Seaweed Anatomy and Photosynthetic Performance: The Ecological Significance of Light Guiges, Heterogeneous Absorption and Multiple Scatter¹²

Light absorption by two green seaweeds with similar photophysiology but different anatomies are compared: i) Ulva lactuca var. rigida (C. Ag.) Le Jolis, an optically translucent species of two cell layers both bearing chloroplasts; and, ii) Codium fragile subsp. tomentosoides (van Goor) Silva, an optically opaque species with a colorlelss medulla surrounded by a cortex of choloroplast‐bearing utriclels. Thallus absorptance (fraction of incident light absorbed) was measured for various pigment contents. Absorptance by U. lactuca was dependent on pigment concentration in an exponential manner and never exceeded 0.6, whereas absorptance by C. fragile was independent of pigment concentration and always approached a balue of 1.0. Water in the medullary tissue of C. fragile is often of the utricles. The utricles appear to be “integrating spheres” enhancing the capture of incident light, aided by the wave‐guide function of the thin peripheral layer of cytoplasm and a reflector function at their base. Photosynthitic performance for U. lactuca saturates at high light intensities and attenuates rapidly with decreasing intensities. In contrast, photosynthetic performance for C. fragile saturates at low light intensities and attenuates slowly with diminishing radiation. Extrapolated diel variation in photosynthesis shows that U. lactuca's anatomy is adaptive for high light intensity environments, whereas C. fragile's anatomy is adaptive for low light intensity environments. Both seaweeds fit into the ecological category of “fugitive” species, and compete in the Long Island Sound (Atlantic Ocean) rocky intertidal for free‐space. Predictions are presented for relative species abundances along a monotonic gradient of light intensity.