Implications of thallus thickness for growth-irradiance relationships of marine macroalgae

Abstract

We have tested the importance of thallus thickness in marine macroalgae for the growth-irradiance relationship measured in the laboratory and for the maximum growth rate measured in the field. We found large differences in the growth-irradiance relationship depending on the biomass parameter (surface area, fresh weight, carbon content) applied. Use of carbon content yielded meaningful reproducible patterns and it appeared to be the most suitable descriptor of plant energetics. Surface area and fresh weight increased in the dark despite loss of carbon in the plant tissue. The growth kinetic parameters were closely related to thallus thickness, here described as thallus-specific carbon (TSC, mol tissue C m−2 surface area). Both the loss rates in the dark and the growth rates in carbon biomass at low and high irradiance decreased with increasing TSC of the species. The decline in growth rate with increasing TSC was described by the allometric equation log(growth rate) = a + b × log(TSC). The slope, b, was between −0·50 and −0·52 in field data derived from 30–34 species of macroalgae. In similar descriptions of maximum photosynthesis at high light (as a function of TSC) derived from the literature, b was between −0·47 and −0·80. The relationships between growth and TSC and photosynthesis and TSC were, however, weak for thin species, and thus resemble the weaker size dependence of growth previously described for unicellular algae. TSC is recommended as a suitable continuous variable to describe the relationship between thallus form and growth and metabolism in marine macroalgae which can be extended to microalgae. © 1996 The British Phycological Society.