Temporal Ammonium Patchiness and Growth Rate in Codium and Ulva (Ulvophyceae)¹

The growth rates of two chlorophyte macroalgae, Codium fragile and Ulva curvata, are compared in response to varied, but non‐random, NH 4 + enrichments (pulses). The species were chosen to contrast radically different morphologies. Pulse frequency and pulse duration were varied independently; however, an equivalent mass of NH 4 + was added in each treatment. The growth rate of Codium varied neither as a function of pulse frequency nor duration; the growth rate of Ulva varied with pulse frequency, but not pulse duration. These data are combined with life form and physiological characters, and are discussed in the context of the “function form” hypothesis. From the evidence we argue that by virtue of its life form, Ulva is capable of utilizing transiently high NH 4 + concentrations and is capable of high growth rates, attributes contributing to its role as a ruderal species. In contrast, Codium's life form does not allow utilization of transiently high NH 4 + concentrations or high growth rates, thereby contributing to its role as a persistent species.