UNIVERSITY OF GOTHENBURG
Department of Earth Sciences
Geovetarcentrum/Earth Science Centre
The distribution of benthic
foraminifera in the
Gullmar Fjord deep basin,
station S65142B
A comparison with hydrographic parameters
and the North Atlantic Oscillation
Patricia Jusslin
ISSN 1400-3821
Mailing address
Geovetarcentrum
S 405 30 Göteborg
Address
Geovetarcentrum
Guldhedsgatan 5A
B828
Bachelor of Science thesis
Göteborg 2014
Telephone
031-786 19 56
Telefax
031-786 19 86
Geovetarcentrum
Göteborg University
S-405 30 Göteborg
SWEDEN
Abstract
In this study the distribution of benthic foraminifera was investigated in the previously
unexplored, inner part of the Gullmar fjord. A Gemini corer was used in order to obtain an
undisrupted 41 cm long sediment core (S65142B). The purpose was to investigate the
correlation between the distribution of different species of benthic foraminifera (through the
depth of the sediment core) and the climatic parameters (concentration of dissolved oxygen,
salinity and temperature), which in turn are influenced by the North Atlantic Oscillation
(NAO). The results showed two distinct shifts in regime of dominating species, from the
mid-1930s until present. The older and the younger sediments are dominated by species
included in the Skagerrak-Kattegat fauna (S-K fauna), while the middle part of the core is
characterized alternately by the opportunistic species Stainforthia fusiformis and the S-K
fauna.
The same pattern is found in several previous studies; Nordberg et al. (2000), Filipsson
and Nordberg (2004), Polovodova Asteman and Nordberg (2013) and Davidsson et al.
(2013), among others. In periods when S. fusiformis was most prominent, several hypoxic
events occurred in the deep waters of the fjord. Meanwhile, the general NAO trend for this
period is known to have been positive, unlike when S-K fauna was most dominant. In
contrast to periods when the concentrations of dissolved oxygen were generally higher the
NAO was negative.
The investigated sediment core was also compared to three other cores, namely B88141B
studied by Lindstrom (2014), AE73142B, studied by Davidsson (2014) and G113142B
(Course core, 2014). These four cores together provided a transect through Gullmar fjord,
from the innermost part of the deep basin towards west. It appears that the number of
benthic foraminifera increases towards the western part of the basin. The lowest amount of
individuals per gram sediment was found in station S65, in the innermost part of the deep
basin. This new information on benthic foraminifera in Gullmar fjord, together with the
hydrography data confirms the close connection between hydrography, climate variations
and the benthic communities in the Gullmar fjord deep basin.
Key words: benthic foraminifera, Gullmar Fjord, Sweden, hydrography, hypoxia, North Atlantic
Oscillation
2
Sammanfattning
I föreliggande kandidatarbete har utbredningen av bentiska foraminiferer undersökts i ett
område i inre delen av Gullmarsfjordens djupbassäng, Detta område har tidigare inte
studerats. En Gemini corer användes för att ta upp en ostörd 41 cm lång sedimentkärna
(S65142B). Avsikten var att undersöka huruvida det råder ett samband mellan utbredningen
av olika bentiska foraminiferarter (med djupet i sedimentkärnan) och de hydrografiska
parametrarna (syrehalt, salinitet och temperatur), vilka i sin tur påverkas av den
Nordatlantiska Oscillationen (NAO).
Resultaten visar två tydliga skiften mellan de mest frekventa arterna, från mitten av 1930talet till nutid. De äldsta och yngsta sedimenten domineras av arter från SkagerrakKattegattfaunan (S-K faunan), medan den mellersta delen av kärnan karaktäriseras dels av
den opportunistiska arten Stainforthia fusiformis och dels arter av arter från S-K-faunan, vilka
växlar i dominans mellan varandra. Detta karaktärsdrag återfinns i flera tidigare studier,
bland andra i Nordberg m.fl. (2000), Filipsson och Nordberg (2004), Polovodova Asteman
och Nordberg (2013) och Davidsson m.fl. (2013). Under perioden då S. fusiformis var mest
rikligt förekommande, uppträdde flera episoder av hypoxi i fjordens djupvatten. Samtidigt
var den allmänna NAO-trenden under denna period positiv, till skillnad från när S-K faunan
var som mest frekvent. Då var NAO-trenden istället negativ och syrehalterna allmänt
högre.
