Trophic Cascades in Aquatic Systems

Julie Seward

Background

In lake ecosystems, microscopic algae called phytoplankton undergo photosynthesis to form carbohydrates and oxygen from carbon dioxide and water filling much the same role as terrestrial plants. When the production of phytoplankton increases, the phytoplankton are said to bloom. Such blooms are the cause of much concern to lake managers. When large phytoplankton blooms begin to die, bacteria feed on the dead cells. Bacteria respire producing carbon dioxide and water from carbon from ingestion of phytoplankton and oxygen. Plenty of food cause the bacteria population to increase. More bacteria means more respiration occurs which depletes the water of oxygen. This can lead to massive fish kills from oxygen deprivation. Managers also worry about taste and odor problems of the water as many lakes are utilized as our drinking water sources. A type of phytoplankton called blue-green algae or cyanobacteria are abundant in high light, nutrient rich waters. This type of phytoplankton cause the taste and odor problems in the lakes. As a result of such concerns, lake managers and ecologists seek to understand how such blooms are controlled.

Productivity of phytoplankton is dependent on the supply of nutrients, primarily phosphorus, in the lake. When there are not enough nutrients to sustain a large phytoplankton population, growth of phytoplankton is said to be nutrient limited. Light is also necessary to the phytoplankton as it is the energy source of photosynthesis. Temperature of the water effects how fast the phytoplankton grow and reproduce. Based on such knowledge, the theory stands that production in the lake is controlled by the "bottom up." That is, abiotic factors such as nutrient supply control primary production of the phytoplankton. If nutrients are limited from entering the lake, production of the phytoplankton will decrease.

How do the critters that eat the phytoplankton fit into this picture? A tomato plant in your backyard will grow and grow with enough light and fertilizer. But, if a munching caterpillar comes in and helps himself, the plant is reduced. In a lake ecosystem, the little animals which eat phytoplankton (herbivores) are called zooplankton. It follows that zooplankton are eaten by minnows (planktivores) which are eaten by top predators (piscivores) which are commonly bass. The top down theory holds that phytoplankton productivity is controlled by the population of its predator (herbivores in this case) which are in turn controlled by their predators, etc. Based on this theory of how ecosystem production is limited, the hypothesis of trophic cascades was born. It states that an increase in piscivore biomass brings a decline in planktivore biomass, increase in herbivore biomass, and decrease in phytoplankton biomass. The cascade hypothesis suggests that abiotic factors such as nutrients set the stage for the potential for phytoplankton production, but that actual productivity is ultimately determined by food web structure.

Outline of this Lab

In this lab you will be presented with a 25 m deep water column model of a lake ecosystem. This water column is within the upper mixed layer where light and temperature do not limit phytoplankton growth. In this water column live bass (piscivores), minnows (planktivores), zooplankton (herbivores), and phytoplankton (primary producers). Zooplankton fill two different trophic levels, predaceous and grazing, dependent on their size. Large zooplankton, such as Cladocerans, feed primarily on small zooplankton such as Rotifers and Copepods. These small zooplankton are grazers which feed on phytoplankton via filter feeding.

Each species immigrates into and out of the given area of lake on the screen. Phytoplankton abundance is dependent on how many phytoplankton individuals there are using resources such as nutrients to grow and multiply. Therefore phytoplankton settlement in the lake in habitat density dependent. The zooplankton immigrate into the lake on the screen in a random fashion. They reproduce when sufficient energy in accumulated from eating from the trophic level below them. Each time a zooplankter reproduces, it splits its energy with its offspring. Energy needed for metabolic costs and reproduction can only be gained by eating individuals in the lower trophic level. Lack of sufficient energy gain to maintain daily activities results in death. The fish are modeled in the same manner. The fish species (planktivore and bass) need much more energy to reproduce due to their larger size.

You are going to be the lake manager in this model. Your function is to prevent an algal bloom from occurring by manipulating the size of the two top predator populations (Bass and planktivores). We'll assume that in this lake ecosystem, that nutrients are not limiting to phytoplankton growth, thus production can be controlled from the top-down. You will recruit local fisherman to catch all the Bass population or the minnow population out of the lake in order to control the phytoplankton production.

The Lab

1. Run EcoBeaker by double clicking on its icon.

2. Open the situation "Trophic Cascades."

Several windows will appear on the screen. The black screen in the upper left shows a profile of the water column of the lake you will be managing. When the model is run, you will see squares dancing in and out of the water column. These squares represent the different species whose colors are given in the species box just to the right of the grid. Two population graphs, a bar and line graph, will show the abundance of the different species as you run the model.

