Explanation:

Gross primary production (GPP) is the amount of chemical energy as biomass that primary producers create in a given length of time.Some fraction of this fixed energy is used by primary producers for cellular respiration and maintenance of existing tissues (i.e., "growth respiration" and "maintenance respiration"). The remaining fixed energy (i.e., mass of photosynthate) is referred to as net primary production (NPP).
In the oceans, light and nutrients are important controlling factors for productivity. Light penetrates only into the uppermost level of the oceans, so photosynthesis occurs in surface and near surface waters.However, among aquatic ecosystems, algal beds and coral reefs have the highest net primary production, while the lowest rates occur in the open due to a lack of nutrients in the illuminated surface layers.