This model of marine plankton growth has several options. All options require that you first run the test to determine the effects of light and depth on the rate of photosynthesis. If you have not run that option, you will first be required to input these data before continuing with your selected model. All models output their data graphically as gram carbon fixed per square meter of ocean vs. depth. Depth can range potentially to 150 m, and is limited by the rate of photosynthesis. When carbon fixation falls below 0.1 grams/ sq. meter/ day, productivity is assumed to be zero and the depth at which this occurred becomes the maximum depth for photosynthesis.

The light model requires that you input the solar radiation at the surface of the water. Reasonable values range between 0.0 and 0.2 g Kcal/ sq. cm/ d. You must also input the extinction coefficient. This value should range from 0.1 for very murky water to 0.01 for clear water. The model then draws the profile for carbon fixation (productivity) as related to depth of the water. The model will halt when you reach either a productivity of 0.1 g carbon fixed per day or a depth of 150 m. In both cases, you will be provided with the mean rate of photosynthesis calculated over all depths. You may then choose to input a second data set, or choose the current one as the baseline for any further models you will run.

If nutrient levels need to be considered, choose the option from the menu. You are asked to input both the observed nutrient concentration and that concentration which would be limiting for photosynthesis. You need not input actual values, but can set up a proportion. If, for example, you wish to explore the effect of having half the available nutrients, enter "1" as the observed nutrient concentration and "2" as the limiting concentration. To simulate an abundance of nutrients, enter "2" as the observed nutrient concentration and "1" as the limiting concentration to indicate that you have twice as many nutrients as required.

Vertical mixing losses can also be modeled. Vertical mixing moves phytoplankton down from the surface to lower levels and reduces the rate of production. Enter a depth greater than the lower level of photosynthesis to simulate these phenomena. If your depth is less than the lower limit for photosynthesis, you will be asked to enter a new depth. The vertical mixing factor is printed as a ratio on the screen.

The ratio is calculated as:

Temperature also affects productivity by speeding up the respiration rate. To simulate a 10-degree rise in temperature, choose the third option and enter 10. The productivity will be reduced by the increased respiration.

You can also use a simple model to test the effects of grazing on the phytoplankton community. This model assumes that the zooplankton filter a constant volume of phytoplankton from the water. Input a relatively low value (less than 0.5) for the grazing rate and a value less than 50 for the density of zooplankton per square meter of ocean surface.