Growth seasonality, photosynthetic pigments, and carbon and nitrogen content in relation to environmental factors: a field study of Ulva olivascens (Ulvales, Chlorophyta)

Relative growth rate, photosynthetic pigment content, and internal carbon a nd nitrogen levels of Ulva olivascens P.A. Dangeard, were examined on a sea sonal basis under natural conditions from December 1994 to February 1996. B iotic variables were correlated with abiotic factors, including phosphate a nd nitrate/nitrite [(NO3- + NO2-)] concentrations in the seawater, temperat ure, photosynthetically active radiation (PAR), and UV-A and UV-B radiation . The relative growth rate of the alga was 68% higher in spring and early s ummer than in mid summer. In the same spring-to-summer period, photosynthet ic pigment concentrations (chlorophyll a, chlorophyll b, and carotenoids) d ecreased by 70-80%. From March to July, nitrogen and carbon content decline d by 62% and 11%, respectively, which yielded a change in the carbon : nitr ogen mass ratio from 12.9 to 39.4. In late summer, the alga disappeared fro m the field site. Seasonal variations in relative growth rate of U. olivasc ens were governed by changes in primary ecological factors, i.e., [NO3- + N O2-] concentration (70%), PAR (15%), UV-B (5%), and temperature (4%). UV-B and UV-A radiations and PAR, were primarily responsible for the seasonal ch anges in chlorophyll a and b and carotenoid contents. More than 78% of the changes in internal carbon and nitrogen levels could be explained by season al changes in UV-B radiation. These results suggest that the metabolic stre ss due to UV-B radiation has little impact on the seasonal growth cycle of U. olivascens but has significant impact on pigments and internal carbon an d nitrogen content.