Exchange of nutrients and oxygen between sediments and water column [Elektronische Ressource] : the role of the benthic boundary layer / von Moritz Holtappels

Mholtapp

Informations

• Publié par: universitat_bremen
• Publié le: 01 janvier 2009
• Nombre de lectures: 22
• Langue: English
• Poids de l'ouvrage: 3 Mo

Extrait

Exchange of Nutrients and Oxygen between
Sediments and Water Column:
the Role of the Benthic Boundary Layer
Dissertation
zur Erlangung des
Doktorgrades der Naturwissenschaften
dem Fachbereich Geowissenschaften
der Universität Bremen vorgelegt
von
Moritz Holtappels


Bremen, August 2009

Die vorliegende Arbeit wurde in der Zeit von Dezember 2005 bis August 2009 am Max-
Planck-Institut für marine Mikrobiologie in Bremen angefertigt.



Gutachter:
Prof. Dr. Bo Barker Jørgensen
Prof. Dr. Michael Schlüter

Prüfer:
Prof. Dr. Achim Kopf
Priv. Doz. Dr. Volker Brüchert

Weitere Mitglieder des Prüfungsausschusses:
Dr. Heiko Sahling
Meri Eichner



Datum des Promotionskolloquiums: 18. September 2009



Table of Contents 3
Table of Contents
Abstract..................................................................................................................................... 5

Chapter 1: Introduction......................................................................................................... 11
Preface.................................................................................................................................. 11
The Carbon Cycle................................................................................................................. 11
The Nitrogen Cycle .............................................................................................................. 12
Nitrogen loss processes ........................................................................................................ 14
Mineralization in marine sediments ..................................................................................... 15
Measuring the exchange of solutes across the sediment surface ......................................... 17
The benthic boundary layer - interface between sediment and water column ..................... 18
Turbulent flow...................................................................................................................... 20
The turbulent boundary layer ............................................................................................... 21
Aim and outline of the thesis................................................................................................ 24
References ............................................................................................................................ 27
Overview of Manuscripts....................................................................................................... 33
Chapter 2: Nutrient fluxes across the benthic boundary layer reveal direct coupling
of nitrification-denitrification in Baltic Sea sediments ................................. 35

Chapter 3: Estimating turbulent diffusivity in a benthic boundary layer –
the relevance of boundary layer mixing for the mass transfer
across the sediment-water interface ............................................................... 63

Chapter 4: Water column versus sedimentary nitrogen loss in the Arabian Sea ............ 97
Chapter 5: Aerobic respiration rates in the benthic boundary layer
of the Baltic and North Sea ........................................................................... 131

Conclusions and Outlook..................................................................................................... 153
Acknowledgements............................................................................................................... 155
Titles and Abstracts not included in the Thesis................................................................. 157

4

Abstract 5
Abstract
This thesis largely focuses on the transport and transformation of nutrients and oxygen across
the benthic boundary layer (BBL). The BBL denotes the water layer that is directly influenced
by the interface between the sediment and the overlying water column and which is
characterized by intense turbulent mixing, gradients in solutes and increased concentrations of
suspended particulate matter. So far, the rates and regulation of nutrient exchange across and
microbial activity within the BBL are poorly understood. The aim of this thesis is to
characterize and quantify the vertical transport of nutrients and oxygen across the BBL as
well as the microbial activity within the BBL. The vertical turbulent transport in the benthic
boundary layer is subject of two Chapters of which one focuses on the measurement and
interpretation of nutrient and oxygen concentration gradients in the BBL while the other
focuses on the dynamics of turbulent flow in the BBL and its mixing efficiency. Microbial
activity in the BBL is described for several different sites exhibiting oxic as well as suboxic
bottom waters.
The thesis presents the first study that evaluates sediment-water fluxes from solute
gradient measurements in the BBL. A newly built sampling gear was deployed to measure
nutrient and oxygen concentration gradients in the BBL of the Baltic Sea with a high depth
resolution. The concentration profiles identified the sediment as a source of ammonium,
nitrite and nitrate and a sink for oxygen. These concentration profiles were used to calculate
the ratio of nutrient flux to oxygen flux across the sediment water interface. On the basis of
nutrient-oxygen flux ratios and sedimentary C/N ratios the amount of sedimentary nitrogen
which was nitrified, denitrified and exported to the overlying water column was determined.
The results of this study indicate that nitrification at the sediment surface in Baltic Sea is an
important source of water column nitrate as well as the sole source of nitrate for sedimentary
denitrification.
To study the turbulent diffusive transport a new approach, which is based on Taylor’s
theory of turbulent diffusion, was developed to calculate turbulent diffusivity from acoustic
Doppler velocimetry (ADV) data. The new approach was tested during a study of the
dynamics of turbulent diffusivity in the BBL of Lake Constance. With common ADV, flow
velocities were measured in the BBL at variable current velocities and density stratifications.
Turbulent diffusivities were determined by using three different approaches; first, the new
approach based on Taylor’s theory, second, the logarithmic Law of the Wall and third, direct
Abstract 6
gradient-flux measurements. The results obtained from Taylor’s theory agreed well with
results from the gradient-flux measurements. In contrast, the logarithmic Law of the Wall
failed to predict plausible diffusivities whenever the boundary flow exhibited decreased
current velocities or stable density stratification. Under these conditions, the turbulent
diffusivities calculated from the gradient-flux measurement and from Taylor’s theory were as
-6 2 -1
low as 10 m s . A simple flux model was developed to predict the effect of low diffusivities
on the oxygen flux into the sediment. Results from the flux model suggest that low
diffusivities in the benthic boundary layer have the potential to decrease oxygen
concentrations and oxygen flux at the sediment surface.
As nutrients are transported across the BBL, the microbial processes in the BBL may
alter the nutrient flux and the nutrient stoichiometry and as such control the primary
production and the species composition of the phytoplankton community in the surface
waters. Microbial aerobic respiration and anaerobic nitrogen loss processes in the BBL and
the adjacent sediment and water column were studied during a cruise to the Pakistani Margin
in the Arabian Sea in Oktober 2007. Anammox and denitrification rates in sediment, benthic
boundary layer and overlying suboxic waters were determined from incubation experiments.
These experiments indicate that sedimentary nitrogen loss was dominated by denitrification
and that the contribution of anammox to the total sedimentary N-loss increased with water
depth. The areal rates of nitrogen loss from the Pakistani Margin sediments exceeded those
from the overlying suboxic water column. In the water column, anammox was the dominant
nitrogen loss process. Anammox rates in the water column were low and were detected
exclusively in the benthic boundary layer and in the oxycline of the upper water column.
Oxygen consumption rates in the oxygen minimum zone were determined from a simple flux
model. The low oxygen consumption rates in the water column corresponded with the low
anammox rates.
Aerobic respiration rates were directly measured in the benthic boundary layer of the
North Sea and the Baltic Sea. Applying oxygen optodes, oxygen respiration rates were
measured in samples from various settings that contained different amounts of particulate
organic matter. The results indicate that quantity and quality of particulate organic matter can
only partly account for the observed oxygen respiration rates and that a considerable amount
of aerobic respiration in the BBL of the Baltic and North Sea is due to the mineralization of
dissolved organic matter.

Kurzfassung 7
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