Föreliggande sedimentkärna jämfördes även med tre andra kärnor, nämligen B88141B
studerad av Lindström (2014), AE73142B, studerad av Davidsson (2014) och G113142B
(Kursmaterial, 2014). Dessa tre kärnor utgör tillsammans en transekt genom
Gullmarsfjorden, från den innersta delen av djupbassängen och ut mot bassängens
västligaste delar. Det förefaller som om antalet bentiska foraminiferer ökar i riktning mot
tröskeln. Lägsta antal individer per gram återfinns i station S65, i den innersta delen av
djupbassäng. Den här nya informationen om bentiska foraminiferer i Gullmarsfjorden
bekräftar tillsammans med hydrografisk data, ett nära samband mellan hydrografi,
klimatvariationer och det bentiska samhället i Gullmarsfjordens djupbassäng.
Nyckelord: bentiska foraminiferer, Gullmarsfjorden, Sverige, hydrografi, hypoxi, Nordatlantiska
Oscillationen
3
Contents
Abstract..................................................................................................................................................... 2
Sammanfattning ...................................................................................................................................... 3
1. Introduction............................................................................................................................................ 5
2. Study area................................................................................................................................................. 5
2.1. Hydrography and NAO............................................................................................................. 5
2.2. Hydrographic measurements ................................................................................................... 7
3. Benthic foraminifera .............................................................................................................................. 8
4. Materials and Methods ...................................................................................................................... 11
5. Results.................................................................................................................................................... 12
5.1. Lithological description.......................................................................................................... 12
5.2. Benthic foraminiferal fauna in core S65142B .................................................................... 12
5.3. Combined results from the transect studies ...................................................................... 13
6. Discussion ............................................................................................................................................ 18
6.1. North Atlantic Oscillation and its influence on fjord’s hydrography .......................... 18
6.2. Estimated time scale ............................................................................................................... 18
6.3. The distribution of benthic foraminifera ............................................................................ 18
6.3.1. Hypoxic events and predating macro fauna .......................................................... 18
6.3.2. Foraminiferal fauna in respond to food availability ............................................ 19
6.3.3. Possible effects of taphonomic processes ............................................................... 20
6.3.4. Correlations between present, previous and parallel studies ............................ 20
7. Conclusions ........................................................................................................................................... 21
8. Acknowledgements ............................................................................................................................. 22
9. References ............................................................................................................................................. 22
10. Appendix ............................................................................................................................................. 24
10. Appendix continued. ......................................................................................................................... 25
4
1. Introduction
The Gullmar Fjord is one of several fjords at the Swedish west coast (Fig.1). The stagnant
conditions, caused by its characteristic bottom topography and hence lack of bioturbation by
benthic organisms, provide the Gullmar Fjord with excellent high-resolution sediment
records, which favorably can be used for environmental investigations (Filipsson &
Nordberg, 2004).
Due to its location, the fjord is exposed to the inter-annual variability of atmospheric
circulation, caused by the North Atlantic Oscillation (NAO) (Hurrell, 1995). This variability
is an important indirect factor in the process of deep-water exchange in the fjord
(Polovodova Asteman & Nordberg, 2013). According to Björk and Nordberg (2003),
limitations of the deep-water exchange is the main cause of hypoxia in the fjord’s bottomwater, which makes the upwelling one of the major factors influencing the fjord
environment.
Benthic foraminifera are sensitive indicators for changes of the environmental factors, with
extra consideration to the physical water-mass properties and to the characteristics of the
sediment (Conradsen et al., 1994). Even though the distribution of benthic foraminifera, as a
proxy for paleo-environmental reconstruction, has been widely applied for over a century,
there are still gaps in the knowledge of the organisms’ response to environmental changes
(Bergsten et al., 1996).
The purpose of this study is to investigate the former and more recent distribution of
benthic foraminifera with depth in the sediment core S65142B, collected in the innermost
part of the Gullmar fjord (Fig. 1), and to compare the results with variations in the NAO,
salinity, dissolved oxygen and temperature in the water column of the fjord. Similar
investigations in the Gullmar Fjord have previously been carried out by Nordberg et al.
(2000), Filipsson & Nordberg (2004), Davidsson et al. (2013) and Polovodova Asteman &
Nordberg (2013). The study of climate variability and its influence on the meiofauna at the
station S65142B has not been investigated before. A comparison between this study and
previous studies will therefore be performed.
Furthermore the results from this study will be compared with the ongoing investigations
of the benthic foraminiferal down-core distribution from several stations taken at a transect
along the fjord (Fig. 1). The investigations were carried out by Davidsson (2014) and
Lindström (2014), simultaneously with the current study.
