NUTRIENT CONTROL OF ALGAL GROWTH IN ESTUARINE WATERS - NUTRIENT LIMITATION AND THE IMPORTANCE OF NITROGEN REQUIREMENTS AND NITROGEN STORAGEAMONG PHYTOPLANKTON AND SPECIES OF MACROALGAE

Nutrient enrichment of shallow coastal waters changes the composition of plant communities so that slow-growing, benthic macrophytes are rep laced by fast-growing algae such as phytoplankton and ephemeral macroa lgae. This scenario suggests that fast-growing algae suffer more from nutrient limitation than slow-growing algae at low nutrient availabili ty. We tested this hypothesis by comparing the effect of in situ nutri ent enrichment on the phytoplankton community, 4 ephemeral macroalgae (Ulva lactuca, Cladophora serica, Chaetomorpha linum and Ceramium rubr um) and 1 perennial macroalga (Fucus vesiculosus). Nitrogen was the ma in limiting nutrient to algal growth and fast-growing algae were N lim ited for a longer period during summer than slower-growing species. Di fferences in the temporal extent of N limitation were related to speci es-specific variations in N requirements for growth and in N storage p ools. 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 con centration needed to sustain maximum growth (i.e. critical concentrati ons). The pools of N reserves only varied 2-fold among algal species a nd could support maximum growth for 0.5 d in the phytoplankton communi ty and for 12 d in F. vesiculosus. Growth of phytoplankton and F. vesi culosus could proceed at reduced rates for another 2.6 and 34 d, respe ctively, based on other internal pools of N. The results suggest that the species-specific differences in growth rate and critical N concent rations account for a substantial part of the variation in the duratio n of nutrient limitation among different algal types and, therefore, p rovide further clarification of the reasons why fast-growing algae are stimulated by increased nutrient availability while slow-growing alga e remain unaffected or are hampered due to shading.