Basin Scale Variability and Photoreactivity of Chromophoric Dissolved Organic Matter

1. Basin Scale Variability and Photoreactivity of Chromophoric Dissolved Organic Matter Jenna Robinson1, Rob Upstill-Goddard1 & Guenther Uher1: Carol Robinson2 1School of Marine Science & Technology, Univ. of Newcastle NE1 7RU, UK2Plymouth Marine Laboratory, Plymouth PL1 3DH, UK University of Newcastle Introduction Chromophoric Dissolved Organic Matter (CDOM) is important for a variety of ecological and biogeochemical processes because of its optical and photochemical characteristics. Renewed interest in CDOM absorbance at visible wavelengths has been fuelled by demands from optical remote sensing and modelling work. The importance of CDOM absorbance characteristics in the UV-region has also been recognised, particularly with regard to its roles in UV-B protection [1] and aquatic photochemistry [3,4]. Photochemical processes of particular interest in global biogeochemical processes include the formation of nutrients, climatically active trace gases and CO2 from DOM [3, 4, 6]. However, distribution patterns and seasonal changes of CDOM absorbance and photoreactivity are still poorly defined and are therefore constraining our ability to formulate realistic photochemical models [e.g. 8]. Large scale patterns of CDOM photoreactivity may be reflected through photochemical oxygen consumption, as has recently been suggested by Andrews et al. and Obernosterer et al. [5, 7]. Our objectives were to Investigate basin-scale distribution patterns of spectral CDOM absorbance and photochemical oxygen consumption. On three Atlantic Meridional Transect (AMT) cruises we collected spectral CDOM absorbance in vertical hydrocasts and examined photochemical oxygen consumption in on-deck incubations of selected samples. Our overall aim was to progress further towards a biogeochemical / photochemical classification of oceanic biomes along the AMT transects. Here, we present preliminary data from our ongoing work. On AMT 12, results indicated that there was bacterial contamination in some of the experiments as oxygen uptake was measured in the dark incubated samples. Therefore 0.1µm filters were used on AMT13 and AMT14. Contamination of samples occurred on AMT13 and the filtering procedure was changed for AMT14 by improving equipment and sample handling protocol. Data presented in figure 3 are values where the control samples indicated no bacterial activity, and for AMT14 where there were no bacterial communities measured in the samples. Results from AMT 12, 13 and 14 showed maximum rates of oxygen uptake of 0.8 µmol O2 L-1 10h-1, 0.3µmol O2 L-1 10h-1and 1.1µmol O2 L-1 10h-1 respectively. Previous photochemical oxidation experiments in the Atlantic ocean sampled surface seawater between 34°N, 35°W and 12°N, 48°W. Photochemical oxygen uptake of 0.9µmol O2 L-1 d-1 to 2.8µmol O2 L-1 d-1 was found after filtering through 0.2µm filters and incubating for 1 to 2 days [7]. Discussion Data gathered during the AMT cruises are still being calibrated. Evidence based on the data presented here is that profiles of CDOM absorbance may be very distinct in different biogeochemical ocean provinces; overall CDOM absorbance was highest and peak CDOM absorbance was shallowest in the upwelling. Ongoing data analysis includes a comparison of CDOM profiles from all of the biogeochemical provinces sampled during the cruises. These data will be used in conjunction with chlorophyll concentrations, bacterial abundance and activity, zooplankton community and size spectra, and other optical data in order to elucidate the controls of CDOM distribution and reactivity. The ultimate goal is to provide data that can aid in developing protocols for the remote sensing of CDOM [2]. CDOM profiles and spectral characteristics will also be examined in relation to the photo-oxidation experiments carried out during AMT 13 and 14, in order to examine any links between CDOM optical properties and the measured rates of oxygen uptake. Photochemical oxygen uptake has been measured on AMT cruises at similar rates to those found by Obernosterer [7] at the latitudes shown by figure 4. However, much lower rates were more