Methane and nutrient dynamics in a tidal flat area of the Southern North Sea [Elektronische Ressource] : time-series and budgets = Dynamik von Methan und Nährstoffen im Wattenmeer der südlichen Nordsee / von Maik Grunwald

Maik Grunwald

Sujets

• Naturwissenschaften

Informations

• Publié par: carl_von_ossietzky_universitat_oldenburg
• Publié le: 01 janvier 2009
• Nombre de lectures: 9
• Langue: English
• Poids de l'ouvrage: 7 Mo

Extrait

Methane and nutrient dynamics in a tidal flat
area of the Southern North Sea:
Time-series and budgets

Dynamik von Methan und Nährstoffen im
Wattenmeer der Südlichen Nordsee:
Zeitserien und Budgets


An der
Fakultät für Mathematik und Naturwissenschaften
der Carl von Ossietzky Universität Oldenburg
angenommene


Dissertation zur Erlangung des Grades
Doktor der Naturwissenschaften
(Dr. rer. nat.)


von

Maik Grunwald
geboren am 02.06.1972 in Cloppenburg


































Erstreferent: Prof. Dr. H.-J. Brumsack
Koreferent: Prof. Dr. J. Rullkötter

Eingereicht am: 03.06.2009
Disputation am: 08.07.2009



Abstract

Abstract
To investigate the dynamics of biogeochemical processes in temperate tidal flat
ecosystems a time-series station was established in the tidal inlet between the islands of
Spiekeroog and Langeoog (NW Germany). Realisation of this station was carried out within
the scope of the DFG Research Group “Biogeochemistry of tidal flats” which was established
in 2001 at the ”Institute for Chemistry and Biology of the Marine Environment” in
Oldenburg. Among others, one major goal of this Research Group was to investigate
exchange processes of dissolved and particulate matter between the tidal flat area and the
open North Sea.
The time-series station provides the opportunity to determine continuously physical,
chemical, and metrological parameters in a high resolution throughout the year, even during
storms and ice winters. Inside the pole of the station, sensor tubes are installed at different
water depth in direction of the prevailing tidal currents. The tubes are equipped with sensors
to determine temperature, conductivity, and pressure. Furthermore, the uppermost sensor tube
allows retrieval of surface water for nutrient analysis. In order to study the dynamics of
nutrients (silica, phosphate, nitrite, and nitrate) and methane, a setup for continuous
determination of these metabolites of organic matter degradation has been established.
Nutrients were determined hourly from April 2006 by autonomous nutrient analysers,
whereas methane was measured during several months in 2005 by a sensor mounted outside
the pole.
In addition to measurements at the station, methane was detected during several
sampling campaigns in the German Bight, the River Weser, in pore waters of tidal flat
sediments, tidal creeks, and in the freshwater contributed to the study area via a flood-gate in
Neuharlingersiel. The resulting spatial distribution of methane demonstrated a clear trend of
decreasing concentrations with increasing distance from the coast, which points towards a
source for dissolve methane in the coastal areas. This assumption was supported by high
resolution measurements of methane at the time-series station revealing distinctly higher
concentrations during low tide. Pore water concentrations down to 4.5 m depth emphasised
deep sediments to be the loci for enhanced methane production. Mathematical modelling
showed that pore waters of deeper sediment layers are transported advectively to the tidal flat
margin and finally into the open water column of the back barrier area. Two further model
approaches, which estimated export budgets from the study area into the open North Sea,
resulted in a net export of dissolved methane in an order of magnitude between the Wash
estuary and River Elbe. Therefore, it can be concluded from this study, that methane
- I - Abstract

