity occurs in the open ocean. Making up the larger life-forms are free-swimming nekton, ranging from small fish to sharks and whales. Benthic organisms (those living on the floor of the deep ocean) vary with depth and substrate. They are strictly heterotrophic and depend on organic matter that drifts to the bottom. They include filter feeders, collectors, deposit feeders, and predators.
Hydrothermal Vents 24.11
Along volcanic ridges are hydrothermal vents inhabited by unique and newly discovered life-forms, including crabs, clams, and worms. Chemosynthetic bacteria that use sulfates as an energy source account for primary production in these hydrothermal vent communities.
Coral Reefs 24.12
Coral reefs are nutrient-rich oases in nutrient-poor tropical waters. They are complex ecosystems based on anthozoan coral and coralline algae. Their productive and varied habitats support a high diversity of invertebrate and vertebrate life.
Ocean Productivity 24.13
Primary productivity in marine environments is limited to regions where the availability of light and nutrients can support photosynthesis and plant growth. The areas of highest productivity are coastal regions and areas of upwelling. In open oceans, especially in tropical areas, productivity is low because the permanent nature of the thermocline slows the upward diffusion of nutrients. Primary productivity in temperate oceans is strongly related to seasonal variation in nutrient supply, driven by the seasonal dynamics of the thermocline.
Dead Zones Ecological Issues & Applications
Inputs of nutrients to aquatic ecosystems from human activities result in eutrophication. The result is increased net primary productivity. With the death of autotrophs, increased decomposition can function to decrease the concentrations of dissolved oxygen, leading to anoxia and the development of dead zones in which low oxygen levels result in the death of marine organisms.
