Applications of ocean transport modelling Hanna Corell

Applications of
ocean transport modelling
Hanna Corell
1
To Mikael, Tore and Elsa.
Applications of ocean transport modelling
Doctoral Thesis
Hanna Corell
Cover image: Condado beach, San Juan, Puerto Rico. Photo by Hanna Corell, 2011
ISBN 978-91-7447-496-1
©Hanna Corell, Stockholm 2012
Printed in Sweden by SU-AB, Stockholm 2012
Distributor: Department of Meteorology, Stockholm University
2
List of papers
The thesis consist of an introduction and the following papers
I.
Corell H., Nilsson J., Döös K. and Broström G. (2009) Wind sensitivity of the inter-ocean heat exchange. Tellus A, 61:5
II.
Corell H. and Döös, K. (in review) Difference in particle-transport
patterns between an open and a closed coastal area in the Baltic Sea;
high resolution modelling with advective particle trajectories.
Ambio.
III.
Corell H., Moksnes, P.O., Engqvist, A., Döös, K. and Jonsson, P.R.
(in review) Larval depth distribution critically affects dispersal and
the efficiency of marine protected areas. Marine Ecology Progress
Series.
IV.
Moksnes, P.O., Corell H., Tryman K. and Jonsson, P.R. (Manuscript) Larval behaviour and dispersal mechanisms in shore crab
larvae: Local adaptations to different tidal environments?
Reprints are made with the permission of the publishers.
In Paper I the original idea comes from Johan Nilsson. I performed some of
the model runs and the majority of the data analysis and writing. Paper II, III
and IV are all based on models that are further developments of a Lagrangian particle-tracking model originally set up by Kristofer Döös. The ideas and
implementations of these developments are mine. In Paper II I performed the
model runs, the major part of the data analysis and wrote the paper. The idea
behind Paper III originates from Per Jonsson and Per Moksnes. I did parts of
the data analysis and Jonsson, Moksnes and I wrote the paper in equal parts.
In Paper IV I did the modelling and parts of the data analysis and writing.
The idea of Paper IV originates from Per Moksnes and me. I have not been
involved in the field surveys and lab experiments in Papers III and IV.
3
4
Abstract
The advective motion of seawater governs the transport of almost everything, animate or inanimate, present in the ocean and those lacking the ability to outswim the currents have to follow the flow. This makes modelling of
advective ocean transports a powerful tool in various fields of science where
a displacement of something over time is studied. The present thesis comprises four different applications of ocean-transport modelling, ranging from
large-scale heat transports to the dispersion of juvenile marine organisms.
The aim has been to adapt the method not only to the object of study, but
also to the available model-data sets and in situ-observations.
•
•
•
The first application in the thesis is a study of the oceanic heat
transport. It illustrates the importance of wind forcing for not only
the heat transport from the Indian to the Atlantic Ocean, but also for
the net northward transport of heat in the Atlantic.
In the next study focus is on the particle-transport differences between an open and a semi-enclosed coastal area on the Swedish
coast of the Baltic Sea. The modelled patterns of sedimentation and
residence times in the two basins are examined after particles having
been released from a number of prescribed point sources.
In the two final studies the transport-modelling framework is applied
within a marine-ecology context and the transported entities are larvae of some Scandinavian sessile and sedentary species and noncommercial fishes (e.g. the bay barnacle, the blue mussel, the shore
crab and the gobies). The effects of depth distribution of dispersing
larvae on the efficiency of the Marine Protected Areas in the Baltic
Sea are examined. Further, the diversity in dispersal and connectivity depending on vertical behaviour is modelled for regions with different tidal regimes in the North Sea, the Skagerrak and the Kattegat.
The spatial scales dealt with in the studies varied from global to a highly
resolved 182-metres grid. The model results, excepting those from the global
study, are based on or compared with in situ-data.
5
6
Contents
List of papers ................................................................................................. 3 Abstract .......................................................................................................... 5 Contents ......................................................................................................... 7 1. Introduction ................................................................................................ 9 Heat transport in the world ocean ................................................................................. 10 Sediment as a potential carrier of radionuclides ........................................................... 11 Larval dispersal and connectivity .................................................................................. 13 Larval behaviour: the European shore crab ................................................................. 16 2. Methods ................................................................................................... 19 General circulation models ........................................................................................... 19 The TRACMASS Lagrangian trajectory model ............................................................. 22 Stream functions and the heat-flux potential ................................................................ 23 Bio-physical modelling in marine ecology ..................................................................... 27 3. Summary of Papers ................................................................................. 29 Paper I: Wind sensitivity of the inter-ocean heat exchange .......................................... 29 Paper II: Difference in particle-transport patterns between an open and a
closed coastal area in the Baltic Sea; high-resolution modelling with advective
particle trajectories. ...................................................................................................... 31 Paper III: Larval depth distribution critically affects dispersal and the efficiency
of marine protected areas. ............................................................................................ 32 Paper IV: Larval behaviour and dispersal mechanisms in Shore Crab larvae:
Local adaptations to different tidal environments. ........................................................ 33 Outlook ......................................................................................................... 35 Acknowledgements ...................................................................................... 37 References ................................................................................................... 38 7
8
1. Introduction
Everything residing in the ocean that does not have the ability to outswim
the currents has to follow the flow. This goes for everything from small particulate matter such as aggregated clay particles to the individuals of the
ocean sunfish (Mola Mola, Fig. 1). Formed like a wind-dispersed seed this
giant fish use the ocean currents as its main means of transportation. Even
chemical substances will have a life cycle that is partly determined by the
circulation. But just the advection will rarely gives the full story of the
transport. Vertical and horizontal changes in position, due to chemical dispersion, swimming, changes in buoyancy or other processes, will change the effect of the advective transport.
This thesis consists of papers on subjects as far apart as
ocean heat transport and the dispersal and connectivity
of the European shore crab. The common thread is the
transportation modelling, and how computational methods in physical oceanography can contribute to
knowledge in research disciplines not entirely focused
on the dynamics of the water. The methods span from
“old-school” Eulerian circulation models to individualFigure 1.
based Lagrangian particle tracking in different settings,
Mola Mola
the choice of modelling tool depending on the characteristics of the object under consideration.
