LIGHT-INTENSITY AND THE INTERACTIONS BETWEEN PHYSIOLOGY, MORPHOLOGY AND STABLE-ISOTOPE RATIOS IN 5 SPECIES OF SEAGRASS

The effects of light intensity on stable isotope ratios, physiology an d morphology of five seagrass species were investigated in an outdoor, light controlled experiment. Seagrasses were maintained in flowing se awater aquaria, with each seagrass species exposed to different light regimes (5, 15, 20, 30, 50, and 100% full sunlight) using shade screen s. After 30 days exposure to the various light regimes the five specie s of seagrass showed markedly different delta(13)C signatures, with va lues ranging from -17.6 to -5.5 parts per thousand. Marked responses t o light intensity were also shown by each species, with leaf delta(1)3 C 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. Les s negative delta(13)C values at high light intensities could be primar ily 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 incr eased 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 lig ht intensity. Higher delta(15)N values were found at the more eutrophi c site (western Moreton Bay = 8.6 to 8.8 parts per thousand) than at t he site further from anthropogenic influence (eastern Moreton Bay = 2. 6 to 4.5 parts per thousand).