Nutrient control of algal growth in estuarine waters. Nutrient limitation and the importance of nitrogen requirements and nitrogen storage among phytoplankton and species of macroalgae

Nutrient enrichment of shallow coastal waters changes the composition of plant communities so that slow-growing, benthic macrophytes are replaced by fast-growing algae such as phytoplankton and ephemeral macroalgae. This scenario suggests that fast-growing algae suffer more from nutrient limitation than slow-growing algae at low nutrient availability. We tested this hypothesis by comparing the effect of in situ nutrient enrichment on the phytoplankton community, 4 ephemeral macroalgae (Ulva lactuca, Cladophora serica, Chaetomorpha linum and Ceramiurn rubrum) and 1 perennial macroalga (Fucus vesiculosus). Nitrogen was the main limiting nutrient to algal growth and fast-growing algae were N limited for a longer period during summer than slower-growing species. Differences in the temporal extent of N limitation were related to species-specific variations in N requirements for growth and in N storage pools. The N requirements per unit biomass and time were up to 30-fold higher for fast-growing algae compared to slow-growing species due to 10-fold faster growth and 3-fold higher demands for the internal N concentration needed to sustain maximum growth (i.e. critical concentrations). The pools of N reserves only varied 2-fold among algal species and could support maximum growth for 0.5 d in the phytoplankton community and for 12 d in E vesiculosus. Growth of phytoplankton and E vesiculosus could proceed at reduced rates for another 2.6 and 34 d. respectively, based on other internal pools of N. The results suggest that the species-specific differences in growth rate and critical N concentrations account for a substantial part of the variation in the duration of nutrient limitation among different algal types and, therefore, provide further clarification of the reasons why fast-growing algae are stimulated by increased nutrient availability while slowgrowing algae remain unaffected or are hampered due to shading.