Paper I reports a model experiment of how the wind in the southern hemisphere affects the global heat transport, and in particular the heat exchange
between the Atlantic and the Indian Ocean. In Paper II the transport patterns of fine sediments in two coastal areas with different oceanographic
setting is examined. Papers III and IV concern the dispersal of marine organisms in early life stages, and how the vertical behaviour of the drifting
organisms affects the connectivity between populations. The spatial resolution of the studies ranges from coarse to very high. In the heat-transport
experiment a resolution of 2° (about 220 km) is used. The regional modelling in The Baltic and the North Sea in Papers III and IV have a resolution
of 2 nautical miles (3.7 km) and the local sediment modelling uses 182metre grid boxes, a study which must be considered as very highly resolved.
9
Heat transport in the world ocean
A considerable part of the large-scale ocean circulation is driven by density
gradients created by fluxes of heat and freshwater at the surface. This is
frequently denoted the thermohaline circulation, with “thermo” and “haline”
referring to the temperature and salinity in the ocean determining the density. This circulation and its pathways play an important role for the climate
and a simple model describing the interbasin exchanges was introduced by
Broecker (1987,1991) and Gordon (1986). In short: the far-reaching northward extension of the Atlantic together with a larger evaporation than precipitation in the tropical Atlantic makes the Labrador Sea and the Nordic
Seas waters very cold and saline. This dense water sinks and flows southward as a deep current. Part of the water is transported by the Antarctic circumpolar current into the Indian and Pacific Oceans. The flow moves
northward and upwells in the northern parts of these oceans. This drives a
warm, shallow return-flow moving from the Northern Pacific through the
Indonesian Archipelago and the Indian Ocean towards the South Atlantic via
the Agulhas Current at the southern tip of Africa. There it is joined by the
water having remained in the Antarctic circumpolar current, which entered
the South Atlantic by the Drake Passage just south of the southern tip of
South America. The water then flows northward in the Atlantic Ocean (Fig
2). This descriptive model is a somewhat over-simplified explanation of the
global overturning. It was, when it was made, an attempt to provide an overall picture of the interbasin water exchange, and though the research community agrees on it being too simple, there is no complete agreement in
which ways.
Figure 2. A simplified picture of the global overturning circulation. From
Kuhlbrodt et al. (2007), reprinted with permission.
10
The Atlantic overturning exerts strong control on the amount of heat that
is transported by the ocean and on the cycling and storage of chemical species such as carbon dioxide in the deep sea, and thus has a strong influence
on the Earth’s climate (Kuhlbrodt et al. 2007). The net northward heat
transport in the Atlantic is the reason for the comparatively mild climate in
north-western Europe. The sources of this heat and the mechanisms behind
the transport are still being debated. Without going into detail, two main
mechanisms are presently discussed. One suggests that it is mixing of heat,
downward across surfaces of equal density, into the abyssal waters, by winds
and tides that drive the overturning (Munk and Wunsch 1998). The other
view states that it is wind-driven upwelling in the Southern Ocean that is the
main driver (Toggweiler and Samuels 1995, 1998). In Paper I a number of
different wind-forcing scenarios are tested in a model to study the effect of
the wind on the heat transport between the Indian Ocean and the Atlantic.
Sediment as a potential carrier of radionuclides
Sedimentation and resuspension are controlling factors of the productivity in
shallow-water ecosystems, through water enrichment by nutrients originating from the sediment. This input is related to desorption of nitrogen and
phosphorus from resuspended particles and from mixing of pore-water nutrients into the water column (Simon 1989, Wainright and Hopkinson Jr.
1997). Resuspension might also be responsible for transport of sedimentbound nutrients from shallow to deeper waters in coastal areas (Håkanson
and Floderus, 1989). A consequence of this redistribution of nutrients can be
enhanced phytoplankton growth during the summer season. In other regions
the resuspension may lead to normal nutrient concentrations where the terrestrial supply of nutrients is cut short due to efficient wastewater treatment.
Excess nutrients can be considered as pollutants, and together with heavy
metals and other polluting substances a large portion of them enter the sea
by the river run-off. This riverine load of pollutants consists of pollution
from different sources within the rivers catchment areas, such as industrial
plants, waste-water treatment plants, farmlands and managed forests, as well
as the natural background load. The polluting substances can be transported
long distances with the particles, and thus knowledge of the sediment distribution in some ways represents knowledge of the distribution of the pollutants. In Paper II the distribution of fine sediment in the coastal basins outside two of Sweden’s nuclear power plants is studied. The pollution scenario
motivating the model study is that of a leakage from a potential repository
11
for nuclear waste. Radionuclides following the ground water from the repository would leak out through the seabed in the coastal zone. Adsorbing to
sediment particles the radionuclides then follow the particle flux out into the
sea basin.
The two investigated regions, Simpevarp and Forsmark, are both located
within the Baltic Sea, along the coast of the Swedish mainland (Fig. 3). In
the beginning of the 1970s the Swedish Nuclear Fuel and Waste Management Co started the process of finding a possible site in Sweden for a nuclear repository. Forsmark and Simpevarp, already being the locations of nuclear power plants, were the final two candidates. Up until 2011, when the
decision to choose Forsmark as the primary candidate was made, both areas
were thoroughly investigated with field sampling programs and with extensive modelling efforts in order to describe the physical and biological settings in the areas and to evaluate present and future land use and risk management.
From an oceanographic point of view the two areas are very different.
Forsmark, situated about 200 km north of Stockholm, is a rather closed area,
dominated by Öregrundsgrepen. It is a funnel-like sound with a wider end
toward the north, shielded by the island of Gräsö. The largest part of the area
is shallow, but in a few places in the channel running along Gräsö the depth
reaches 50 metres. In contrast, the Simpevarp area is almost completely open
to the Baltic Sea. Located just in level with the northern tip of the island
Öland it faces open water in every direction but the south-east. The area is
somewhat deeper than Forsmark; most of the eastern part of the domain is
deeper than 30 meters and in the north-eastern part depths of around 100
meters are reached.
12
1. Forsmark
Gulf
of
Bothnia
2. Simpevarp
1
Skagerrak
2
Kattegat
North Sea
Wadden
Sea
Baltic
Proper
Danish
Straits
Fig 3. The study areas in papers II, III and IV: the North Sea, Kattegat,
Skagerrak and the Baltic Sea.
