SEASONAL GROWTH OF CODIUM-BURSA, A SLOW-GROWING MEDITERRANEAN MACROALGA - IN-SITU EXPERIMENTAL-EVIDENCE OF NUTRIENT LIMITATION

The seasonal growth of Codium bursa J. Agardh was examined in relation to ambient and internal nutrient concentrations to elucidate whether the low tissue nutrient concentrations observed are an intrinsic feature of this species or reflect nutrient-limited growth. Growth rates were rather low (mean +/- SE: 2.2 +/- 0.4 x 10(-3) d(-1); doubling time = 1.24 +/- 0.1 yr), and were significantly correlated to seasonal changes in ambient and internal nutrient concentrations, suggesting nutrient, probably P, limitation. This suggestion is supported by the rather high atomic tissue C/P ratios (1712), compared to C/N ratios (31.8) observed in summer. This was confirmed by experimentally injecting nutrients into the internal lumen of the plants in situ, which resulted in a doubling of growth rate, photosynthetic efficiency, and a reduction in light compensation irradiance (39.6 and 29.9 mu mol photons m(-2) s(-1) in control and fertilised specimens, respectively). These results provide, along with the strong correlation between growth rate and ambient and internal phosphorus concentrations, clear evidence of a tight nutrient (likely phosphorus), rather than Light or temperature, control of C. bursa growth rates. Hence, even inherently slow-growing organisms in the oligotrophic Mediterranean may be resource limited.