NUTRIENT CONTROL OF ESTUARINE MACROALGAE - GROWTH STRATEGY AND THE BALANCE BETWEEN NITROGEN REQUIREMENTS AND UPTAKE

The ability to sustain growth at low availability of nitrogen (N) was examined in 6 species of macroalgae with different growth strategies b y comparing substrate dependent growth kinetics. The N required to sup port optimal growth and the N uptake kinetics of 2 slow-growing algae, Fucus vesiculosus and Codium fragile, and 4 fast-growing species, Cha etomorpha linum, Cladophora serica, Ceramium rubrum and Ulva lactuca, were experimentally determined in summer when the algae were N limited . The N required to support maximum growth varied 16-fold among specie s, with fast-growing algae having the highest N demands. The high N re quirements of ephemeral algae were caused by up to 13-fold higher grow th rates and 2- to 3-fold higher N content at maximum growth. Also, th e fast-growing species took up ammonium and nitrate 4 to 6 times faste r per unit of biomass than slow-growing species at both low and high s ubstrate concentrations, but the ratios of maximum N uptake to require ments were larger among the slow-growing algae. Thus, the fast-growing species tended to require relatively higher external concentrations o f inorganic N to saturate their growth. Under N Limited conditions, al l 6 macroalgae were able to exploit pulses of high concentrations of a mmonium by taking up ammonium at transiently enhanced rates (i.e. surg e uptake). Uptake was, however, only marginally enhanced at low, and n aturally occurring, concentrations of ammonium, suggesting that surge uptake is of minor ecological importance. Our results show that large, slow-growing macroalgae may be better able to meet their N requiremen ts at low N availability than fast-growing species. This is consistent with the common observation that nutrient-poor coastal areas are domi nated by slow growing macroalgae rather than ephemeral species, althou gh ephemeral species have higher N uptake capacities.