Dr. Wade H Jeffrey

The Ross Sea polynya is one of the most productive areas of the Southern Ocean; however, little is known about how plankton there respond to inhibitory solar exposure, particularly during the early-spring period of enhanced UVB (290--320 nm) due to ozone depletion. Responses to solar exposure of the phytoplankton and bacterial assemblages were studied aboard the research ice breaker Nathaniel B. Palmer during cruises NBP0409 and NBP0508. Photosynthesis and bacterial production (thymidine and leucine incorporation) were measured during in situ incubations in the upper 10 m at three stations, which were occupied before, during, and after the annual peak of a phytoplankton bloom dominated by Phaeocystis antarctica. Near-surface production was consistently inhibited down to 5-7 m, even when some surface ice was present. Relative inhibition of phytoplankton increased and productivity decreased with increasing severity of nutrient limitation as diagnosed using F sub(v)/F sub(m), a measure of the maximum photosynthetic quantum yield. Relative inhibition of bacterial production was high for both the high-biomass and postbloom stations, but sensitivity of thymidine and leucine uptake differed between stations. These results provide the first direct evidence that solar exposure, in particular solar ultraviolet radiation, causes significant inhibition of Ross Sea productivity.