Assessment of macroalgal nitrogen limitation in a seasonal upwelling region

Abstract

The relationship between nitrogen (N) availability and the growht of macroalgae in a seasonal upwelling region (Yaquina Head, Oregon, USA) was investigated. Water column nutrient concentrations were relatively high and stable during the winter, decreased in early spring, and were highly variable in the summer and early fall. Periods of high nutrient availability due to upwelling commenced in May and alternated with periods of low nutrient availability until September. Dissolved inorganic nitrogen (DIN) concentration was a poor predictor of N limitation of macroalgal growth due to the ability of macroalgae to store N for continued growth when ambient DIN concentrations were low. The relationship between growth and internal N content for Pelvetiopsis limitata (Phaeophyta: Fucaceae) and Ulva rigida (Chlorophyta: Ulvales) depended on the N source and culture conditions. Results of an in situ N enrichment experiment indicated that P. limitata growth may have been N-limited in April and May. A nitrogen budget for P. limitata indicated that NH4+ was the major N source for growth at Yaquina Head, due to its rapid uptake, despite the greater abundance of NO3-. The critical N value (tissue N level during the transition from N limiting to non-N limiting conditions) for growth of U. rigida on NO3- (.ltoreq. 24% dry wt) was much lower than that for growth on NH4+ (3.0% dry wt). Ammonium was the major N source for growth of U. rigida at Yaquina Head, and growth was probably N-limited in April and May, as indicated by the critical N level for growth on NH4+. Growth of U. rigida may also have been intermittently N-limited between upwelling events. Problems with the use of critical N levels and nutrient monitoring to assess N limitation are discussed. Use of critical tissue N levels is complicated by variations in this parameter with the N source used for growth.