Aquatic Life Zones

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Randall Phillis

Ch. 6 (cont.): The Aquatic Equivalents of Biomes III. In aquatic ecosystems, important environmental factors include salinity,amountof dissolved oxygen,availabilityof light for photosynthesis, and concentrations of dissolved nutrients (e.g., N, P, Fe) needed by the phytoplankton. A. Aquatic life is ecologically divided into plankton(free-floating), nekton(stronglyswimming, e.g., fish), and benthos (bottom-dwelling). 1. The microscopic phytoplanktonare photosynthetic and are the base of the food web in most aquatic communities. Although they are not as strongly mobile as fish, many phytoplankton are capable of considerable diurnal vertical movement. Some important phytoplankton groups, like dinoflagellates (including the red tides but also many benign species), have flagella to power their motion. Others, such as diatoms, some of the most abundant algae in the ocean, use oil droplets that can be varied in size. They move upwards to take advantage of the light during the day, but their activities deplete the nutrients near the surface. Consequently, they move downwards at night to find higher nutrient concentrations. 2. Zooplanktonare nonphotosynthetic organisms that include protozoa, rotifers, tiny crustaceans (like copepods), and the immature stages of many animals (such as polychaete worm larvae). 3. Unlike terrestrial ecosystems, aquatic systems (particularly marine) may actually have any more biomass or energy present in the zooplankton (1° consumers) than is present in the all the phytoplankton (producers) in the same volume of water. This is possible because many of the phytoplankton have generation times of about one day, whereas zooplankton have generation times of several days or a few weeks. Consequently, a single volume of water generates a far greater total gross production of phytoplankton biomass per year than it generates of total gross zooplankton. Net primary productivity still greatly exceeds net secondary productivity. B. Freshwater ecosystems include flowing-water (rivers and smaller streams), standing-water (lakes and ponds), and freshwater wetlands. 1. In flowing-water ecosystems, the water flows in a current, which is swifter in headwaters than downstream. Flowing-water ecosystems have little phytoplankton and depend on detritus from the land for much of their energy. Slower-flowing rivers are often murky and harbor some of nature’s drabest fish species. Slower flowing wide rivers may also generate more of their own productivity, especially by benthic algae and rooted plants near their banks. 2. Large standing-water ecosystems (freshwater lakes) are divided into zones on the basis of water depth. a.Themarginal littoral zone contains both emergent vegetationand algae and is very productive. b. The limnetic zone is open water away from the shore that extends down as far as enoughsunlight penetrates to permit photosynthesis. Organisms in the limnetic zone include phytoplankton, zooplankton, and larger fishes. c. The deep, dark profundal zone holds little life other than anaerobic bacterial decomposers in some lakes. In other lakes, O is available to support 2 a variety of decomposers in lake bottom sediments. 3. Lakes often become thermally stratified (Fig. 6.15a). At such times, a thermocline (a depth where water temperature abruptly changes) separates warmer, less dense water near the surface (the epilimnion) from the deeper, colder, denser water of the hypolimnionbelow the thermocline. Water achieves its greatest density (1.000 g/cm ) at 4°C. The difference in density at 15°C (0.999 g/cm ) 3 may seem small, but it means that trying to exchange a m of 4° water at a 3 depth of 30 m with a m of 15° water at the surface takes as much work as would be needed to raise 1 kg by 30 m, which is a lot of energy to ask wind to pass to lake water. 4. Lakes are also categorized based on whether and how often they are vertically mixed, which is the only significant way to recycle algal nutrients from the profundal zone back to the limnetic zone. a. Amictic lakes never mix or “turn over”, because ice shields the water from mixingbywind. b. Dimictic lakes turn over twice per year - once in fall and once in spring. These are the seasons when a temperate lake’s temperature is nearly uniform from top to bottom so the wind can mix it easily. In winter, the lake freezes so no mixing, and in summer it’s strongly thermally stratified. Fig. 6.15b shows a dimictic lak. c. Monomictic lakes come in two varieties: i. Warm monomictic - the lake turns over only at the coldest time of the year in climates warmer than those where dimictic lakes predominate. ii. Cold monomictic - turn over occurs only at the hottest time of the year in climates colder than those of dimictic lakes. d. Polymictic and oligomictic types occur in the tropics and differ prima-rily in the frequency at which the irregularly timed mixing occurs. e. Meromictic lakes have a chemocline, a depth where salinity abruptly changes. Small changes in salinity can give water density changes that would result from huges changes in temperature. Unlike thermoclines which can be eroded by seasonal temperature changes, chemoclines are highly persistant. The water below chemoclines frequently becomes completely anaerobic. Many deep tropical lakes have chemoclines. Lake Tanganyika with a maximum depth of 1,470 m, has a chemocline between 100 and 250 m, below which it is anaerobic. 5. Freshwater wetlands, lands that are transitional between freshwater and terrestrial ecosystems. Although many types of wetlands have been defined, two useful named wetland types are marshes,inwhichthevegetationismostly grasses and sedges, and swamps,inwhich trees are the dominant plants. Fig. 6.16 shows a cypress swamp. a. They are usually covered by shallow water and have characteristic soils and vegetation. For example, their soils are often covered by a thick layer of accumulated organic matter. Below this layer the soil has exotic colors like muted blues and grey greens. The colors reflect the absence of oxygen, which results, in turn, in the absence of oranges and browns
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