A mini-model of a tropical marine lagoon is given in Figure III-14. The lagoon is bordered by a belt of mangroves (M) with area A which surrounds a shallow basin containing productive algal communities in the water and on the shallow bottom I m deep or less. Larger organisms such as young shrimps and fishes consume the detritus from the mangroves and algae. The lagoon water exchange with the sea through a small narrow channel that flows more out than in because of the fresh water received from the land runoff.

The mini-model illustrates food chain patterns in ecosystems and the recycle of mineral nutrients. As already of detritus storage permits the higher consumers to develop a prey-predator oscillation. These cause the nutrient cycle and production oscillate and auto catalytic pathways to the higher consumers. This is an example of the ways the larger species can control the periodicities of ecosystems. The external sources, sun, nutrient inflow and tidal patterns are held constant. They are source limited inputs which set the general carrying capacity of the system so far as support of higher organisms.

For a scale of time of months and years, the ups and downs of phytoplankaton due to their own storage fluctuations are too short to affect the components higher in the food chain with longer turnover times. Therefore the nutrients and the algal production rates are shown without storages, responding without delay to any changes in their inputs and outputs.

Examples of Other Ecosystems with Recycle Oscillations

In some ways, especially in the way prey-predator oscillations control the longer period oscillations of nutrients and plant production, the model illustrates shallow aquatic ecosystems of freshwater as well as marine type. Some terrestrial ecosystems such as the Arctic tundra may operate similarly but with time in years rather than in months.

"What if" Experimental Problems

1. In the model in Figure III-14 and program LAGOON increase the nutrients running off the land into the lagoon and through the mangrove border. Increase NR from 1 to 3. What happens to the levels and the oscillations?
   


2. Suppose the area of mangroves is reduced by cutting. Change A from 0.1 to 0 and M to 0. What effect was observed on the detritus and consumers?
   


3. The shrimp production are dependant on larvae from offshore reproduction exchanging back into the lagoon with the tide. Suppose there are no larvae available. Set L=0 and S=0. What is the effect on the shrimp in the lagoon? What other effects were observed?

COMPUTER MINIMODELS AND SIMULATION EXERCISES FOR SCIENCE AND SOCIAL STUDIES

Howard T. Odum* and Elisabeth C. Odum+
* Dept. of Environmental Engineering Sciences, UF
+ Santa Fe Community College, Gainesville

Center for Environmental Policy, 424 Black Hall
University of Florida, Gainesville, FL, 32611
Copyright 1994

Autorização concedida gentilmente pelos autores para publicação na Internet
Laboratório de Engenharia Ecológica e Informática Aplicada - LEIA - Unicamp
Enrique Ortega
Mileine Furlanetti de Lima Zanghetin
Campinas, SP, 20 de julho de 2007