Biomass Layering and Metabolism in Mats of the Macroalga Ulva lactuca L

Studies of photosynthesis and respiration in the sheet-like green macroalga Ulva lactuca L. have typically made use of cut discs or individual thalli. Because this species accumulates in layered mats in the field, its structure has the potential to significantly alter metabolic rates compared to those measured for a single layer. The effect of biomass layering on photosynthesis and respiration in U. lactuca was assessed by incubating stacks of 1, 2, 4, and 8 thalli across a range of surface irradiance and in the dark. The photosynthesis-irradiance curve for a single thallus was hyperbolic in shape with maximum photosynthetic rates of 16-24.5 mg O2 g-1 dry weight h-1, a photosynthetic efficiency of 0.18 mg O2 g-1 dry weight h-1(μ E m-2 s-1)-1, and dark respiration rates of 0.95-1.2 mg O2 g-1 dry weight h-1. Weight-specific photosynthesis and respiration rates decreased as the amount of layering increased, with rates approximately twice as high in a single layer as for 8 layers. A simple model revealed that the reduced photosynthetic rates were due to attenuation of light through the stack. Each layer of Ulva reduced the light available to the next layer by approximately 55%, and variability in this absorption was directly related to thallus chlorophyll content. Model runs suggested that acclimation of the thalli by changes in chlorophyll content has the potential to reduce some, but not all of the effect of layering. Reduced respiration rates were attributed to the depletion of oxygen from the interstices between the layers. Results were incorporated into a model of an Ulva mat, which predicted decreasing rates of weight-specific total mat production with increasing mat thickness as additional layers are added to the bottom of the mat that neither produce nor consume significant amounts of oxygen. The model predicted area-weighted mat production to increase up to a thickness of 10 layers, after which the rate was constant with increasing mat thickness for the same reason. Since Ulva has a tendency to accumulate in layers under natural conditions, these results improve upon metabolic measurements made with single thalli and will be useful for calculations of macroalgal production as well as in ecosystem models in which Ulva is an important primary producer.