Final answer:
The gross annual primary productivity of the ecosystem is calculated by adding the net annual primary productivity to the respiration by producers, resulting in 20,000 kcal/m2/year.
Step-by-step explanation:
The student asked about calculating the gross annual primary productivity of an aquatic ecosystem, given the net annual primary productivity and respiration by aquatic producers. To find the gross primary productivity, you add the value of the net primary productivity (8000 kcal/m2/year) to the value of the respiration by the producers (12,000 kcal/m2/year). Therefore, the gross annual primary productivity for this ecosystem is 20,000 kcal/m2/year.