increase in mean winter air temperature since 1950 (Ducklow et al, 2007). The ocean

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1 Exploring the relationship between Chlorophyll a, Dissolved Inorganic Carbon, and Dissolved Oxygen in the Western Antarctic Peninsula Ecosystem. Katie Coupland December 3, 2013 Since the start of the industrial revolution, humans have increased the atmospheric carbon dioxide concentration from 280 parts per million (ppm) to over 400ppm (NOAA, 2013). This rapid change in atmospheric composition is altering ecosystems worldwide, with the most dramatic effects occurring around the Weddell Peninsula in Antarctica, a 6 C increase in mean winter air temperature since 1950 (Ducklow et al, 2007). The ocean absorbs roughly 30% of all CO 2 emitted into the atmosphere naturally and in order to combat climate change, it has been proposed that we stimulate the ocean to absorb even more CO 2 by fertilizing with iron. The idea harnesses the biological pump, the fixation of inorganic carbon into organic carbon and subsequent sinking of the organic carbon to the sediments, to transport atmospheric CO 2 to the bottom of the ocean (Strong et al, 2009). In some areas of the ocean, photosynthesis is limited by iron or other micronutrients, instead of nitrogen, phosphorus or silica. These areas termed, High Nitrate, Low Chlorophyll (HNLC), have the ability to increase primary productivity with a supply of iron and possibly sequester more carbon into oceanic sediments. The Western Antarctic Peninsula (WAP) Ecosystem, as well as the majority of the Southern Ocean, is defined as HNLC. There have been multiple studies trying to quantify the amount of carbon exported to a predetermined depth (variable between studies) (Strong et al, 2009) in able to quantify the efficiency of iron fertilization. In the WAP region, there has been a long- term monitoring program, Pal LTER, conducting cruises during the summer months since 1990 (Waters and Smith, 1992). The cruises cover an area 900km X 200km and collect many parameters including temperature,

2 salinity, chlorophyll a, DIC, and dissolved oxygen samples at each station. This information can be used to compute gross primary productivity (GPP) as well at net community production (NCP) throughout the region. During photosynthesis molecular oxygen is released and carbon dioxide is absorbed. Therefore, the concentrations of these two chemicals can be studied in the surface ocean to quantify how much primary productivity is occurring. However, processes other than biological ones, such as heating, cooling, or wind driven air- sea gas exchange, can alter the saturation state of both gases in the water (Carrillo et al, 2003). These processes complicate our ability to measure primary productivity. Oxygen reaches equilibrium between the air and sea much quicker than carbon dioxide, dampening the biological signal. Here we look at the relationship between oxygen, DIC, and Chlorophyll a concentrations in the surface waters over a 6 year period in the WAP ecosystem. Is the biomass of chl a more closely related to oxygen or DIC and how do these parameters change spatially? The data was collected during January of each year from during the Pal- LTER cruises to the WAP region. Figure 1 shows the sample stations, the majority of which were sampled each year. Only data from the upper 3 meters of the water column are being considered here. For all years, there is no relationship between chlorophyll a and oxygen. Figure 2 shows the regression of the two, with an R 2 = and a correlation of The relationship between DIC and Chl a can be seen in Figure 3, R 2 = 0.38 and correlation This relationship is much more consistent with what we would expect to see. As plankton photosynthesize and grow, the CO 2 in the water is removed. The slow equilibrium rate of the gas makes this draw down by phytoplankton detectable.

