A simple model of salt marsh in a temperate latitude is simulated under annual rhythmic influences of sun, river inflow, and seasonal changes in the magnitude of the tidal flushing. Nutrients and organic matter are received from the river inflow and flushed out by tidal exchange. Organic production of the marsh plants is a function of the nutrient levels and the sun; nutrients are recycled from plants and consumers.

Five years are plotted in the graphs. The uppermost graph has sinusoidal rise and fall of sunlight with season being high in summer; the third graph from the top has strongest river inflow at the end of winter each year. The tidal flushing is shown with highest flushing in the spring and fall of each year.

The results of the simulation are a seasonal pulse of marsh plant stock (Q2 in lower graph). The pulse in binding and releasing of nutrients (Q1 in the 4th graph from the top) is out of phase with the live plant stock. Each year some of the dead plant matter is added to the peaty substrate so that it increases gradually during the 5 years (Q3 in the lower graph).

This model has three driving functions and two export pathways. It shows the way organic production of marshes can be a pulse of food for the aquatic organisms in the waters of the estuary. Factors that may be important that are not included in this simple model are the effect of higher summer temperature in increasing consumption, the migratory input of larvae and out-migration of post-larval fishes and shrimp.

Examples of Marsh and Rhythmic Inputs

Many systems have rhythmic inputs such as those from sun, climate and tide. As in Figure III-11 the storages absorb and filter out some of the input variation. A system with this ability is called a "filter". Wetlands have inflows of water from rains and floods that give those ecosystems pulses. Organisms capable of growing with the right timing to gain from these inputs tend to prevail.

Environments with rhythmic inputs are said to have frequency niches. Adapted species have the right responses to prevailing frequencies of inputs. The principle may apply to ecosystems of land or water. Other kinds of systems, such as economic systems, may respond to external input variations also.

"What if" Experiments

1. What is the effect of a small initial storage of organic matter on the oscillations in the system due to the external input oscillations? Start Q3=10.
   


2. What is the effect of increasing the tidal range? Multiply the equation in the program for E by 2.
   


3. To which is the system more sensitive, a change in sun or a change in river borne nutrients? Multiply the equation for sun in the program by 2.
   Then compare with result of doing this to the equation for nutrient inflow J.

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