Larval dispersal and connectivity
How are different populations of marine organisms connected with each
other and how does connectivity affect population persistence? In computer
science and mathematics, connectivity is one of the basic concepts of graph
theory. A graph is a mathematical structure used to describe the pairwise
relations between objects (nodes), and the connectivity of a graph is a measure of its robustness as a network. For example, how many and which connections can be removed without destroying the whole network, and if one
more node were put in the network where would it have the most effect? In
marine ecology, population connectivity can be defined as “the exchange of
individuals among geographically separated sub-populations that comprise a
meta-population (Hanski, 1999); set in the context of benthic-oriented marine species, population connectivity encompasses the dispersal phase from
reproduction to the completion of the settlement process” (Cowen and
Sponaugle, 2009). Most of this exchange takes place during the pelagic lar13
val phase that most marine organisms undergo (Fig 4). This is the developmental stage when eggs or newly hatched organisms drift with the currents
and can be transported long distances. An understanding of marinepopulation connectivity requires knowledge of the biological and physical
processes that govern this larval dispersal (Cowen et al. 2007). There are
numerous evolutionary explanations for the pelagic dispersal phase. The
larvae can exploit different food sources compared to the adults, and the
pelagic phase is a way of avoiding benthic predators (even though the larvae
expose themselves to a whole new set of predators in the open water). However, a major explanation for pelagic larvae is that many marine organisms
are more or less stationary as adults, either sessile, such as barnacles and
mussels, or sedentary as crabs and reef fish. This makes them dependent on
a mechanism to distribute their young to colonize new areas. The populations are connected to other populations through larval dispersal, forming
meta-population networks. Understanding the source-sink dynamics of meta-populations is therefore of paramount importance for conservation and
restoration of marine populations (Lipcius et al. 2008). Connectivity may
also have evolutionary consequences where increased gene flow through
larval dispersal may prevent loss of genetic diversity through inbreeding and
genetic drift. On the other hand, high gene flows may constrain evolution of
local adaptation.
To reduce the stress of over-fishing and loss of biodiversity, restrictions
to the human use of the ocean are needed. To achieve this, implementation
of Marine Protected Areas (MPAs) and no-take nature reserves are considered effective instruments (Lester et al. 2009). MPAs in this sense are areas
where human activities have been restricted for management and/or conservational purposes. If an MPA is to be effective in sustaining a population,
the reproduction of the species must either take place through local recruitment within the MPA or through network persistence, where larvae are imported from other MPAs (Hastings and Botsford 2006). Since single MPAs
are rarely made big enough to sustain a population effectively, networks of
several MPAs are needed. It is, however, important that these networks are
designed as efficiently as possible with regard to the behaviour and dispersal
distance of target species. This becomes particularly complex when the species show long-distance larval dispersal, like many marine invertebrates and
fish (Kinlan and Gaines 2003).
14
Fig 4. Marine larvae of different species and in different phases of evolution.
Pictures from Cowen et al. (2007) and Werner et al. (2007). Printed with
permission from Oceanography.
The influence of larval vertical behaviour on the efficiency of MPAs in the
Baltic Sea is studied in Paper III. The Baltic is a shallow, brackish, intracontinental sea in northern Europe. It is the world’s second largest brackish
sea and consists of a number of sub-basins, divided by sills and other mor-
15
pho- and bathymetrical formations (Fig 3). Three narrow straits between the
Danish mainland and islands and the Swedish mainland, limit the water exchange with Kattegat and the North Sea. Due to the morphology and proximity to the saline inflow the oceanographic features of the sub-basins differ.
To a large extent the circulation is topographically constrained in all basins.
The salinity varies from marine conditions (≤ 34‰) in the inflow water from
Kattegat, to brackish (6.5-8.5‰) in the surface waters of the Baltic Proper,
to almost fresh (2-4‰) in the northern part of the Gulf of Bothnia
(Leppäranta and Myrberg 2009). Due to this gradient, the distribution of
many organisms ends at the border of these sub-basins, and the biodiversity
decreases with the salinity. This brackish estuarine setting with the different
sub-basins, the salinity gradient and the fact that the Baltic is almost unaffected by the tide makes the Baltic a very special sea.
Larval behaviour: the European shore crab
The European shore crab Carcinus maenas (Fig. 5), modelled in Paper IV,
is native to the European and North African coasts from the Baltic Sea to
Iceland and Morocco. It is also a successful invasive species travelling with
ballast water. It has been sighted all around the world and has settled in locations in North America, South Africa and southern Australia. The shore crab
has a pelagic larval phase lasting between 25 and 40 days, depending on
temperature. It settles in shallow coastal areas where it actively chooses
certain types of habitats (Moksnes 2002).
Fig. 5. European shore crab, Carcinus maenas, from Rathbun (1930)
16
Depending on the tidal amplitude in the area where the shore crab lives the
larvae show different behaviours during their larval phases. In areas with a
large tidal amplitude, such as in Portugal and Wales, the larvae are hatched
mainly just in the beginning of ebb after a spring tide during the night. On
these occasions the off-shore transport is especially strong and the larvae
take advantage of the tidal motion of the water, changing drifting depth in
phase with the M2-tidal rhythm. By residing at the surface during ebbing
tide, when the water moves away from the coast, and closer to the bottom at
flood tide they are moved out of estuaries and near-shore areas. After about
three quarters of the pelagic stage they switch phase and swim in the surface
at high tide, thus being transported back to shore. It has been shown in laboratory experiments that larvae hatched in aquaria with a constant environment show this tidal rhythm and accumulate in the surface every 12
hours 25 minutes (Zeng & Naylor 1996c). In Kattegat and along the Swedish
coast of Skagerrak, where the tidal amplitude is small, this behaviour has not
been found either in the laboratory or in situ (Quieroga et al. 2002). Instead
most of the larvae are concentrated below the thermocline (15-30 m) during
the day and in the surface at night. According to the "diurnal sea breeze hypotheses" (Shanks 1995) this behaviour in combination with the sea breeze
disperses larvae in their early stages of development off the coast and after a
shift the late-stage larvae back to shore.
The study area where the effect of tides on the dispersal of the shore crab
has been investigated comprises regions with tidal influence spanning from
very strong (> 2 m) in the area around the English Channel (Huthnance
1991) to almost negligible (< 10 cm) in the southern parts of Kattegat (Fonselius 1995). The maximum speed of the tidal current is measured to around
100 cms-1 at the outlet of the English Channel and close to the coast in the
southern parts of Wadden Sea, declining northward to less than 20 cms-1 at
the Skaw (Otto et al. 1990).
The North Sea on the north European continental shelf is a shallow sea
with mean depth of about 70 meters and the dominant dynamical feature is
the tidal motion. Together with the net effects of the wind-driven circulation,
the tidal residual forces the basic circulation pattern, creating a preference
for cyclonic (counter-clockwise) circulation for the wind-induced currents.
(Otto et al. 1990). On the mainland side of the sea this results in a number of
coastal currents in north/north-easterly direction, e.g. the Jutland current,
which transports North Sea water into Skagerrak and Kattegat.
