Phosphorus and Nitrogen Limitation of Phytoplankton Growth in the Freshwaters of North America: A Review and Critique of Experimental Enrichments

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Abstract
While phosphorus is generally considered to be the primary nutrient limiting algal growth in lakes, limitation of algal growth by nitrogen has been observed in freshwater. It is also commonly observed that the most pronounced phytoplankton responses to enrichment occur when both N and P are added together. This led us to re-evaluate nitrogen's status as a secondary nutrient in freshwater through a systematic literature search. In our survey of enrichment bioassays, we found considerable deficiencies in the degree to which investigators have applied sufficient replication, performed and reported statistical tests, and assessed seasonal and spatial differences in algal nutrient limitation. Given these limitations, however, we found that combined N + P enrichment enhanced algal growth much more frequently and more substantially than did addition of N or P singly. On average, the frequency and degree of algal response did not differ for P vs. N enrichment. From our review of whole-lake fertilizations, we concluded that the roles of N and P in constraining algal growth at the whole-lake scale have not been completely separated. However, examination of the distribution between treatments (+N, +P, +N, and P) of lake-years in which substantial algal growth response occurred indicated that combined N and P enrichment was required to consistently produce substantial algal growth response. A more important role than has previously been recognized for N as a limiting nutrient in freshwaters is indicated. We suggest that greater attention should be given to both P and N in the future.