3 Overall, Chl a concentrations are higher in coastal areas where ice melt is more prevalent, creating stratification and a shallower mixed layer depth (Ducklow, 2007). The spatial distribution of DIC is shown in Figure 4, all samples were taken between latitude 64.2 S and 64.8 S during January Figure 5 shows the stations sampled and general bathymetry. The sample taken at longitude W is located directly over the shelf break and the high DIC concentration indicates an intrusion of Upper Circumpolar Deep Water onto the shelf at this point. Moving eastward from this point the DIC drops off as expected. The oxygen change is also depicted in Figure 4, with no discernable trend. Interannual variability can have a large impact on biomass and subsequently DIC and oxygen, to a lesser extent. In addition to sea ice extent controlling biomass on a yearly basis, Montes- Hugo et al links the strength of the summer phytoplankton bloom to the direction and magnitude of wind during the spring months. For springs (September- November) with strong northerly wind, there is an overall increase in productivity throughout the WAP region. Figure 6 shows the average annual chlorophyll a and DIC concentrations for all stations, 2006 had the highest concentration of Chl a and the lowest DIC concentration. Oxygen concentrations varied from year to year but did not follow any observable trend. Using either change in oxygen or DIC over time to calculate DIC is not with out faults. Rates of change for these molecules can be determined by bottle incubations or in situ monitoring (not employed widely in WAP due to seasonal ice cover and depth) and from there, with some assumptions, a rate of carbon production can be made. The general assumptions for calculating primary productivity are productivity decreases exponentially with depth following light availability, the sun intensity remains constant over a given

4 length of time (1 day typically), wind strength remains constant for a given length of time and area (again 1 day most often), and that photosynthesis occurs at maximum rate provided ample nutrients are available. As stated earlier, the wind stress is more important when using oxygen than DIC due to the differences in the exchange rates of the two gases, however it does still need to be considered for changes to DIC in surface waters. As seen in this examination of the data, using raw oxygen change is insufficient to calculate primary productivity in this ecosystem. In response, one method that has been previously applied is to measure not only the dissolved oxygen concentration but the argon concentration as well (Huang et al, 2012). Argon is a biologically inert gas that has a very similar gas exchange rate to oxygen. By using argon as a proxy for oxygen flux into and out of surface waters, it is possible to separate the change due to air- sea exchange and the change due to biological activity. From this rate of change for oxygen, a measurement of primary productivity can be calculated, eliminating one of the assumptions. Calculating gross primary production is the first part of determining carbon export to the deep sea. A sediment trap has been located offshore from Palmer station in 150m of water since Unfortunately this data was not available to analyze at this time. The data from the sediment trap may help to understand how much organic carbon is being exported from the surface waters to deeper depths. However, the average depth of the Southern Ocean is 4500m, thus using a sediment trap on the shelf (300m) is not representative of true carbon sequestration to deep sediments. As expected, using a sediment trap has its limitations as well including selective particle exclusions (based on size), particle degradation within trap, and extreme area coverage limitations. Nonetheless, the sediment traps allow for a better understanding of the export from the euphotic zone to

5 deeper depths. Ducklow 2006 showed the sedimentation rates are weakly, inversely correlated (significant) to krill abundance in the WAP ecosystem and significantly and directly correlated to salp abundance since Salps produce larger fecal pellets that sink rapidly (Bruland and Silver, 1983), potentially increasing carbon sequestration. During the summer months, the northern WAP region is dominated by cryptophytes, a preferred food source of salps (Moline et al, 2004) while the southern WAP ecosystem is dominated by diatoms, the preferred food of krill (Haberman et al, 2003). Diatoms largely benefit from retreating sea ice cover where cryptophytes do not, continuing towards a reduced ice environment could change the community composition in the favor of salps (Huang et al, 2012). Quantifying how much carbon export is naturally occurring in the oceans is critical to understanding how it is enhanced/diminished with iron fertilization. At this point there is not a well supported baseline to compare against (Carrillo et al, 2003) and many iron limitation studies have stated vastly different export values. (Strong et al, 2009). Furthermore, there are many side effects to iron fertilization that have not been studying in great depth. Increased carbon dioxide levels in the ocean cause the ph to decrease. The ocean has already experienced an average 0.1units decrease in ph over the last century due to increased CO 2 in the atmosphere. Deliberately causing more to dissolve into the ocean could further acidify the surface ocean ph since not all additional CO 2 absorbed would be sequestered. Future studies in iron fertilization as well as natural carbon export are imperative to deciding if fertilization is even a cost effective mechanism for sequestering carbon and if the overall changes in the ocean ecosystems are beneficial or detrimental. REFERENCES

