In oceans productivity is generally limitated by nitrogen.

Answer

**Step-by-Step Solution:**

1. **Understanding Ocean Productivity**: Ocean productivity refers to the rate at which marine organisms, primarily phytoplankton, produce organic compounds through photosynthesis. This process is crucial for the marine food web and overall ocean health.

2. **Identifying Limiting Factors**: In ecology, a limiting factor is an environmental condition that limits the growth, abundance, or distribution of an organism or a population. In oceans, the primary limiting factors for productivity are often nutrients.

3. **Role of Nitrogen**: Nitrogen is a vital nutrient for marine organisms. It is a key component of amino acids, proteins, and nucleic acids. While nitrogen is abundant in the atmosphere (making up about 78% of air), it is not readily available in a form that marine organisms can utilize.

4. **Nitrogen Deficiency in Oceans**: The concentration of nitrogen in ocean waters is relatively low compared to other nutrients. This deficiency limits the growth of phytoplankton, which are the primary producers in marine ecosystems. Without sufficient nitrogen, these organisms cannot produce enough organic matter, thus limiting overall ocean productivity.

5. **Comparison with Other Nutrients**: While phosphorus is also a limiting nutrient in some marine environments, nitrogen is often considered the primary limiting nutrient in open oceans. The balance of these nutrients is crucial for maintaining productivity.

6. **Conclusion**: Therefore, the statement that "ocean productivity is generally limited by nitrogen" is true. Nitrogen deficiency in ocean waters restricts the growth of phytoplankton, which in turn limits the overall productivity of the ocean.

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