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 by comparing substrate dependent growth kinetics. The N required to support optimal growth and the N uptake kinetics of 2 slow-growing algae, Fucus vesiculosus and Codium fragile, and 4 fast-growing species, Chaetomorpha 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 species, with fast-growing algae having the highest N demands. The high N requirements of ephemeral algae were caused by up to 13-fold higher growth rates and 2- to 3-fold higher N content at maximum growth. Also, the fast-growing species took up ammonium and nitrate 4 to 6 times faster per unit of biomass than slow-growing species at both low and high substrate concentrations, but the ratios of maximum N uptake to requirements were larger among the slow-growing algae. Thus, the fast-growing species tended to require relatively higher external concentrations of inorganic N to saturate their growth. Under N Limited conditions, all 6 macroalgae were able to exploit pulses of high concentrations of ammonium by taking up ammonium at transiently enhanced rates (i.e. surge uptake). Uptake was, however, only marginally enhanced at low, and naturally 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 requirements at low N availability than fast-growing species. This is consistent with the common observation that nutrient-poor coastal areas are dominated by slow growing macroalgae rather than ephemeral species, although ephemeral species have higher N uptake capacities.