commonly found over the three cruises. When there were no bacterial communities present in the samples, rates of oxygen consumption were measured to range from 0 µmol O2 L-1 10h-1 to 1.1µmol O2 L-1 10h-1. Rates of oxygen uptake will be compared with CDOM data, and then compared to published data to search for any trends in the rates of photochemical oxygen consumption with regard to CDOM concentration and characteristics. AMT 13 Mauritanian upwelling AMT 14 CTD 59 Latitude 18.6°N CDOM absorbance at 350nm CDOM absorbance at 350nm Fig.2 CDOM profiles from AMT 13, upwelling off Mauritania, Africa and from AMT 14 at a similar latitude. Depth in m Depth in m CTD 24 Chlorophyll CTD 25 CDOM CTD 63 22.3°N CTD23 26.9°N CDOM absorbance at 350nm CDOM absorbance at 350nm Fig.3 AMT 14 CDOM profiles form North and South Gyre. Chlorophyll Depth in m Depth in m CDOM AMT Cruises AMT 12, 13 and 14 took place in April and September of 2003 and April of 2004 respectively. The objective of the cruises was to contrast variability in CDOM absorbance properties and reactivity between different biogeochemical provinces. AMT12 and AMT14 concentrated on sampling as far into the Gyres as possible, whereas AMT 13 focused on a region of upwelling off the west African coast. Figure 1. shows the cruise tracks of AMT 12 13 and 14. Photochemical Oxygen Consumption On three AMT cruises on deck incubations were used to determine the amount of oxygen consumed during photochemical degradation of CDOM. Sea water samples from 55% light depth and the chlorophyll maximum were filtered through 0.2µm filters on AMT12, and through 0.1µm filters on AMT13 and 14. 18 bottles were then filled with the filtered seawater in replicate. One third of the bottles were chemically fixed as time=0, one third were in quartz bottles incubated in natural sunlight and the final third were incubated in the dark as controls. The samples were incubated over 10 hours of daylight, then removed and fixed for subsequentoxygen titrations using theWinkler method. AMT12 Photochemical uptake of Oxygen CDOM profiles CDOM was measured on AMT 13 and 14. Samples were taken from niskin bottles mounted on a CTD rosette which collected water between the sea surface and 300m. Sampling was carried out twice daily, during early morning (pre-dawn) and at mid-day. Following filtration through 0.2µm filters, CDOM absorbance was measured with a Tidas II UV-Visible Spectrophotometer with an Ultrapath multiple path length liquid core waveguide (World Precision Instruments). AMT13 Photochemical uptake of Oxygen Acknowledgements: This study was supported by the UK Natural Environment Research Council through the Atlantic Meridional Transect consortium (NER/O/S/2001/00680). References [1] Scully et al. FEMS Microbiology Ecology 46 353-357 2003. [2] Blough and Green Role of None Living Organic Matter in the Earth’s Carbon Cycle 23-45 1995 [3] Mopper et al Letters to Nature, Nature 353 60-621991. [4] Moran and Zepp Limnology and Oceanography42 1307-1316,1997. [5] Andrews et al Limnology and Oceanography 45 (2) 267-277, 2000 [6] Johannessen and Miller Marine Chemistry 76 271-283, 2001. [7] Obernosterer et al Limnology and Oceanography 46 (3) 632-643 2001 [8] Preiswerk D. and Najjar R. G. (2000) Global Biogeochem. Cycles 14(2), 585-598. AMT14 Photochemical uptake of Oxygen Figure1. Cruise tracks for AMT12, 13 &14 Figures 2 and 3 show representative CDOM profiles for AMT13 and 14. Profiles in figure2 are from the west African upwelling (AMT13) and from the corresponding latitude during AMT14 but located outside the upwelling. The upwelling data (AMT13) show higher overall CDOM absorbance than outside the upwelling, and the peak absorbance was located at a shallower depth. Figure 3 shows CDOM profiles taken in the North and South Atlantic Gyres during AMT14. For both gyres the chlorophyll maximum is located at a depth of about 120m. The northern gyre profile shows peak CDOM absorbance (350nm) close to the chlorophyll maximum, whereas in the southern gyre maximum CDOM absorbance occurs at about 25 m depth. Figure 4. Photochemical uptake of oxygen on AMT12, 13 and 14 55% light depth sampled 1% light depth sampled 0.1% light depth sampled