The dynamic system of an ecosystem depends on which of the following?

The given food web contains some missing organisms $A, B, C$ and $D$. Identify these organisms and select the correct answer.

$A$: $A$ stable community should show much variation in year-to-year productivity.
$R$: It must not be resilient to occasional disturbance.

Read the following statements carefully and answer the question given below:
$(1)$ $NPP = GPP - R$
$(2)$ Productivity of an ecosystem can be measured in $g/m^2/yr$ or $kcal/m^2/day$.
$(3)$ The total amount of energy produced by all producers in an ecosystem is called Gross Primary Productivity $(GPP)$.
$(4)$ The net primary productivity of the oceans is $55$ billion tons.
State the number of correct statements.

Zooplanktonic forms are

Provide information about the aquatic weed known as the 'Terror of Bengal'.