6 Bruland, K. W., and M. W. Silver (1983), Sinking rate of fecal pellets from gelatinous zooplankton (salps, pteropods, doliolids), Mar. Biol., 63, Carrillo, C. R.C. Smith, D. M. Karl (2003) Processes regulating oxygen and carbon dioxide in surface waters west of the Antarctic Peninsula, Marine Chemistry 84 (2004) Ducklow, H. K. Baker, D. G. Martinson, L. B. Quetin, R. M. Ross, R. C. Smith, S. E. Stammerjohn, M. Vernet and W. Fraser (2007), Marine Pelagic Ecosystems, the West Antarctic Peninsula, Phil. Trans. R. Soc. B Haberman, K. L., L. B. Quetin, and R. M. Ross (2003), Diet of the Antarctic krill (Euphausia superba Dana): II Selective grazing in mixed phytoplankton assemblages, J. Exp. Mar. Biol. Ecol., 283(1 2), Huang, K., H. Ducklow, M. Vernet, N. Cassar, and M. L. Bender (2012), Export production and its regulating factors in the West Antarctica Peninsula region of the Southern Ocean, Global Biogeochem. Cycles, 26, GB2005, Moline, M., H. Claystre, T. Frazer, O. Schofield, and M. Vernet (2004), Alteration of food web along the Antarctic Peninsula in response to a regional warming trend, Global Change Biol., 10, Montes- Hugo, M., C. Sweeney, S. C. Doney, H. Ducklow, R. Frouin, D. G. Martinson, S. Stammerjohn, and O. Schofield (2010), Seasonal forcing of summer dissolved inorganic carbon and chlorophyll a on the western shelf of the Antarctic Peninsula, J. Geophys. Res., 115, C03024 Trends in Atmospheric Carbon Dioxide - Mauna Loa. National Oceanic & Atmospheric Administration Strong, A.L., J. J. Cullen, S. W. Chisholm (2009), Ocean Fertilization: Science, Policy, Commerce, Oceanography, Vol. 22, No Waters, K.J., Smith, R.C., Palmer LTER: a sampling grid for the Palmer LTER program. Antarctic Journal of the United States 27,

7 APPENDIX Figure 1. The study site for the Pal- LTER series. The sites are located off the Western Antarctic Peninsula. Sampling sites are aligned in rows with 20km between each site and 100km between each row, with some near shore sites added.

8 Dissolved Oxygen (mg/l) Chlorophyll a compared to dissolved oxygen concentrations R² = Chlorophyll a (µg/l) Figure 2. The relationship between oxygen and chlorophyll a for all stations, all years. There is no correlation (- 0.03). Chlorophyll a compared to Dissolved Inorganic Carbon concentrations DIC (µmol/kg) R² = Chlorophyll a (µg/l) Figure 3. The relationship between dissolved inorganic carbon (DIC) and chlorophyll a for all stations, all years. There is a correlation of

9 DIC (µmol/kg) DIC and oxygen change from offshore to inshore Longitude Figure 4. Change in DIC and Oxygen while moving from offshore to inshore. All samples taken between 64.2 S and 64.8 S, during January Longitude is plotted on the x- axis with the more western stations to the left. DIC Oxygen Oxygen (mg/l) Figure 5. Stations included in transect for spatial analysis of DIC and Oxygen in above graph. The bathymetry, including shelf break is visible.

10 Chlorophyll a (µg/l) Mean Chlorophyll a and DIC for January in WAP DIC Chlorophyll a DIC (µmol/kg) 0.00 Jan- 03 Jan- 04 Jan- 05 Jan- 06 Jan- 07 Jan Year Figure 6. DIC and Chl a concentrations averaged for January of each year. When Chlorophyll a is highest, DIC is at the lowest average.