What limits primary productvity in tropical oceans?

Primary productivity in tropical oceans is primarily limited by nutrient availability, particularly nitrogen and phosphorus. Despite abundant sunlight and warm temperatures, these nutrients are often depleted in surface waters due to stratification, where warm, less dense water sits above cooler, nutrient-rich water. This stratification can hinder the upward mixing of nutrients, resulting in lower productivity compared to other regions like upwelling zones. Additionally, factors such as grazing by herbivores and competition among phytoplankton can further influence productivity levels.