| The aim of this work was to examine how ultraviolet radiation (UV, 290--320 nm) impacts the physiology of two temperate phytoplankton species, as well as natural phytoplankton assemblages, off Coastal California. Cultures of the cyanobacteria Synechococcus sp. (WH7803), and the dinoflagellate Hetrocapsa pygmaea, were each incubated in a UV-PAR growth facility at UC Santa Barbara, consisting of 3 chambers: S&barbelow;olar P&barbelow;AR + UV-A&barbelow; + UV-B&barbelow; (SPAB, ≥290 nm), S&barbelow;olar P&barbelow;AR + UV-A&barbelow; (SPA, ≥325 nm), and S&barbelow;olar P&barbelow;AR (SP, ≥400 nm). There was also an I&barbelow;ndoor P&barbelow;AR (IP) control culture. Experiments on natural assemblages were conducted in situ inside UV drifters deployed in two distinct water masses of the Santa Barbara Channel. The drifters consisted of eight incubation chambers that systematically eliminated shorter wavelengths of UV.; In the Synechococcus sp. experiment, UV-B caused a decrease in Chl a&barbelow; concentration, affected Chl a&barbelow; periodicity, damaged phycobilisome proteins, enhanced production of the photosynthetic pigments zeaxanthin and phycouroblin, and caused a decrease in photosynthetic efficiency. In experiments on H. pygmaea UV did not alter the ratio of photosynthetic pigments within the cell, but did decrease growth, and caused an increase in cellular pigment concentrations. Photoprotective pigments in H. pygmaea increased when cells were exposed to UV-A, and decreased or did not change when exposed to UV-B. In natural phytoplankton assemblages off Coastal California, UV-B caused a change in community structure on short time scales. Prymnesiophytes were most heavily impacted, declining 15--17% after exposure to in situ irradiances. Depending on the integrated exposures and dose rates, community responses varied when released from the pressures of UV.; It is evident from results of this work that UV affects the physiology of temperate phytoplankton species, and can cause changes in community structure of natural assemblages. However, while UV-B caused significant cellular damage, UV-A had little effect and was beneficial in some cases. As concentrations of stratospheric ozone continue to decrease, leading to increases in UV-B, it is essential that UV impacts on phytoplankton are included in any models or ecosystem predictions. |
