WOOSTER LAKE FIELD TRIP
Aquatic ecosystems comprise about 71% of the earth's surface. The majority of all oxygen occurring in
the atmosphere and most of the biosphere's carbon fixation occur through photosynthesis in the world's
aquatic systems. Most of the liquid water on earth is found in the world's oceans, but freshwater lakes,
rivers, streams, ponds, and wetlands are also important in the world's water economy, especially in
areas such as Kansas. The study of the biology, physics and chemistry of freshwater systems is called
limnology, whereas the same studies in marine systems are part of the science of oceanography.
The productivity of both freshwater and marine ecosystems depends on light penetration, temperature,
nutrient concentrations, underwater topography, water depth, and the direction and degree of water
movement. These factors change daily and they change with seasonal variations
in climate. In turn, these changes influence the nature and distribution of aquatic life. Wooster Lake,
on the ESU campus, illustrates some important principles of limnology.
The Watershed
Aquatic systems like Wooster Lake are actually comprised of two community-level units: 1. the lake
community; and 2. the terrestrial community surrounding the lake. Collectively, the lake itself and its
drainage basin comprise the watershed, which in effect is the ecosystem-level unit of aquatic systems.
The type of watershed in which a lake is situated profoundly influences the type of biota (living
organisms) found in the lake. For instance, many Kansas lakes are actually reservoirs that drain
farmland and pastures. A large quantity of suspended clay reaches the lake with every rainfall. As a
result, such lakes become turbid and light is prevented from penetrating the lake's waters to any great
depth.
Rain falling on the ground around Wooster Lake will eventually find its way into the pond. What types
of vegetation and ground cover are found in the immediate vicinity of the lake? From an examination of
the land around the pond, you can see that a great deal of debris (leaves, twigs, grass, etc.) produced
in the terrestrial community gets washed into the lake. This debris is called detritus and is very
important to the biology of the lake. Detritus forms the basis of heterotrophic food webs comprised of
bacteria, fungi, and aquatic invertebrates. These decomposers process organic matter from the
watershed and cycle energy and nutrients to other trophic levels in the lake. All organic matter is
eventually processed as detritus by bacteria and fungi. Although you can see large bits of detritus in the
lake, be aware that much more is actually entering in dissolved form, having already been partially
processed by terrestrial decomposers.
The Lake
Take a look at the lake itself. What color is it? The water's greenish cast is due to the large
populations of phytoplankton in Wooster Lake. Phytoplankton include many species of blue-green
algae, green algae, and diatoms. Collectively, phytoplankton are important components of the
primary producer trophic level. They form the basis for other trophic levels above them.
Phytoplankton are consumed by primary consumers such as invertebrates and small fish, which are
in turn consumed by secondary consumers.
The phytoplankton of Wooster Lake are particularly numerous because of two factors: 1. the lake
contains an abundance of dissolved nutrients, especially nitrogen and phosphorous, which are
necessary for algal growth, and 2. the lake is relatively free from suspended clay materials
imported from the surrounding watershed. Thus, light and nutrients play important roles in algal
abundance in lakes. Another important ecological factor in algal abundance is water temperature.
At cold temperatures, algae grow more slowly because their metabolic rate is lower. As the water
warms in early spring, the populations of algae in the pond will increase rapidly, producing a
phytoplankton bloom.
Lakes can be classified according to how productive they are. Productivity is a function of how
much biomass is produced per unit lake volume per unit time. Highly productive lakes are termed
eutrophic. Eutrophic lakes are often formed from less productive systems by human intervention.
Moderately productive lakes are termed mesotrophic and highly unproductive lakes are termed
oligotrophic. Lakes in Kansas are mesotrophic or eutrophic. In view of the apparently high
populations of plankton in Wooster Lake, how would you classify this lake?
When you look at Wooster Lake in the late spring, summer or fall, you will notice that its shallow
margins are populated by plants. Aquatic vascular plants are numerous in many lakes and are
termed aquatic macrophytes. Evidence suggests that aquatic vascular plants descended from
terrestrial ancestors that were able to colonize the shallows at the water's edge. Some
macrophytes, like the cattail (Typha sp.) and sweetflag (Acorus sp.) grow with their leaves and
flowers above water. Others, like the common water milfoil (Myriophyllum sp.) are totally
submersed, while some such as duckweed (Lemna sp.) and lotus (Nelumbo sp.) float on the water
surface.
The shallows of a lake, where light can penetrate all the way to the bottom is called the littoral
zone. The open water region is called the limnetic zone. As you look at Wooster Lake and
consider its physical structure, what region of the lake do you suppose might be most productive?
The sediments in both the littoral and limnetic zones are populated by a community of heterotrophic
organisms. These benthic invertebrates, such as insect larvae, worms, protozoa, and other
organisms, represent important links in the dynamics of aquatic systems. When lakes become
eutrophic, the sediments frequently become anoxic, meaning that all the oxygen has been depleted
through heterotrophic respiration. Many species of benthic invertebrates can become excluded from
the sediments when anoxia occurs. Fish kills can also result when this anoxic zone penetrates into
the water column itself. Aeration, such as provided by the pumps in Wooster Lake, is designed to
prevent the formation of anoxia by circulating bottom waters upward, thereby exposing them to
oxygen in the air.
Field Collections and Lab Analysis
On your trip to Wooster Lake, you will examine several sites and collect organisms present. Unlike
terrestrial communities, the organisms present in the aquatic community will not be immediately
obvious because most aquatic organisms are microscopic. Consequently, it will be necessary to
collect samples from each site and then return to the laboratory to examine them microscopically,
with dissecting and/or compound microscopes.
Note that in the littoral zone, there are both attached or rooted as well as suspended organisms.
Consequently, when collecting in these areas, scrapings from the surfaces of rocks should be
included. The littoral zone will also be sampled with a seine. To sample the limnetic zone, we will
use a plankton net made of fine nylon mesh which traps the plankton. By towing the net through
the water several times, plankton will be concentrated for easier viewing. In the benthic samples,
sediments and associated organisms will be collected with a benthic dredge net, also known as a
D-net.