In Kattegat, which is very shallow with a mean depth of only 23 m, the
wind-induced surface transport dominates the circulation. A strong underlying component of the flow is the Baltic current that brings the outflow wa-
17
ters from the Baltic Sea northward. It flows northward, but can be temporarily reversed at times with strong westerly winds. The Baltic current continues
in Skagerrak where the Norwegian coastal current carries the transport of the
Baltic Sea outflow to the Norwegian Sea. It is the strongest permanent current in the whole modelled area, with velocities up to 150 cms-1 (Fonselius
1995).
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2. Methods
General circulation models
A circulation model is a mathematical idealization of the circulation of a
fluid on a rotating sphere. In this case the fluid is the ocean. The circulation
model uses the Eulerian specification of the flow field, i.e. it follows the
development of the fluid at a fixed location or within a specified control
volume (Fig. 6). In this volume the horizontal and vertical velocities, temperature, salinity and other descriptive variables are calculated at each time
step. But only one control volume will rarely do the trick; the more control
volumes, or grid boxes, the fluid basin is divided into, the higher spatial
resolution the calculations will have. The alternative way of describing the
fluid is Lagrangian, further discussed in the next section, which follows the
development of the descriptive variables around a fluid parcel as it moves
through the fluid. An analogy describing the two different specifications
would be (Eulerian) to observe a river from a position on the riverbank or
(Lagrangian) from a boat moving on the river.
Euler Lagrange Fig 6. The Eulerian and Lagrangian ways to describe the flow.
19
The basic concept behind circulation models is that a fluid in an enclosed
system, governed by a set of equations describing the physics of the fluid, is
bound to behave in a fairly predictable way. The fluid will try to redistribute
heat, salt and momentum in the water body with motion as a result. The
governing Navier-Stokes equations are discretized on a grid and solved to
obtain the state of the ocean at the next time step (Fig 7). To start up, the
descriptive variables can be set to 0, to some mean value, or the state of the
ocean from another experimental run. After a spin-up period applying constant external forcing the fluid reaches a state from which the simulations
can start. Model experiments can be crudely divided into three major categories; forecasts, hindcasts and hypothetical experiments. The forecasts are
much less common for the ocean than the atmosphere, but most major modelling centres produce them. Hindcasts are descriptive model runs of times
passed that use measured values of the forcing, e.g. wind speed, precipitation, temperature and pressure at the sea surface, where they are available.
The output from these models is used to increase our understanding of the
dynamics of the oceans. A special type of hindcast dataset is the reanalysis.
It is a model run where in situ validation data, such as salinity, temperature
and/or water velocity, is incorporated in the model run. The validation
measurements can be from ships carrying oceanographic devices, from
buoys, drifters or other instruments measuring oceanographic variables. The
reanalysis output datasets can give a more accurate representation of the
state of the ocean than the regular hindcast, but are much more costly to
produce.
Fig. 7. A tri-polar global model grid. It has two “north-poles”, placed on
land, instead of the real one and by this avoiding the trouble of having a
singular point in the ocean.
20
Since there are important processes in the ocean occurring on scales smaller
than the models can resolve, down to millimetres and less than seconds, they
need to be represented in some other way. This is done by parametrizations,
viz. describing something by the effect it has rather than calculating the actual process. A commonly discussed example is clouds in climate models. The
clouds are too small to be resolved in the calculations, but since they have an
effect on the heat and energy budgets in the climate system they cannot be
ignored just because they are small. Describing these effects in a way that
captures the dynamics of the clouds is presently a field of intense research.
An example of parametrized processes in ocean models is the transport and
mixing due to eddies smaller than the grid size. The various ways in which
parametrizations of sub-grid processes are undertaken are the main reason
different models with the same resolution can yield somewhat different results. (Other factors contributing to these differences can be the methods
used to do forward-in-time integrations of the equations and the choice of
data to force the model at the borders, among other things.)
As computational resources become less expensive, the temporal and spatial resolution of the models increases. At the time of writing several global
models have versions with a spatial resolution of 1/12° (about 9.25 km) or
below. A model with 1/4° resolution is said to be “eddy-permitting”, while a
resolution if 1/12° is “eddy-resolving”. However, depending on what is being analysed, the highest possible resolution and a realistic forcing and bathymetry might not be the best way to conduct the study. Simplifying the
set-up of the model can be a way to filter the outcome and thus facilitate the
interpretation. An example could be using a simplified morphology, such as
having the land in the model domain in the form of a narrow border dividing
a basin, or using a flat bottom bathymetry. The MITgcm model (Marshal et
al. 1997) set-up in Paper I uses rectangular basins with flat bottoms and thin
strips of land representing the American and Eurasian/African continents
(Fig 8). This set-up captures the dominant features of importance to this
study, i.e. a narrower "Atlantic" basin and a broader "Indo-Pacific" connected through a periodical "Southern Ocean", allowing for circumpolar flow.
Simplified forcing fields are another common way to make experiments
more idealised. The reference wind forcing in Paper I is a zonal-mean wind
profile, applied at all longitudes. The effect of this wind forcing is then
compared to those of a number of other zonal wind profiles, with everything
else equal. To examine the response to the forcing the model is run until a
steady state is achieved, i.e. when the changes in the descriptive variables
between two time steps are smaller than some threshold value and thus
“nothing more happens” in response to the wind field.
21
Fig 8. Conceptual 3D image of the idealized world-ocean domain from Paper I, with an American and an African continent with different southward
reach, a circumpolar channel at the Drake Passage and an Atlantic basin
that extends farther northward than the Indo-Pacific. (The proportions are
grossly exaggerated, the actual domain can be seen in figure 10c,d).
The TRACMASS Lagrangian trajectory model
The amount of data produced by the circulation models increases along with
the resolution. The more available, the more detailed studies can be made,
but one can also end up in a situation where one cannot see the forest for the
trees. Here the Lagrangian trajectories can be a useful tool for analysing the
data. Apart from being a very graphical tool, making it possible to visualize
(in 3D if so required) where the tracked water parcels actually go or come
from, the technique also has the benefit of being mass conservative. To be
able to model fluid dynamics at all, you make the assumption that the fluid
is incompressible. This means that the same amount of fluid that flows in to
a control volume, as described in the previous section, also has to flow out
of it. There is no way of putting more water in by compressing the fluid that
is already in the volume. Because of this the Lagrangian trajectories can
each be attributed a specific volume, and the volume transport through any
chosen section in the ocean can be calculated by integrating over all trajectories passing.