Below the grid is the control panel box. This allows you to start and stop the model showing how much time goes by in the process. Each time unit represents a day. Phytoplankton blooms are temperature dependent. We will assume that this lake has six months in which the temperature is optimal for a bloom to occur (approximately 180 time steps).

The last window on the screen is the Change Parameters box. This is where you decide which species of fish is removed from the lake.

3. Run the model (hit the GO button in the Control Panel).

In the water column, you will see all the different species immigrating in and out as well as reproducing. Notice that in the Change Parameters box the numbers of bass and planktivores are proportionate. You will soon change this.

4. Observe the graphs for about 1 season (about 180 time steps). Describe the abundance patterns of the species over time. This is the first season in which the lake was monitored. The plans are in the works for controlling the phytoplankton production.

5. When you have observed the lake for a sufficient number of "monitoring season", stop the model by hitting the STOP button.

6. Your job is now to vary the populations of planktivores and bass in order to reduce phytoplankton production in the lake.

(a) Change the number of planktivores in the lake to "0" next to the # planktivores in the Change Parameters window. Click the CHANGE button to reset the model to the new parameters. Click to RESET button on the Control Panel to clear the previous data. Now run the model again (click GO).

(b) Now "restock" the lake with planktivores and harvest the bass population. Follow the same steps as above to reset the model.

(c) Stop the model in the middle of the warm season (about 90 timesteps) and add some bass to the lake in the Change Parameters window. Click the CHANGE button. Do NOT reset the model, but click the GO button and allow the model to continue. This simulates young of the year (YOY) bass which are planktivores maturing into top predators.

7. Which model modification best controlled phytoplankton production? Explain the abundance patterns in this scenario. Why did this scenario work better?

8. What occurred when the YOY matured filling the top predator role? What facets of fish behavior would modify this outcome?

9. Hypothesize on the difference in the phytoplankton community composition in the different model simulations. How would the size of the phytoplankton change?

10. Summarize your results from this model. This is your report which you will submit as you scientific opinion as to what technique would be most advantageous to prevent phytoplankton blooms from occurring in this lake ecosystem.

Notes and Comments

The issue of bottom-up versus top-down control of ecological system is a source of controversy in Ecology. Many challenge that the top-down trophic cascade hypothesis is too simplistic to describe the complexities of aquatic systems. One particular contention is that certain species feed on more than one trophic level (omnivory). Large zooplankton called Cladocerns are omnivores. Certain species of Cladoceran are predaceous and others are herbivorous. As such, the large zooplankton species used in the model could be feeding on the small zooplankton and the phytoplankton species. This hypothesis also ignores the bacterial component of aquatic ecosystems. Bacterial are the only species that can eat dissolved organic matter. Once eaten, the dissolved component once again is made available to higher trophic levels as particulate organic matter as bacterial biomass. Small zooplankton and heterotrophic algae feed on bacteria. These two major omissions from the trophic cascade hypothesis cause some scientist concern.

Scientific studies have experimentally demonstrated the trophic cascade hypothesis. Whole lake manipulations and stream studies have shown phytoplankton productivity controlled by the abundance of the top predator. Shifts in zooplankton dominance from small to large species has been shown to affect the relative abundance of nutrients in lake ecosystems. Cladocern (large zooplankton) are good planktivore food. As such, they have a high growth rate in order to quickly grow to a size too large to be eaten. High rates of protein synthesis are necessary to sustain such high rates of growth, therefore RNA is abundant in Cladoceran. RNA is phosphorus rich which means Cladoceran require a high concentration of P to sustain protein synthesis. Much of the lake P in stored in Cladoceran biomass. They are able to outcompete phytoplankton for P, causing a shift in the lake to P-limitation. Conversely, Copepods are nitrogen sinks forcing the lake toward N-limitation. A shift in zooplankton composition can affect the stoichiometry of nutrients in the lake ecosystem. The question of what controls the lowest trophic level production is an intriguing question which will stimulate more experimental work in the field of Ecology.

References

Carpenter, S.R., J.F. Kitchell, J.R. Hodgson, P.A. Cochran, J.J. Elser, M.M. Elser, D.M. Lodge, D. Kretchmet, X. He, and C.N. von Ende. 1987. Regulation of lake primary productivity by food web structure. Ecology, 68:1863-1876.

Carpenter, S.R., J.F. Kitchell, and J.R. Hodgson. 1985. Cascading trophic interactions and lake productivity. Bioscience, 35:634-639.

Power, M.E. and W.J. Matthews. 1983. Algae-grazing minnows, piscivorous bass, and the distribution of attached algae in a small prairie-margin stream. Oecologia (Berlin), 60:328-332.