Ultraviolet B radiation effects on the structure and function of lower trophic levels of the

GUSTAVO A. FERREYRA, BEHZAD MOSTAJIR, IRENE R. SCHLOSS, KHALED CHATILA, MARTHA E. FERRARIO

PHOTOCHEMISTRY AND PHOTOBIOLOGY

Año: 2006 p. 887 - 897

0031-8655

The impact of ultraviolet-B radiation (UVBR: 280-320 nm) on lower levels of a natural plankton assemblage (bacteria, phytoplankton and microzooplankton) from the St. Lawrence Estuary was studied during 9 days using several immerged outdoor mesocosms. Two exposure treatments were used in triplicate mésocosms: natural UVBR (N treatment, considered as the control treatment) and lamp enhanced UVBR (H treatment, simulating 60% depletion of the ozone layer). A phytoplankton bloom developed after day 3, but no significant differences were found between treatments during the whole experiment for phytoplankton biomass (chlorophyll a and cell carbon) nor for phytoplankton cell abundances from flow-cytometry and optical microscopy of three phytoplankton size classes (pico, nano and microplankton). In contrast, bacterial abundances showed significantly higher values in the H treatment, attributed to a decrease in predation pressure due to a dramatic reduction in ciliate biomass (~70-80%) in the H treatment relative to the N one. The most abundant ciliate species were Strombidinium sp., Prorodon ovum and Tintinnopsis sp., and all showed significantly lower abundances under the H treatment. P. ovum was the less affected species (50 % reduction in the H treatment as compared with N control), contrasting with ~90 % for the other ones. Total specific phytoplanktonic and bacterial production were not affected by enhanced UVBR. However, both the ratio of primary to bacterial biomass and production decreased markedly under the H treatment. In contrast, the ratio of phytoplankton to bacterial plus ciliate carbon biomass showed an opposite trend than the previous results, with higher values in the H treatment at the end of the experiment. These results are explained by the changes in the ciliate biomass and suggest that UVBR can alter the structure of the lower levels of the planktonic community by  selectively affecting key species. On the other hand, linearity between particulate organic carbon (POC) and estimated planktonic carbon was lost during the post-bloom in both treatments. In previous works, this had been related to the aggregation of carbon released by cells to the water column in the form of transparent exopolymer particles (TEP) under nutrient limiting conditions. Unexpectedly, POC during such period was higher in the H treatment than in controls. It is hypothesized that such result may be related to a decrease in the ingestion of TEP by ciliates, in coincidence with increased DOC release by cells under enhanced UVBR. The consequences of such results for the carbon cycle in the ocean are discussed.