Why there is low primary productivity in oceans than on land?

Q. Which of the following statement(s) about productivity is/are incorrect?
I. Gross primary productivity of an ecosystem is the rate of production of organic matter per unit area during photosynthesis.
II. Gross primary productivity is measured in terms of kcal/m²/year
III. Net primary productivity of oceans is much higher than that of land masses on earth.
IV. Gross primary productivity = Net primary productivity - Respiration losses (R)

Q. Why are oceans least productive?

Q. Discuss the main reason for the low productivity of the ocean.

Q. Assertion :Oceans are a low productivity ecosystems despite occupying about $70\%$ of the earth's surface. Reason: In aquatic ecosystems, productivity is limited by light which decreases with increasing water depth.

Q. Consider the following four statements (A-D) and select the correct option stating which ones are true (T) and which ones are false (F). Statements :

(A) Vertical distribution of different species occupying different levels in a community is called stratification.
(B) Net primary productivity minus respiration losses is the gross primary productivity.
(C) Rate of biomass production is called decomposition
(D) Annual net primary productivity of oceans is 55 billion tons