2. Study area
2.1. Hydrography and NAO
With a length of 25 km and a width of 1-3 km the Gullmar fjord is by far the largest
amongst several silled fjords in the Bohuslän province on the Swedish west coast
(Gullmarsfjorden-naturvårdsområde, 2014). It has a sill depth of 42 m and a maximum depth
of 120 m in the deepest basin, Alsbäck (Fig. 1). The water column is stratified in four layers
due to differences in density, which in turn is caused mainly by the differences in salinity
(Polovodova Asteman & Nordberg, 2013 and references therein). With a thickness of
approximately 1 m, the uppermost layer of the stratified water column consists mostly of
fresh river water, derived from the Örekils Älv (Fig. 1). The second layer reaches a depth of
about 15 m and consists of brackish water (24-27 psu) from both the Skagerrak and the
Baltic Sea. Between 15 and 50 m a denser, more saline third layer (32-33 psu), which
originates from the Skagerrak, is characteristic. Deeper than 50 m, the forth layer occur. It is
more stagnant, with a residence time of approximately 1 year and a salinity similar to that of
5
the intermediate Skagerrak water (34,4 psu). The second and third layers have a residence
time of 20-38 days and 29-60 days respectively (Arneborg, 2004).
Figure 1. Location of the study area.
The smaller map shows a regional
view and location of the study area
on the Swedish west coast, and the
larger map shows the Gullmar fjord.
The sampling site for the core used
in this study is marked with a black
circle. The sampling site for the
other cores used in the transect
study; Ae73142B (Davidsson, 2014)
and B88141B (Lindström, 2014) are
indicated with grey circles. The
grey diamond indicates the location
of the core G113142B (Course
material, 2014) and the location of
Alsbäck, the deepest part of the
Gullmar fjord. Modified from
Polovodova Asteman & Nordberg
(2013).
As mentioned earlier, upwelling is a controlling factor of the environmental situation in the
fjord, since this process results in deep-water exchange and thereby causes the reoxygenation, which is critical for the benthic marine life. Upwelling is caused by vertical
movements in the water-column stratification outside the Gullmar fjord (Arneborg, 2004).
Studies show that there is a link between the water exchange and the wind direction and
strength, which in turn is controlled by the North Atlantic Oscillation (Polovodova Asteman
& Nordberg, 2013). A positive phase of the NAO index causes predominant westerly winds,
which result in more mild and humid winters. During a negative phase, the dominating wind
direction is northeast-east, resulting in a transport of the surface water out of the fjord,
which enables upwelling, and hence, the deepwater exchange (Filipsson & Nordberg, 2004).
According to Hurrell et al. (2003), the NAO phenomenon refers to the reallocation of
atmospheric masses, oscillating back and forth between the subtropical Atlantic and the
Arctic. These variations alter the weather over the North Atlantic Ocean and adjacent
continents. The NAO is detected by measuring differences in sea-level pressure between two
6
pressure systems, the Azores High and the Icelandic Low. Such measurements have been
carried out since the year 1864 and the data on NAO index (Fig. 2) is available from that
time until present (Hurrell, 1995).
Several severe low-oxygen events have been recorded over the last century in Gullmar
Fjord. According to previous studies, frequent events of stagnation and seasonal hypoxia
became characteristic features in the Gullmar fjord during the second part of the 20th
century (Filipsson & Nordberg, 2004; Polovodova Asteman & Nordberg, 2013). The most
severe and long lasting hypoxia took place between the 1996 and 1998 and prior to that
another critical period of hypoxia took place in 1979/80, when the deep-water exchange
almost failed to occur (Filipsson & Nordberg, 2004).
Climatic factors may not be the only force controlling the environmental situation.
According to Nordberg et al. (2000), the inner part of the Gullmar fjord received large
amounts of oxygen-consuming substances from the paper- and sulfur industries located
nearby. However, since the mid 1960s, the fjord has not been exposed to any heavier
pollution from anthropogenic sources.
2.2. Hydrographic measurements
The hydrographic data of dissolved oxygen, salinity and temperature measurements have
been taken at Alsbäck Deep (SMHI, 2014) from depths below 110 m. The content of
chlorophyll a was measured at Björkholmen at the depth of 10 m (SMHI, 2014). The results
of these parameters together with the NAO index (downloaded from NWS, 2014) are
displayed in Figure 2. The measurements of dissolved oxygen, salinity and temperature
were not carried out regularly before the year of 1980. From 1980 on, the measurements
were performed more frequently, giving the graphs a higher resolution for the last 30 years
or so. The saw-tooth shape of the curves corresponds to their shifts between stagnation
periods and annual bottom-water exchange events in the fjord.