The dispersal of the larvae in Paper III and Paper IV, as well as the
transport of sediment particles in Paper II, are calculated using various developments of the Lagrangian trajectory scheme TRACMASS (Döös 1995,
Blanke and Raynaud 1997, De Vries and Döös 2001). This is a particletracking model that calculates transport of water using data fields of velocity, temperature and salinity from 3D general-circulation models. The model
is run “off-line”, i.e. it uses stored data as input. The benefit of the off-line
working mode is that it permits calculation of a vast number of trajectories
22
to a small computational cost compared to making the same calculations online within the circulation model. TRACMASS can determine trajectories
both forwards and backwards in time between any sections or regions in the
ocean. Given a stationary velocity field, the model calculates exact solutions
for the trajectory paths. When using time-dependent velocity fields the velocity is assumed to be constant over successive periods equal to the sampling time. The velocities from the GCM data sets are given at the sides of
each grid box, and to determine the trajectory of a given particle they are
interpolated to the position of the particle, and the successive transport of the
particle within the box is calculated analytically (Döös 1995).
Stream functions and the heat-flux potential
Two special tools in Paper I are the stream function in the latitudetemperature plane and the heat-flux potential. Both are used to examine the
effect of the wind forcing on the heat transport.
Stream functions are often used to simplify equations for incompressible
flow that satisfies the law of conservation of mass. They can also be used to
make a two-dimensional visualisation of a flow by calculating and plotting
streamlines. Streamlines are instantaneous curves tangent to the direction of
the flow field and the flow is constant along the streamlines. If the flow is
steady the streamlines coincide with the path-lines/trajectories (Kundu
1990). In oceanography the meridional overturning stream function is one of
the most common ones (Fig 9a). It is a zonal integration of the average velocity field at every depth, shown with latitude on the x-axis and depth on
the y-axis. It is used to give a visual representation of the overturning circulation; how cold saline water sinks at high latitudes in the northern hemisphere, flows southward and then turns back north again, while water from
the Southern Ocean flows northward underneath the south-going North Atlantic water, past the equator and then back. However, this average picture
provides little information of water mass transformation through changes in
buoyancy and may also give a false impression of the movements of the
various water masses, e.g. the Deacon cell in the Southern Ocean (Döös and
Webb 1994). An alternative representation is the stream function in densitylatitude coordinates (Fig 9b). It yields a thermodynamic picture of the flow,
where the velocity is averaged along lines of constant potential density rather than constant depth. This allows for a better representation of the water
masses than the depth-latitude stream function, as ocean currents tend to
follow isopycnal surfaces rather than surfaces of constant depth (Döös and
Webb 1994; Nycander et al. 2007).
23
0.
1.
2.
A
3.
4.
5.
6.
90. 80. 70. 60. 50. 40. 30. 20. 10. 0. -10. -20. -30. -40. -50. -60. -70. -80.
29.5
30.5
31.5
32.5
B
33.5
34.5
35.5
36.5
37.5
90. 80. 70. 60. 50. 40. 30. 20. 10. 0. -10. -20. -30. -40. -50. -60. -70. -80.
Fig 9. Meridional overturning streamfunction from the OCCAM model as
(A) a function of latitude and depth and (B) as a function of latitude and
potential density at 2000m. The transport is given in Sverdrups and the
stream function is counted positive (red shading) in counter-clockwise cells
and negative (blue shading) in clockwise cells. From Nycander et al. (2007),
©American Meteorological Society, reprinted with permission.
A close relative of the density-latitude stream function is the temperaturelatitude stream function used in Paper I (Fig 10). It also gives a thermodynamic picture of the flow, but contrary to the standard stream function it
lacks information about water transformations due to salinity gradients. The
purpose of using it in the analysis in Paper I is to examine the effect of the
wind on the poleward transport of heat in the idealised Atlantic and IndoPacific ocean basins.
24
a. Atlantic Ocean
realistic wind
Temperature ( C)
%#
o
Southern
Ocean
!
('
('
(#
(#
'
'
#
Southern
Ocean
c. Atlantic Ocean
no wind
%#
!
#
!
"#&
%#&
#
L atitude
%#$
"#$
!
"#&
%#&
#
Latitude
%#$
"#$
Fig 10. Stream functions in the temperature-latitude plane resulting from
model cases with (left) and without (right) wind forcing in the idealized
world ocean in Paper I.
In steady-state the vertical exchange of heat between the ocean and the atmosphere at the surface can be seen as the response of the whole fluid, and
not just the top layer. It gives a measure of the sources and sinks of heat in
the ocean (Fig. 11). Since mass and energy are conserved in the system there
will be a net transport of heat from the sources to the sinks. This horizontal
net transport is affected by the wind forcing, and is one of the things that are
investigated in Paper I. The magnitude of this transport is described mathematically by the scalar potential of the divergent component of the horizontal heat flux, the “heat-flux potential”. This is not an output from the circulation model but is found by calculating the divergence of the vertical heat
flux, i.e. the divergence of the ocean-atmosphere heat exchange. The divergence is found by solving the Poisson equation for the vertical heat flux at
every surface-point in the grid. To confirm that our idealized circulation
model captures the relevant features of the ocean-atmosphere exchange the
same calculations were made using output from the eddy-permitting OCCAM model (Coward and de Cuevas 2005; Marsh et al. 2005a). This model
is not in steady state but is still a realistic hindcast, and the time-average of
the surface heat fluxes for the years 1991-2002 was used as a proxy for a
steady-state field (Fig. 11a). The method has previously been used by Trenberth and Salomon (1994), with the difference that they used top-of-theatmosphere radiation and atmospheric reanalysis data to get a surface heat
flux estimate, while our calculations are solely based on circulation-model
outputs (Fig. 11b).
25
A
-23
-19
-15
-11
-7
-3
1
5
9
13
17 PW
B
D
C
No wind
Realistic wind
60N
C
40N
40N
20N
20N
Latitude
Latitude
60N
0
PW
D
0.1
0
-0.1
0
-0.2
20S
20S
40S
40S
-0.3
-0.4
60S
0
30
60
90
Longitude
120
150
180
60S
0
30
60
90
120
150
180
Longitude
Figure 11. (A) The heat potential field with streamlines from the OCCAM
model surface heat flux, from Paper I. (B) The heat potential in the world
from Trenberth and Solomon (1994), printed with permission. (C) and (D)
The heat potential fields, resulting from cases without and with wind forcing
respectively, from the idealized world ocean in Paper I. White potential lines
(25 1012 W apart) and black lines (5 1012 W apart) are added.
