Light intensity and the interactions between physiology, morphology and stable isotope ratios in five species of seagrass

The effects of light intensity on stable isotope ratios, physiology and morphology of five seagrass species were investigated in an outdoor, light controlled experiment. Seagrasses were maintained in flowing seawater aquaria, with each seagrass species exposed to different light regimes (5, 15, 20, 30, 50, and 100% full sunlight) using shade screens. After 30 days exposure to the various light regimes the five species of seagrass showed markedly different delta(13)C signatures, with values ranging from -17.6 to -5.5 parts per thousand. Marked responses to light intensity were also shown by each species, with leaf delta(1)3C values becoming at least 3 to 4 parts per thousand less negative in full sunlight. Other common responses to light intensity were: higher productivities, higher C:N ratios, larger lacunal areas and more root biomass under full sunlight compared with lower light intensities. Less negative delta(13)C values at high light intensities could be primarily due to (a) increased uptake of C-13 from the external C source or (b) increased internal recycling of CO2 in the lacunae due to the increased lacunal size. The increase in size of lacunae may be related to the need to supply more oxygen to the increased root biomass occurring in seagrasses under high light conditions. In contrast to delta(13)C, the delta(15)N values of seagrass leaf tissue appeared to be affected by the site of collection, rather than the species of seagrass or light intensity. Higher delta(15)N values were found at the more eutrophic site (western Moreton Bay = 8.6 to 8.8 parts per thousand) than at the site further from anthropogenic influence (eastern Moreton Bay = 2.6 to 4.5 parts per thousand).

Keywords: carbon, light, nitrogen, seagrass, stable isotope ratio, zostera-marina l, izembek lagoon, organic-matter, food web, carbon, , eelgrass, nitrogen, discrimination, plants, variability