Nutrient control of algal growth in estuarine waters. Nutrientlimitation and the importance of nitrogen requirements and nitrogen storageamong phytoplankton and species of macroalgae

Abstract

Nutrient enrichment of shallow coastal waters changes the composition ofplant communities so that slow-growing, benthic macrophytes are replaced byfast-growing algae such as phytoplankton and ephemeral macroalgae. Thisscenario suggests that fast-growing algae suffer more from nutrientlimitation than slow-growing algae at low nutrient availability. We testedthis hypothesis by comparing the effect of in situ nutrient enrichment on thephytoplankton community, 4 ephemeral macroalgae. (Ulva lactuca, Cladophoraserica, Chaetomorpha linum and Ceramium rubrum) and 1 perennial macroalga(Fucus vesiculosus). Nitrogen was the main limiting nutrient to nigel growthand fast-growing algae were N limited for a longer period during summer thanslower-growing species. Differences in the temporal extent of N limitationwere related to species-specific variations in N requirements for growth andin N storage pools. The N requirements per unit biomass and time were up to30-fold higher for fast-growing algae compared to slow-growing species clueto 10-fold faster growth and 3-fold higher demands for the internal Nconcentration needed to sustain maximum growth (i.e. criticalconcentrations). The pools of N reserves only varied 2-fold among algalspecies and could support maximum growth for 0.5 d in the phytoplanktoncommunity and for 12 d in F. vesiculosus. Growth of phytoplankton and F.vesiculosus could proceed at reduced rates for another 2.6 and 34 d,respectively, based on other internal pools of N. The results suggest thatthe species-specific differences in growth rate and critical N concentrationsaccount for a substantial part of the variation in the duration of nutrientlimitation among different algal types and, therefore, provide furtherclarification of the reasons why fast-growing algae are stimulated byincreased nutrient availability while slow-growing algae remain unaffected orare hampered due to shading.