LIGHT-ABSORPTION BY MARINE MACROPHYTES

Tissues of 338 marine macrophytes comprising 103 species, collected fr om the Atlantic, Mediterranean, South China, and Caribbean Seas, and e ncompassing a broad range in thallus form and pigmentation, were exami ned to quantify the importance of phylogenetic differences, spectral v ariability, and plant form and pigment content to account for differen ces in the absorption of light by marine macrophytes. Phylogenetic dif ferences accounted for 2.5% of the variance in absorption observed, no n-phylogenetic spectral differences being much larger (26%). Differenc es among individual specimens were much larger (72%), absorption at 67 5 nm increasing non-linearly as chlorophyll a density1/2, indicating t hat light absorption increases with increasing chlorophyll a density f ollowing a law of diminishing returns, as predicted by theory. The ene rgy return per unit tissue produced (i.e. light absorption per unit pl ant weight) increased linearly with increasing chlorophyll a concentra tion. However, the light absorbed per unit weight decreased, for a giv en chlorophyll a concentration, as plant thickness increased. This ind icates that while increasing thickness may increase chlorophyll a dens ity and, hence, the light absorbed by marine macrophyte thalli, this s trategy represents a burden limiting potential carbon turnover and pla nt growth. These results indicate that the diverse repertoire of light absorption by marine macrophytes can be adequately modeled as a conti nuum, dependent on plant thickness and pigment content, independent of phylogenetic differences.