26
Bio-physical modelling in marine ecology
A fundamental part of marine ecology research is the understanding of the
sources of variability in recruitment; how marine organisms spread their
young and the various reasons for the success or failure of these recruits to
make it to adulthood. The link between physical processes and biological
conditions as a key component in describing this variability was recognised
a hundred years ago (e.g Helland-Hansen and Nansen, 1909). The recent
progress in computer science has, indirectly through the advances in physical oceanography and directly via individually based ecological modelling,
made it possible to quantify this link (Werner and Quinlan 2002).
The use of Individual Based Models (IBMs) to explore marine connectivity is based on coupled models, consisting of a particle-tracking module
working on the data from or within an ocean circulation model of some sort.
The opposite modelling approach is to consider whole populations at a time,
and fluxes of organisms rather than the dispersal of individuals. Here the
IBMs have been the preferred choice since they are “bottom-up”-models
starting with the parts (i.e. the individuals) of the system and thus allows for
more explicit biological detail than is generally possible if the whole population is handled simultaneously (Grimm 1999). Nowadays spatial explicit
IBMs are considered the best possible way to gain knowledge of marine
connectivity since empirical methods are often costly and suffer from limited spatial and temporal coverage (Paris et al. 2007, Cowen and Sponagule
2009). In some cases empirical studies are almost impossible, how can the
dispersal distance of an organism a few millimetres big that drifts with the
currents for two months be measured? In these cases the models may be the
only possible choice for a study.
Yet, the IBMs also face a range of problems. The large variation of the
governing spatial and temporal scales is one of them. Physical and biological
processes occur on multiple scales and they generally interact and overlap
(Werner et al. 2007). The scale of turbulent mixing and near-shore dynamics
(100-102 m) and the scale of boundary currents and climate-related processes
like the North Atlantic Oscillation (105-107 m) all influence the outcome of
the biological processes. The models should be able to handle the time it
takes to shift position in the water column (minutes-hours) and the full larval
duration time (weeks to months), maybe even the time for the consecutive
dispersal of several generations. To resolve everything a model with centimetre-minute-resolution that is fast and sufficiently storage-efficient to allow for yearlong integrations in model time would be needed. Unfortunately,
there are no such models yet, nor will there be in the near future. This calls
for different kinds of parametrizations of all relevant processes, both physical (as discussed earlier) and biological. However, if the focus of the study is
27
on e.g. the basin-wide dispersal of some species, as in Paper IV, the lack of
resolution at the centimetre-scale or the parameterization of absolutely everything unresolved may not make any significant difference. As a first-order
approximation the large-scale patterns of dispersal will be driven by the
large-scale ocean circulation (as opposed to small-scale coastal dynamics)
and as long as that is resolved properly answers may be obtained. As always
in modelling, you have to use the appropriate model for the question you
ask, or (maybe more commonly) ask questions that you can actually answer
with your model.
The dispersal of larvae among local populations is a complex function of
ocean circulation, spawning dynamics, the duration of the planktonic stage
and larval behaviour (Shanks 1995) and most pelagic larvae are not passively transported but show vertical swimming behaviour that leads to speciesspecific vertical distributions of larvae that may change with ontogeny or
with diurnal or tidal cycles (Forward and Tankersley 2001; Sale and Kritzer
2003; Queiroga and Blanton 2005). Most model studies that integrate larval
behaviour have focused on ontogenetic changes, how the development stage
determines behaviour and vertical position. Recent examples are dispersal of
oyster larvae in Chesapeake Bay (North et al. 2008) and in Tasman Bay,
New Zeeland (Broekhuizen et al. 2011). Studies focusing on tidal migration, such as the one in Paper IV, have previously been studied for plaice in
the North Sea. de Graf et al. (2004) used a Lagrangian particle tracking
model, where the plaice changed depth depending on the direction of the
current. Bolle et al. (2009) made a similar study using a Eulerian model, and
tried both current-induced and salinity-induced shifts in vertical positions.
By including field surveys and laboratory-experiments the information on
dispersal and connectivity gained from biophysical models can be further
improved. Here coupled biophysical models offer a most interesting framework to include dispersal and connectivity in management and conservation
issues. Examples are the design of sustainable MPA networks, no-take areas
with sufficient spill-over effects, risk assessment of invasive species and the
identification of demographically independent stocks.
28
3. Summary of Papers
Paper I: Wind sensitivity of the inter-ocean heat
exchange
Paper I concerns the heat transport between the basins of the world ocean,
and how this transport is affected by the wind forcing. Gordon (1986) concluded that the water coming through the Drake Passage south of the southern tip of South America, denoted “the cold-water path”, was not warm
enough to feed the large northward warm-water flow through the Atlantic.
Rather, the bulk of the water had to be of Indian-Ocean origin, and enter the
Atlantic by the Agulhas current rounding South Africa. Later studies confirm this finding (e.g. Döös 1995, Blanke et al. 2001). The model experiments in the present study show that wind forcing is needed to obtain any
net northward transport of heat at all in the Atlantic (Fig. 12d). Furthermore,
with no wind forcing at all the heat transport through the Agulhas ceases and
a small heat transport along the “cold-water path” appears.
The model experiments are made with the MIT general circulation model
(Marshal et al. 1997) subject to idealized basin geometry (see Fig. 8) and
forcing. Three zonal wind fields mimicking the shape of the annual mean
surface wind are applied to the model, which is then run to steady state.
Apart from a “realistic” wind forcing a case with symmetric wind over the
southern and the northern hemisphere is tried, and one case with wind exclusively over the Southern Ocean. As a reference case the model is run with no
wind forcing at all. The resulting heat transport is analyzed in several ways;
the scalar potential of the divergent component of the horizontal heat flux is
calculated, which gives a "coarse-grained" image of the surface heat flux
that captures the large-scale structure of the horizontal heat transport (Fig.
11c,d). Furthermore, the non-divergent component is examined, as well as
the meridional heat transport and the temperature-latitude overturning
stream function (Fig. 10). A sensitivity analysis examines the heat-transport
response to changes in wind stress at divergent latitudes. The results are
compared with results from the eddy-permitting global circulation model
OCCAM (Coward and de Cuevas 2005; Marsh et al. 2005a).
29
The results show that the westerly wind stress over the Southern Ocean has
two effects: a local reduction of the surface heat loss in response to the equator-ward surface Ekman drift, and a global re-routing of the heat export from
the Indo-Pacific. Without wind forcing, the Indo-Pacific heat export is released to the atmosphere in the Southern Ocean, and the net heat transport in
the southern Atlantic is southward instead of the prevailing northward net
flow. With wind forcing, the Indo-Pacific export enters the Atlantic through
the Agulhas and is released in the Northern Hemisphere. The easterly winds
enhance the poleward heat transport in both basins.