contributed by tidal flat areas has to be considered in budget calculations for the southern
North Sea.
Concentrations of the nutrients silica, phosphate, and nitrite plus nitrate (NO ) x
determined at the time-series station revealed different seasonal patterns as well as alternating
tidal dependencies. Silica concentrations were highest during winter months with a pattern
seemingly influenced by zooplankton grazing pressure leading to a more plateau-shaped or
peak-shaped development, respectively. High winter levels decrease drastically in spring due
to elevated diatom growth. However, the onset of the diatom blooms varied between the years
as indicated by the silica development. Low concentrations during summer increase again
towards autumn and winter due to both decreasing assimilation by diatoms and pore water
contribution. In contrast, phosphate concentrations are highest in late summer, which is
governed by both phytoplankton assimilation/degradation and coupling of phosphate to the
iron-oxyhydroxide cycle in sediments. On the other hand, NO follows a general seasonal x
pattern with highest concentrations in winter and lowest during the vegetation period. On a
tidal scale, the nutrient patterns of the open water column are predominantly controlled by
pore water contributions as organic particulate matter is imported into the Wadden Sea and is
remineralised especially in the sediments.
It could have been shown, that decomposition of phytoplankton affects the dynamics
of redox-sensitive trace metals. In addition to elevated release of nutrients to the open water
column, breakdowns of algae blooms favour aggregation due to intense release of transparent
exopolymer particles (TEP) thus leading to formation of large organic-rich aggregates.
Deposition of such aggregates enhances microbial activity within the surface sediments and
the release of dissolved organic carbon, nutrients and trace metals like Mn, Mo, or V.
The investigation of pore waters, enriched in silica and phosphate, reveal a diffusive as
well as advective transport to the open water column, which leads to a seasonal and tidal
periodicity in the study area. In contrast, the freshwater contribution via the flood-gate is
identified to be of minor importance in terms of nutrient budgets of the back barrier area and
the German Bight. This study highlights the role of tidal flat sediments concerning nutrient
contribution to the German Bight. It could have been demonstrated, that the entire tidal flat
areas of the German Bight Wadden Sea contribute dissolved silica and phosphate in the same
order of magnitude as the sum of Rivers Elbe, Weser, and Ems, while NO is exported in one x
order of magnitude lower. Thus, tidal flat areas act as a ‘bio-reactor’ and contribute largely to
the nutrient budget of the German Bight ecosystem, as organic particulate matter is degraded
and subsequently exported as dissolved nutrients to the open North Sea.
- II - Kurzfassung

Kurzfassung
Um die Dynamik biogeochemischer Prozesse in Wattgebieten gemäßigter Zonen zu
untersuchen, wurde eine Messstation im Seegatt zwischen den Inseln Spiekeroog und
Langeoog (NW Deutschland) errichtet. Diese Station wurde im Rahmen der im Jahr 2001 am
„Institut für Chemie und Biologie des Meeres“ in Oldenburg gegründeten DFG-
Forschergruppe „Biogeochemie des Watts“ realisiert. Eine Hauptzielsetzung dieser
Forschergruppe war, Austauschprozesse gelöster und partikulärer Stoffe zwischen dem Watt
und der offenen Nordsee zu untersuchen.
Die Messstation bieten die Möglichkeit, kontinuierlich physikalische, chemische und
meteorologische Daten in einer hohen zeitlichen Auflösung über das ganze Jahr, auch bei
Stürmen oder Eisgang, zu erheben. Im Pfahlrohr der Station sind Querrohre in verschieden
Wassertiefen in vorherrschender Strömungsrichtung der Tide installiert. Diese Querrohre sind
mit Sensoren ausgestattet, die Temperatur, Leitfähigkeit und Druck messen. Zusätzlich
befindet sich am obersten dieser Querrohre eine Wasserentnahmestelle zur Nährstoffanalyse.
Um die Dynamik von Nährstoffen (Kieselsäure, Phosphat, Nitrit, Nitrat) und Methan zu
untersuchen, wurde ein Messsystem entwickelt, das diese Metaboliten, die beim Abbau
organischen Materials freigesetzt werden, kontinuierlich erfasst. Seit April 2006 werden
Nährstoffe durch automatisierte Analysatoren im stündlichen Intervall gemessen.
Methanmessungen erfolgten während mehrerer Monate im Jahr 2005 durch einen Sensor, der
außerhalb des Pfahlrohres angebracht war.
Neben Messungen an der Station wurde Methan während verschiedener
Schiffseinsätze in der Deutschen Bucht und der Weser, sowie im Porenwasser von
Wattsedimenten, in Tiderinnen und im Süßwasser, das über das Siel in Neuharlingersiel in
das Untersuchungsgebiet gelangt, untersucht. Die sich daraus ergebende räumliche Verteilung
von in Wasser gelöstem Methan zeigte eine deutliche Tendenz abnehmender Konzentrationen
in Richtung offene Nordsee, was auf eine Methanquelle in den Küstengebieten hinweist.
Diese Annahme wurde durch zeitlich hoch aufgelöste Methanmessungen an der Messstation
unterstützt, die merklich höhere Konzentrationen während der Niedrigwasserphase aufwiesen.
Porenwasserkonzentrationen, die bis zu einer Sedimenttiefe von 4,5 m gemessen wurden,
brachten hervor, dass tiefere Sedimentregionen Orte erh&#