Photochemistry of Antarctic Waters in Response to Changing UV-B Fluxes

Our primary objective on the PD94-12 cruise aboard the RV Polar Duke was to conduct several experiments designed to evaluate the photochemical properties of open oceanic and coastal Antarctic waters during summertime, non-ozone hole conditions. The cruise was generally very successful with some very interesting findings. There were also some unanticipated difficulties that were encountered, as will be discussed below.

Experiments conducted

1. Buoy (drifter) deployments. The objective of this experiment was to determine photoreactivity in the water column of biologically productive water (Paradise Harbor) and unproductive waters (Station 600.100 and Crystal Sound). Filtered seawater samples (0.2 µm) were placed in quartz tubes and irradiated under natural light conditions at various depths for about 12 hours. Photochemical production of H₂O₂, OH radical, formaldehyde, and a-keto acids were measured. Sample depths were usually surface, 2, 4, 6, 10, 15 and 20 m. The three buoys were successfully deployed and recovered. All photochemically formed species examined showed an exponential decrease in their production rates with depth, presumably due to the corresponding drop-off in UV. However, the rate of this drop-off varied for the different chemical species (fig. 1), and was consistent with differences that were observed in the action spectra measured in the lab.

   Last year, under ozone hole conditions, a similar depth dependency was observed for the OH radical (aldehydes and a-keto acids were not measured) but not for H₂O₂, which showed a subsurface maximum in photoproduction rate at 2-4 m. We suggest that this result was due to a greater flux of more energetic photons under ozone hole conditions. These more energetic photons may have altered photochemical production or destruction rate of H₂O₂ relative to non-ozone conditions.

2. Ultrafiltration experiments. We performed three on-deck irradiation experiments with seawater fractionated by ultrafiltration. The first two samples were taken from open ocean sites, while the third was taken from a productive coastal site (Paradise Harbor). The purpose of this experiment was to determine which size fraction(s) was primarily responsible for the photochemical reactivity that we measure in the unfractionated seawater. Parameters examined included: photoproduction of H₂O₂, OH radical, formaldehyde and a-keto acids; photobleaching of humic fluorescence and absorbance; and concentrations of nitrate, nitrite, total dissolved protein and carbohydrates. Preliminary results indicate that H₂O₂ and a-keto acid photoproduction arise from dissolved organic matter of molecular weight of < 10,000 dalton. Reasonably good mass balances on the photoproduction of these species, relative to unfractionated seawater, were obtained (within 20%). The preliminary results also indicated that OH photoproduction was dominated by a low molecular weight species, < 1000 dalton (i.e., nitrate). This result is in contrast to our results for low latitude regions, where photoreactions involving DOM dominated OH production.

3. Depth profiles. We conducted detailed depth profiles to characterize the organic and photochemical properties of the water at the major stations that were occupied during PD94-12. Parameters measured were: flavins, aldehydes and ketones, a-keto acids, protein and humic fluorescence, and absorbance. Depth profile data have not been quantified to date.

4. Dark incubations. We determined the biological and/or abiotic removal of photochemically produced chemical species, flavins and hydrogen peroxide, at in situ temperatures. Filtered (0.2 µm) and unfiltered water from open ocean (Crystal Sound) and coastal (Paradise Harbor) sites were used. The samples, incubated in the dark, were analyzed daily. The results indicted that for hydrogen peroxide, open ocean seawater showed slow loss (turnover time ca. weeks) and coastal seawater showed fast loss (turnover time ca. hours). The data for the flavins in unfiltered seawater has not been analyzed yet. No loss or production was observed for hydrogen peroxide or the flavins in the filtered samples.

Problems encountered

Although the cruise was generally very successful, there were some problems that were encountered. In particular, we not able to make the high sensitivity absorption measurements as planned due to equipment failure of the Hewlett Packard spectrophotometer. We suspect that this was due to failure of one of the controller cards in the instrument due to problems with the ships electrical system. We also had a hard drive failure and failure of one of our HPLC absorbance detectors, both of which could be resolved by using backup equipment. We were also not able to conduct some collaborative experiments with Dave Karl, as we had hoped, to ascertain the coupling between the biological uptake of pyruvic acid and its photochemical production rate; both science groups were pressed for time in meeting their primary science objectives, as there was an untimely delay of the SAAM 1B flight. The third difficulty was due to electrical problems that we encountered when the Duke was navigating through the pack ice at Crystal Sound. Essentially, our whole scientific party had to shut down all scientific operations for a day due to the wildly fluctuating ship power, which caused complete failure of the UPS systems that were on-line. Another major problem was the wire and Go-Flo bottles. Rust dripping off the conducting wire may have ruined some of our first experiments and incubations. We were able to partially get around this problem by using water from the ships non-metallic seawater system for our later experiments. Some of our experiments, which involved looking at the photoreactivity of water from different depths, had to be canceled because of the rusty wire problem. We were also plagued with non-functioning Go-Flo bottles. On nearly every cast, 3-4 bottles did not close or were leaking badly, despite efforts to clean the closing mechanisms and o-rings and tightening the elastic bands. Finally, it was difficult for us to find a place on the ship to work up the samples for trace organic analysis. We finally settled on the aquarium room since it appeared to be well ventilated. However, even there fumes from the stacks, incinerator, galley, and welding operations occasionally contaminated our samples when the wind was blowing from certain directions.