modern limnological study revolves around the primary
productivity of lakes. The ecology of plant growth is of great importance
to the character and history of lakes and to all other organisms that
live in lakes. The major threat to lakes involves the excessive growth
of primary producers due to nutrient inputs caused by poor landuse
management. Therefore, it is worth a closer look at these organisms.
zone is defined by the growth of rooted and floating aquatic plants,
Figure 17 provides examples of common macrophytes found in Minnesota
lakes. The macrophyte community can also include large algae,
such as Chara,Nitelle,orCladophora.In shallow,
clear lakes, macrophytes may represent most of the green plant material
present and may account for most of the photosynthesis.
be few macrophytes in a lake when the bottom is too rocky or too sandy
for the plants to anchor themselves, wave action too severe, or the
water too deep. Also, sunlight may not reach the bottom even in shallow
areas if the concentration of algae or silt is high.
the other main group of primary producers (Figure 18). They come in
countless forms and live in nearly all kinds of environments. Most are
microscopic, growing as single cells, small colonies, or filaments of
cells. Suspended algae are called phytoplankton,
while attached algae are called periphyton.
Phytoplankton grow suspended in open water by taking up nutrients from
the water and energy from sunlight. If their populations are dense,
the water will become noticeably green or brown and will have low transparency
are classified into groups by the type of pigments they use to perform
photosynthesis. While chlorophyll-a
is common to all groups there are many other accessory pigments that
allow the algae to capture different types of light. Green algae are
considered the most closely related to higher plants. Within this group
alone there is a great diversity of size, shape, and growth form (single
celled, colonial, filamentous, and flagellated). Diatoms
belong to a large group, classified as the golden-brown algae, which
also includes chrysophytes and dinoflagellates. The most striking characteristic
of diatoms and chrysophytes is the ability to form silica (glass) cell
cell walls are similar to a petri
dish, having two halves that fit together. Some chrysophytes have
elaborate silica scales, spines, or vase-like shells called loricas.
Diatoms are non-motile
(unable to swim), so they depend on water turbulence to remain suspended.
Chrysophytes have flagella
(whip-like appendages) that allow them to control their position in
column. There are other important algal groups containing motile
are another group of golden-brown algae that also have flagella. These
cells are capable of moving very rapidly; positioning themselves where
light and nutrients are optimal for growth. Another flagellated group
called the cryptomonads are very small algae and contain pigments that
enable them to photosynthesize under very low light conditions, either
very deep in the water column or during those times of the year when
sunlight isnt very strong.
"algae" are technically referred to as cyanobacteria
since, except for their chlorophyll-based photosynthesis, they are bacteria.
They generally receive the greatest amount of research and management
attention because of their ability to form nuisance
blooms in eutrophic
lakes. It is important to remember, however, that blue-green algae
are very important primary producers in both freshwater and marine systems,
despite often being a nuisance.
have several characteristics that often enable them to dominate and
create nuisance or noxious conditions. Some blue-green species have
the ability to adjust their buoyancy. They can float or sink depending
on light conditions and nutrient supply. All plants, including all algae,
typically satisfy their nitrogen requirement by absorbing nitrate (NO3-)
and/or ammonium (NH4+) from the water. However,
some blue-greens can fix
molecular nitrogen (N2) derived from the atmosphere and dissolved
in the water and convert it to ammonium in the cell through a process
fixation. This allows them to maintain high rates of growth when
other forms of nitrogen are sufficiently depleted to limit growth by
other types of algae. Blue-green algae typically are well-adapted to
deficiency because of their ability to absorb and store excess phosphorus
when it is available -- enough to last days to weeks in some cases.
green algae and diatoms, the blue-green algae are less suitable food
consumers. This is partly because some blue-greens can form large
colonies of cells embedded in a gelatinous matrix which may pose handling
problems for grazers.
They also may produce chemicals that inhibit grazers or makes them "taste
bad" to the grazers. Consequently, blue-greens have advantages
over other algae at using nutrient and light resources, as well as avoiding
flos-aquae is a common species of filamentous blue-green algae (see
Figure 18) with the ability to regulate its buoyancy, fix nitrogen,
form large inedible colonies, and form algal blooms. Other common bloom
genera are Anabaena (N2-fixing filamentous algae)
and Microcystis (colonial; not a N2-fixer). These
bloom-forming algae are known to produce toxins in farm ponds that can
poison cattle and, more recently, have been found to produce potent
neurotoxins and hepatotoxins that may be a greater public health concern
than previously realized.