Effects of Enhanced UV-B on Pigment-based Phytoplankton Biomass and Composition of Mesocosm-Enclosed Natural Marine Communities from Three Latitudes

SUZANNE ROY, BRUNA MOHOVIC, SÔNIA M.F. GIANESELLA, IRENE SCHLOSS, MARTHA FERRARIO AND SERGE DEMERS

PHOTOCHEMISTRY AND PHOTOBIOLOGY

Año: 2006 p. 909 - 922

0031-8655

A series of three outdoor mesocosm experiments was undertaken in Rimouski (Canada), Ubatuba (Brazil) and Ushuaia (southern Argentina) to examine the effects of lamp-enhanced UV-B (280- 320 nm) on phytoplankton communities isolated from local seawater at each site. Detailed pigment composition was used to identify these communities. Each experiment compared three replicated UV-B treatments, consisting of natural sunlight conditions (NUVB), low level UV-B enhancement corresponding to local 30% ozone depletion (LUVB) and high level enhancement corresponding to 60% ozone depletion (HUVB). Each mesocosm (approximately 2 m-deep) was mixed continuously (turnover time ca 1.3 h) and sampled daily over 7 to 10 days. In Rimouski, a large diatom bloom occurred during the first week. A repeated- measures (RM) ANOVA, with time as the repeated factor, showed slight but significant increases in chlorophyll (Chl) a for the HUVB treatment, especially obvious over the last 3 days of the experiment. A large decrease in grazers (ciliates) that was observed concurrently in this treatment is the most likely explanation for this Chl a increase. The lack of negative effect on algal biomass by enhanced UV-B is attributed to the mixing inside the mesocosms and the relatively low UV-B penetration. In Ubatuba, most pigments decreased through time, particularly fucoxanthin, Chl c3 and alloxanthin. The RM-ANOVA showed no effect of the UV-B treatments, except for Chl c3, with significantly lower concentrations under natural UVB conditions, indicating that enhanced UV-B directly or indirectly favored Chl c3 algae (likely prymnesiophytes). While particulate organic carbon concentration was significantly larger in HUVB compared to the other treatments, Chl a was unaffected, suggesting that enhanced UV-B favored heterotrophs. Lack of algal growth during this experiment was attributed to low nutrient concentrations (lowest of the three sites), high irradiances (highest noon incident PAR and UV of the three sites) and UV-B penetration down to the bottom of the mesocosms. In Ushuaia, a small bloom took place over the first 5 days. The RM-ANOVA showed no overall effect of the UV-B treatments for any of the pigments examined, but on the last 3 days of the experiment several green algae-type pigments such as Chl b and siphonein showed increased concentrations under the HUVB treatment. UV-B enhancement favored green algae, as seen from the stronger increase through time in the Chl b:Chl a ratio from the HUVB treatment. UV-B enhancement also seemed to cause a slight decline in physiological condition, as chlorophyllide a and some pheophorbides that may be the product of dying algae, increased in relative concentration in the HUVB treatments in Ubatuba and particularly in Ushuaia (where UV-B also penetrated to the bottom of mesocosms). For all three sites, changes in phytoplankton biomass due to the UV-B treatments were minor, even though UV-B enhancement was important. This study indicates that effects of enhanced UV-B on the community structure of both phytoplankton and their grazers are more important than effects on overall algal biomass.