A
0.1
0.06
0.04
0.02
symmetric wind
Southern Ocean wind only
realistic wind
15
Ekman transport (Sv)
0.08
Windstress, (N/m2)
20
symmetric wind
Southern Ocean wind only
realistic wind
0
10
5
0
ï5
ï10
ï0.02
60S
40S
20S
0
20N
40N
ï15
60N
60S
50S
40S
30S
Latitude
C
0.4
0.3
0.2
0.1
0
ï0.1
no wind
realistic wind
40S
20S
0
Latitude
10S
20N
40N
60N
D
0.3
0.2
Atlantic
0.1
0
ï0.1
ï0.2
ï0.2
ï0.3
60S
20S
Latitude
Meridional heat transport, (PW)
Meridional heat transport, (PW)
0.5
B
Indo-Pacific
Southern
Ocean
ï0.3
60S
40S
no wind
realistic wind
20S
0
20N
40N
60N
Latitude
Figure 12. (A) The zonal wind-stress cases tested in Paper I in Nm-2. (B) The
Ekman transports in Sverdrups for the three wind cases. The asterisks in A
and B indicates the positions of the end of Drake Passage and the tip of the
African continent in the model (at 44°S and 26°S, respectively). (C) The net
northward heat transport in PW (1015 W) in the whole model ocean for the
realistic wind case and the case without wind. (D) Same as in C but divided
into the sub-basins.
30
Paper II: Difference in particle-transport patterns
between an open and a closed coastal area in the Baltic
Sea; high-resolution modeling with advective particle
trajectories.
In Paper II the differences in sediment-transport patterns between two areas
with contrasting oceanographic and geo-morphological conditions were
examined using a high-resolution Lagrangian particle-tracking model. The
final positions of the particles within the domain, the position of exiting and
the residence times (until exiting or the first settling event) were calculated.
The areas, Forsmark and Simpevarp, situated on the Swedish coast of the
Baltic Sea (Fig 2), are both locations for Swedish nuclear power plants and
have been investigated as possible sites for an underground nuclear waste
repository. The particle release-points in the simulations are locations where
potential leakage of radionuclides by the ground water from such a repository could debouch into the marine coastal zone. In this context the particle
transport patterns give an indication of the proportion of the leakage that will
stay locally, in contrast to leaving the area.
Forsmark is a semi-enclosed region shielded from the Baltic by a number
of islands and islets, while the Simpevarp area leads straight out to the open
sea. This difference is clearly visible in the results. In Forsmark the vast
majority of the particles stayed in the domain in small inlets with low water
velocities. Only about 6% of the clay and less than 1% of the silt exited,
while in Simpevarp more then 80% of the clay and 60% of the silt left the
domain. The residence times were shorter in Simpevarp, 11 days on average
for clay compared to 59 days in Forsmark. The clay that did not exit the
domains tended to stay close to the release points. The silt particles resided
much longer in the domains before exiting, 138 and 99 days for Forsmark
and Simpevarp respectively, and particles that did not exit were transported
further within the domains through successive settling and resuspension. The
Simpevarp silt showed a lot of along-coast transport and bathymetrically
constrained settling.
31
Paper III: Larval depth distribution critically affects
dispersal and the efficiency of marine protected areas.
This study aims to improve estimates of the dispersal of marine larvae by
including information on larval traits such as the depth distribution of the
larvae, pelagic larval duration (PLD, i.e. drifting time) and spawning season.
Particular focus is given to how larval depth distribution affects connectivity
and functionality of Marine Protected Areas (MPA) in the Baltic Sea.
A field survey made in the Baltic Sea and Kattegat showed that both invertebrates and fish differed in their larval depth distribution, ranging from
surface waters to more than 100 m. Most species were clustered at the surface or at depths of ~10 or ~30 m and had distinct spawning seasons. This
information was used in a biophysical particle-tracking model of larval dispersal to test the relative magnitudes of effects of the different larval traits
on the variation in dispersal distance and direction. The results showed that
depth distribution, together with PLD, explained 80% of the total variation
in dispersal distance, whereas spawning season, geographic and annual variations in circulation had only marginal effects. Decreased drifting depth
increased the average dispersal distance 2.5 times. It also decreased the
coastal retention and the proportion of larvae that were recruited to the population where they originated, and increased connectivity between different
areas substantially. Median dispersal distances varied between 8 and 46 km
depending on drifting depth and PLD, with 10% of simulated trajectories
dispersing beyond 30-160 km. In the Baltic Sea, the majority of shallow
Natura 2000 MPAs are smaller than 8 km. In the present study, only one of
the 11 assessed larval taxa would have more than 10% of the dispersed larvae recruited locally within MPAs of this size. The connectivity between
MPAs is expected to be low for most larval trait combinations. Our simulations and the empirical data suggest that the MPA size within the Natura
2000 system is considerably below what is required for local recruitment of
most sessile invertebrates and sedentary fish.
32
Paper IV: Larval behaviour and dispersal mechanisms
in Shore Crab larvae: Local adaptations to different tidal
environments.
Paper IV is a study of the dispersal and connectivity of European Shore
Crab (Carcinus maenas). A laboratory experiment and a number of fieldsurvey results (from Skagerrak, Kattegat and the Wadden Sea) on the vertical distribution of larvae in the water column are combined with a largescale biophysical modelling experiment. The aim is to determine how differing vertical behaviour during the pelagic larval phase affects the dispersion
distance, recruitment and connectivity in tidal and micro-tidal regions in the
North Sea and Kattegat-Skagerrak.
The purpose of the laboratory experiment was to examine the vertical behaviour of newly hatched larvae from the Wadden Sea on the west coast of
Denmark. Whether the bulk of the larvae are changing vertical position with
the tidal phase, as do the "high tidal amplitude" larvae from Wales, or with
the time of day, as the "low tidal amplitude" larvae from Kattegat do, can
give an indication of their genetic origin, and thus the origin of the population. Our results indicate that the shore crab in the Wadden has individuals
showing behaviour consistent with both populations.
