Ecological Growth Strategies in the Seaweeds Gracilaria foliifera (Rhodophyceae) and Ulva sp. (Chlorophyceae): The Rate and Timing of Growth

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Abstract
The red seaweed G. foliifera and the green seaweed Ulva sp. were grown in Plexiglas tubes suspended in an outdoor continuous-flow system. Growth rates (% increase in fresh weight per week) were determined at monthly intervals over 14 mo. The growth rate of Ulva exceeded that of Gracilaria at both ambient incident light (Io) and 0.13Io. At Io, maximum growth rates for Gracilaria (to +37% per week) and Ulva (to +55% per week) were observed from April to Sept. Within this period, growth rate was correlated with the availability of N-nutrients. The difference between the growth rates of the 2 spp. decreased at 0.13Io. Apparently, at still lower light intensities (.apprx. 0.03Io), the growth rate of Gracilaria may exceed that of Ulva. In the intertidal zone of jetties near Beaufort [North Carolina, USA], Ulva occurs above Gracilaria. In the broad zone where the 2 spp. overlap, Ulva seems to be physiologically capable of outgrowing Gracilaria. The fact that this does not normally occur may be due to the greater susceptibility of Ulva to abrasion from wave action and sand scouring.