MACROFAUNAL DIVERSITY AT HAUSGARTEN
36. GRADIENTS IN THE NEAR BOTTOM WATER COLUMN
Eberhard Sauter1, Melanie Bergmann1, Bruno Bombled 2,
The exchange of solutes produced, consumed, or modified by benthic organisms as well as early diagenetic processes follows and governs biogeochemical gradients at the sediment-water interface. Whereas biogeochemical processes within the sediment have been interpreted on the basis of sediment and pore water depth profiles, bottom water gradients had been hardly taken into account in the past. Although the assumption of an equal distribution of solutes within the bottom water column can be a good approximation under certain conditions, it has been recognized during the last decade that dissolved matter fluxes through the sediment-water interface strongly depend on near-bottom flow regime (Huettel and Gust, 1992; Huettel et al., 1996; Asmus et al., 1998; Boudreau et al., 2001). In fact, many especially microbially mediated processes take place within the benthic boundary itself or close to the interface whereby residence times of particles depend on the flow regime close to the seabed. Most of the studies on these processes have been performed in shallow waters whereas there are so far much less data from deep-sea environments.
As the bottom flow regime determines transport, mixing and interfacial exchange rates, one of the HAUSGARTEN projects deals wit the effects of different bottom currents set up experimentally by the installation of a flume at Station HG S-3 in 2003 and the deployment of current meters during this expedition (see following section).
During this expedition, sampling of different individual layers of the near-bottom zone was projected. Bottom water was to be analyzed for oxygen concentration and nutrients. Water was also to be filtered in order to determine the δ15N and δ13C signals at particulate matter in the near-bottom zone. This isotope signature serves as a base line for further food web studies. As particulate organic matter is the ultimate primary food source to fuel benthic life, the investigation of isotope signatures are hoped to allow tracing of the carbon and nutrient metabolism in the food chain of this deep-sea environments.
Work at sea
This was carried out by means of a specific bottom water sampler (BoWaSnapper, Fig. 36.1) developed at the AWI. The system described in more detail by Sauter et al. (2005), basically consists of 6 horizontal sampling bottles arrayed in 20, 37, 66, 96, 150, and 208 cm altitude on a turnable centre pillar of ~2 m height. The sampler
36. GRADIENTS IN THE NEAR BOTTOM WATER COLUMN
is released at the seafloor by a bottom contact switch. However, bottles are closed only ~10 min after the touch down, when the resuspension cloud has disappeared.
Prior to closure, bottles are turned into the bottom current by a current vane in order to guarantee for water exchange while standing at the sea floor.
Fig. 36.1: Bottom water sampler (BoWaSnapper) with open sampling bottles before deployment (a), schematic view of decoupling from the ship’s motion (b).
The bottom water sampler was deployed at the following HAUSGARTEN stations (Tab.
36.1):
Tab. 36.1: Bottom water sampler stations
Station Latitude Longitude Max. cable length [m]
HG Location PS70/160-1 79° 8,08' N 5° 59,31' E 1307 HG1
PS70/169-1 79° 4,42' N 4° 14,35' E 2414 HG IV Central PS70/172-1 78° 36,59' N 5° 3,96' E 2350 HG S-3
PS70/196-1 79° 36,45' N 5° 9,04' E 2802 HG N-3
PS70/221-1 79° 8,07' N 2° 50,57' E 5585 HG 9 Molley Deep
Upon recovery, water samples were taken immediately for oxygen, nutrients and particulate organic matter. While nutrients were stored in the cool room, oxygen samples were analyzed by Winckler titration. For subsequent δ15N and δ13C analysis at particulate organic matter, water sub-samples were filtered on board. Filter samples were stored cool for subsequent isotope analysis.
Preliminary results
While nutrient analyses will be carried out back home (University Gothenburg), oxygen profiles were analyzed immediately by Winckler titration. Most of the profiles do not exhibit a monotonous decrease in oxygen towards the sea bed, indicative of rather large (turbulent) bottom flow velocities, and of possibly highly variable concentrations
ARK-XXII/1A-C
(Nilsen and Tengberg this cruise report). However, at Station HG Central, we found a slight decrease in oxygen towards the sea bed, although the concentrations only varied in a narrow range (305 - 307 µM, Fig. 36.2). In general, near-bottom oxygen values increase on the depth transect from the shallow Station HG I (1,200 m depth; 303 - 305 µM) down to HG IX (Molloy Hole, 5500 m; 304 -309 µM). Highest concentrations were found at the northernmost station HG N-3 (307-316 µM, Fig. 36.2). All values measured during this campaign were lower than those obtained in 2001. Under consideration of other data such as time series optode measurements, it has to be examined, whether the difference of these results is indicative of a long term trend or spatial and temporal dynamics in the benthic boundary system.
Fig. 36.2: Selected oxygen profiles of the near-bottom zone at stations HG IV, Central (left) and N-3, the northern most station (middle). For compa@rison at the right a profile is shown which was
obtained at HG Central station in 2001 (mind different scale).
References
Asmus, R., Jensen, M. H., Jensen, K. M., Kristensen, E., Asmus, H., Wille, A., 1998. The role of water movement and spatial scaling for measurement of dissolved inorganic nitrogen fluxes in intertidal sediments. Estuarine and Coastal Shelf Science 46, 221-232.Huettel, M., Ziebis, W. and Forster, S., 1996. Flow induced uptake of particulate matter in permeable sea beds. Limnology and Oceanography 41, 309-322.
Boudreau, B.P., 2001. Solute transport above the sediment-water interface. In: Boudreau, B.P., Jørgensen, B.B. (eds.) The benthic boundary layer. Oxford University Press, 104-126.
Huettel, M. and Gust, G., 1992. Impact of bioroughness on interfacial solute exchange in permeable sediments. Marine Ecology Progress Series 89, 253-267.
Sauter, E. J., Schlüter, M., Wegner, J., Labahn, E. (2005). A routine device for high resolution bottom water sampling, Journal of Sea Research, 54, 204-210.