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

Abstract

Relative growth rate, photosynthetic pigment content, and internal carbon and nitrogen levels of Ulva olivascens P.A. Dangeard, were examined on a seasonal basis under natural conditions from December 1994 to February 1996. Biotic variables were correlated with abiotic factors, including phosphate and nitrate/nitrite [(NO3− + NO2−)] concentrations in the seawater, temperature, 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 summer than in mid summer. In the same spring-to-summer period, photosynthetic pigment concentrations (chlorophyll a, chlorophyll b, and carotenoids) decreased by 70–80%. From March to July, nitrogen and carbon content declined by 62% and 11%, respectively, which yielded a change in the carbon: nitrogen mass ratio from 12.9 to 39.4. In late summer, the alga disappeared from the field site. Seasonal variations in relative growth rate of U. olivascens were governed by changes in primary ecological factors, i.e., [NO3− + NO2−] concentration (70%), PAR (15%), UV-B (5%), and temperature (4%). UV-B and UV-A radiations and PAR, were primarily responsible for the seasonal changes in chlorophyll a and b and carotenoid contents. More than 78% of the changes in internal carbon and nitrogen levels could be explained by seasonal changes in UV-B radiation. These results suggest that the metabolic stress 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 and nitrogen content.