Final answer:
The primary productivity of a kelp forest is the amount of carbon fixed minus the carbon used in respiration by the kelp, reported in grams of carbon per square meter per year (gC/m2/yr). Gross primary productivity measures the total energy conversion, while net primary productivity accounts for energy used by the kelp for biological functions.
Step-by-step explanation:
The measure of the primary productivity of a kelp forest, which is a highly productive ecosystem found in temperate and arctic climates, is related to how much carbon (designated as gC for grams of carbon) is fixed within this marine ecosystem over a period of one year per square meter (gC/m2/yr).
The primary productivity is often broken down into gross primary productivity and net primary productivity. Gross primary productivity (GPP) is the total rate at which photosynthetic primary producers, such as kelp, convert energy from sunlight into chemical energy in the form of organic compounds. Net primary productivity (NPP) is the portion of energy that remains after the primary producers have used some of it for their own respiration and maintenance (biological functions), which can be calculated as the total carbon fixed minus the carbon oxidized during cellular respiration.
In the example of the Silver Springs aquatic ecosystem, the GPP was measured at 20,810 kcal/m2/yr, but after subtracting energy used for respiration and loss as heat, the net primary productivity available for primary consumers was 7,633 kcal/m2/yr. Therefore, to estimate the primary productivity of a kelp forest in grams of carbon per square meter per year, one would measure the total amount of carbon fixed by the kelp and subtract the amount used by the kelp in respiration.