Final answer:
The productivity in polar regions is limited by sunlight due to the reduced sunlight availability caused by the tilt of the Earth's axis and the extreme angles of the sun's rays.
Step-by-step explanation:
The productivity in polar regions is limited by sunlight. In polar regions, the amount of sunlight received is significantly reduced due to the tilt of the Earth's axis and the extreme angles of the sun's rays. This limited sunlight availability restricts the growth and productivity of plants and phytoplankton, which are the primary producers in polar ecosystems.