Measurement and interpretation of solute concentration gradients in the benthic boundary layer

Abstract

The coastal ocean is characterized by high exchange rates of organic matter, oxygen, and nutrients between the sediment and the water column. The solutes that are exchanged between the sediment and the overlying water column are transported across the benthic boundary layer (BBL) by means of turbulent diffusion. Thus, solute concentration gradients in the BBL contain valuable information about the respective fluxes. In this study, we present the instrumentation and sampling strategies to measure oxygen and nutrient concentration gradients in the BBL. We provide the theoretical background and the calculation procedure to derive ratios of nutrient and oxygen fluxes from these concentration gradients. The noninvasive approach is illustrated at two sampling sites in the western Baltic Sea where nutrient and oxygen concentration gradients of up to 5 and 30 µM m -1 , respectively, were measured. Nutrient and oxygen flux ratios were used to establish a nitrogen flux balance between sediment and water column indicating that 20% and 50% of the mineralized nitrogen left the sediment in form of N 2 (station A and B, respectively). The results are supported by sediment incubation experiments of intact sediment cores, measuring denitrification rates, and oxygen uptake. The presented flux ratio approach is applicable without knowledge of turbulent diffusivities in the BBL and is, therefore, unaffected by non-steady-state current velocities and diffusivities.*Corresponding author: E-mail: mholtapp@mpi-bremen.de AcknowledgmentWe thank Bernd Schneider and Anne Loeffler (Leibniz Institute for Baltic Sea Research, Warnemünde) who provided ship time on RV Professor Albrecht Penck; Gaute Lavik for assistance in mass spectrometry and data analysis, G. Klockgether, M. Meyer, and J. Schmidt for analytical assistance, F. Wenzhoefer, H. Roy, and P. Faerber for technical assistance and the crews of RV Heinke and RV Professor Albrecht Penck for excellent collaboration. This study was funded through DFG-Research Center/Excellence Cluster, "The Ocean in the Earth System," and the Max Planck Society. DOI 10.4319/lom.2011.9.1 Limnol. Oceanogr.: Methods 9, 2011Methods 9, , 1-13 © 2011, by the American Society of Limnology and Oceanography, Inc. LIMNOLOGY and OCEANOGRAPHY: METHODSespecially the bottom water flow (Tengberg et al. 2005). The incubated water needs to be mixed artificially to simulate the flow of the bottom water. However, the degree of stirring can change the concentration of suspended particulate matter, the behavior of the macrofauna, or the porewater flow in permeable sediments.In recent years increasing attention has been paid to the water layer just above the sediment-the so called benthic boundary layer (BBL)-which is characterized by turbulent boundary layer flow (Dade et al. 2001). As this layer intimately links the sediment to the water column, it is well-suited for the noninvasive observation and quantification of concentration gradients and fluxes between sediment and water column. So far, only a few studies (R...

Estimating turbulent diffusion in a benthic boundary layer

“…When combined with solute gradient measurements (Holtappels et al 2010), the new approach allows to determine the solute flux across the BBL according to Fick's law. This noninvasive flux measurement provides an alternative to the eddy correlation approach (Berg et al 2003), which allows flux measurements only for a very limited number of solutes, measurable with high temporal and spatial resolution (e.g., oxygen via microsensors).…”

“…We give two reasons why the measurement of turbulent diffusivity in the BBL is of interest. First, Sauter et al (2005) and Holtappels et al (2010) …”

Nitrite oxidation in the Namibian oxygen minimum zone

“…Water samples for high-resolution vertical nutrient profiling and 15 N-incubation-experiments were collected with a pump-CTD system (Kuypers et al, 2003). Additionally, the benthic boundary layer (BBL) was sampled at 6 depths from 30 cm to 2 m above seafloor using a bottom water sampler (Sauter et al, 2005;Holtappels et al, 2011a). NO 2 À and NH 4 þ were measured on board spectrophotometrically (Grasshoff et al, 1999, detection limit 0.01 mM) and fluorometrically, respectively (Holmes et al, 1999, detection limit 0.01 mM).…”

“…However, the presented crude nitrite budget ( Figure 5) has not taken into account any external inputs and outputs. As BBLs are characterized by intense turbulent mixing and sharp gradient of nutrients (for example, Holtappels et al, 2011a), additional NO 2 À could have been provided by the underlying sediments at these depths. Nitrite might also be transported laterally by both cross-shelf and along-shelf currents (Lass and Mohrholz, 2005).…”

Eddy‐correlation measurements of benthic fluxes under complex flow conditions: Effects of coordinate transformations and averaging time scales

“…3 is vanishing, can be easily monitored by measuring the mean oxygen gradient below the eddy-flux sampling volume (Holtappels et al 2011). These additional data can also be used to account for non-vanishing magnitudes of the storage term in the flux calculations.…”