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
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
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
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
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
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