To assess the influence of vertical behaviour on the dispersal distance and
recruitment success a biophysical model was used. Three different vertical
migration strategies were modelled:
• Sea-breeze, changing vertical position with time of day
• Tidal, changing with the tidal frequency at the release point
• Adaptive-tidal, changing with local tidal frequency
The larvae drifted for 40 days and to mimic an ontogenetic shift they
switched phase at day 26. The experiment also included two types of fixeddepth reference behaviours:
• Surface, drifting at 0.3 m
• Deep, drifting at 20 m (or as deep as possible if the overall
depth is smaller)
Both reference-behaviours retained the same depth during all 40 days of
simulation. Larval trajectories were released from 4 regions (Wadden Sea,
Jutland, Kattegat and Skagerrak) over 4 different months (May-August)
within 3 separate years.
The model results showed that larval behaviour had a dominant role on
the dispersal variables and that different behaviours resulted in specific dispersal patterns that were consistent between months and years, but differed
between regions with different tidal regimes. In general, deep swimming
larvae were dispersed shorter distances, were closer to shore, and had higher
33
recruitment success at the end of the dispersal period compared to surface
swimming larvae. Larvae with cyclic vertical migration behaviour were
dispersed long distances, but were closer to shore and in general had higher
recruitment success than both surface and deep swimming larvae. In the
mesotidal Waddens Sea, tidal-migrating larvae were dispersed further and
had higher recruitment success compared to diel-migrating larvae, whereas
in microtidal Skagerrak no difference was found between vertically migrating larvae. Connectivity analyses indicated an oceanographic dispersal barrier between the tidal North Sea and the microtidal Skagerrak-Kattegat area.
Surface
Deep
Sea-Breeze
Tidal
Figure 13. A random selection of larval trajectories from each region
(Wadden-purple, Jutland-blue, Kattegat-green and Skagerrak-red) from the
July spawning period of 1989 and 1996. Black asterisks show the endpoints
of the trajectories. (The adaptive-tidal behaviour produced a very similar
picture as the tidal behaviour and was omitted.)
34
Outlook
The dispersal and connectivity modelling in Papers III and IV was undertaken with flow data from years with different anomalies in the North Atlantic oscillation (NAO). The NAO is a fluctuation of the atmospheric pressure
gradient over the North Atlantic, measured as the average difference in sealevel pressure between Reykjavik (the Icelandic low-pressure system) and
Lisbon (the Azores high-pressure system). It influences the direction and
strength of the westerly wind field over the Atlantic (Hurell et al. 2003). A
large pressure difference between the two measuring stations (high index,
NAO+) leads to an increase in the westerlies, often giving mild winters and
cool summers in Central Europe. With the opposite anomaly, i.e. small difference in pressure (low index, NAO-) the westerlies are weakened, bringing
cold winters and a southerly shift of the storm tracks over southern Europe.
Choosing years with opposite indices in the studies in Papers III and IV
was a way of attempting to increase the variability in the model runs. This
however, this leads to the question “How much of the climatic variability
due to the NAO can you actually detect in a small basin such as the Baltic or
the North Sea?”
A yearly average of the horizontal velocities from the BaltiX model
(Hordoir et al. 2012), used in Paper IV, for the years 1989 (positive) and
1996 (negative) reveals a pronounced difference in the flow field in the
North Sea, Kattegat and Skagerrak (Fig. 14). During 1989 both the Baltic
and the Norwegian currents were stronger than in 1996, and there is also a
strong eastward flow into Kattegat just north of Jutland that is opposite in
direction when the westerly winds are weaker. A pronounced feature in the
Wadden Sea is the 1996 off-shore direction of the flow at its apex, increasing the oceanographic barrier that restricts the connectivity, as discussed in
Paper IV. In the Baltic Sea the differences are more subtle. These are just
two years, and also average values from a whole year. Investigating shorter
periods, with sudden shifts between strong anomalies, using trajectory modelling could provide an answer to whether the NAO exerts any influence on
the flow closer to shore controlling the dispersal. Furthermore, it would be
35
1989
1989
1996
1996
Figure 14. The horizontal velocity field at 0-10 m depth in the Baltic Sea (A
and B) and the North Sea, Skagerrak and Kattegat (C and D) for the positive
NAO index year 1989 and the negative index year 1996. The grey-scale indicate the amplitude of the signal, darker colour- higher value.
interesting to determine if there are any significant differences in connectivity along the mainland coast of the North Sea between positive and negative
NAO periods.
An interesting further development of the study in Paper I would be to
analyze the heat-transport in one of the new reanalysis datasets, such as ECCO, using the heat flux potential. A sufficiently long average of the vertical
heat transport between ocean and atmosphere could serve as a proxy for the
steady state response of the ocean.
36
Acknowledgements
To begin with, I would like to express my gratitude to my main supervisor
Kristofer Döös. After supervising my master thesis you encouraged me to
continue as a PhD student, and without you I would probably not have chosen this career. Thank you for always making time for me, and for letting me
do whatever I wanted. My assisting supervisor Johan Nilsson is gratefully
thanked for all the help and support during my fist years.
To my Gothenburg co-authors I am much obliged. Per Jonsson, working
with you has taught me a lot about scientific method and how to find and
communicate the significant parts of the results. Thank you for all your help
and for introducing me to connectivity research, you are a great inspiration
to me. Per-Olav Moksnes, even though you do not always understand what I
do you have a fantastic (and annoying) gift for always spotting when I have
done something wrong. For this I am most grateful. I admire your thoroughness and aspire to become as thorough myself.
Peter Lundberg: Summas gratias tibi ago pro diuturna correctione et
emendatione scripti mei, praecipue pro verbis difficilibus, quibus mihi affuisti. Hoc multum apud me valet! Thank you to my co-authors Anders
Engqvist and Göran Broström for fruitful collaboration, and to Ulrik
Kautsky at the Swedish Nuclear Fuel and Waste Management Co, to Stockholm Marine Research Centre, L&E Kinnanders foundation, Bert Bolin Centre for Climate Research and K&A Wallenberg foundation for financial support. Joanna Fransson, thank you so much for proofreading my thesis!
MISU is a wonderful place to work! Thank you to all of you, for making
it such a nice place to come to every day, for all the interesting conversations
at lunch and coffee breaks and for all the help and support that I received
during the years. A special thanks to my office roommates: Jenny Nilsson,
Linda Enmar, Johan Liakka and Linda Megner; you have all made it much
more fun to come to work! Thank you Frida Bender, for all the nice coffee
breaks and interesting conversations, I will definitely miss it all when I
leave. And Joakim Kjellsson, thank you for putting up with all my questions
about your (not always so self-instructive) changes to TRACMASS.
Finally I wish to thank my family for their endless support. Most of all,
thank you Mikael, I love you.
37
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