There have been several periods of severe hypoxia in the fjord waters from the year of
1980 until present (Fig. 2). According to Polovodova Asteman and Nordberg (2013) there is
a distinct limit in the concentration of dissolved oxygen, which is set at 2ml/L. Below this
limit several life forms in the Gullmar fjord no longer find the environment tolerable. In the
Figure 2, this limit is shown by a vertical line. A trend line has also been added to the
oxygen graph to make clearer the interannual variations in O2 concentration.
Neither the salinity nor the temperature shows any significant interannual fluctuations
(Fig. 2), although the general shape of the curves corresponds to the seasonal and annual
hydrographic variations.
The measurements of the chlorophyll a have been carried out frequently since the year
1990. Two distinct peaks are observed at 1990 and at 2011 (Fig. 2).
As it is shown at Fig. 2 the NAO index shows both positive and negative trends, which can
span over several years. The NAO index varies between a negative phase (1950 - 1970) and
a generally positive phase (1985 – 2010).
7
Salinity
Dissolved
oxygen
Temperature NAO index
Chlorophyll A
2013
2003
1993
1983
1973
1963
1953
1943
(ml/L)
(°C )
(psu)
18
12
6
2
0
1
0
-1
-2
8
6
4
35,5
2
34,5
8
33,5
6
4
2
0
1933
(µg/L)
Figure 2. Showing the NAO index (NWS, 2014), hydrographic measurements of dissolved oxygen,
salinity and temperature from Alsbäck, depth > 110 m and chlorophyll a from Björkholmen, depth
10 m (SMHI, 2014).
3. Benthic foraminifera
Benthic foraminifera are one of the most important protist groups when it comes to paleoenvironmental investigations, mainly due to the preservation of a hard fossilized test and the
vast diversity of species. In the fossil record, benthic foraminifera can be traced all the way
back to the early Cambrian period. Depending on species composition and dominance in the
foraminiferal assemblage, conclusions about the paleo-environment and climate can be
drawn (Culver, 1993). The principle “present is the key to the past” is well-known and, perhaps,
most widely used in biostratigraphy and paleo-reconstructions. However, in order to
correctly interpret the history preserved in the sediment records, knowledge about the
ecology of the benthic foraminifera must be attained (Murray, 2006).
The foraminifera have a great capacity of tolerating a variety of environments and adverse
conditions. Many species still exist in small numbers, even though the conditions are
unfavorable and are beyond foraminifera’s optimum tolerance zone. If the conditions change
to foraminifera’s advantage, some species rapidly increase in abundance. These species are
known as opportunists. Species that do not respond to rapidly changing conditions in this way
are known as generalists (Murray, 2006).
8
Benthic foraminifera can be roughly divided into two groups, based on the contexture of the
test. It can either be agglutinated (built by cementing sediment particles together) or
calcareous (built out of calcium carbonate, secreted by the organism). The particles making
up the agglutinated walls can be mineral grains either specifically chosen on the basis of size,
composition or gravity, or simply reflecting the composition of the sea floor. Out of the
calcareous made up tests, three major types are recognized; microgranular, porcelaneous and
hyaline. These types are an important foundation in classifying the foraminifera (Culver,
1993). However, when it comes to classification, it is not unusual to encounter difficulties
due to tests exposure to taphonomic processes. These processes are mainly bioturbation,
time averaging, test destruction and transport (Murray, 2006). Agglutinated tests are more
susceptible for test destruction, but some species are more durable than others, depending
on the morphology of the test structure (Schröder, 1988).
Based on their habitat, benthic foraminifera can be infaunal (live within the sediment) or
epifaunal (adapted to a life on the sediment surface or other substrates). There are also
various feeding strategies of the benthic foraminifera (parasitism, carnivory, herbivory,
suspension and deposit feeding, omnivory), though the vast majority of them are herbivore
(Pawlowski, 2012). Several species show a positive correlation to high food availability and
since algal blooms (measured as content of chlorophyll a) are patchy in occurrence, food is
likely to be a controlling factor for spatial patchiness in foraminiferal distribution (Murray,
2006).
In the stratigraphic column of the Gullmar fjord sediments, some species in the benthic
foraminiferal community are more commonly present. Filipsson and Nordberg (2004), refers
to these species as the Skagerrak-Kattegat fauna (S-K fauna; Fig. 3), since they are typical in
the shelf areas of the Skagerrak and Kattegat. The six species of the S-K fauna are: Hyalinea
baltica, Nonionellina labradorica, Bulimina marginata, Cassidulina laevigata, Textularia earlandi
and Liebusella goësi. The occurrences of the S-K fauna, along with other foraminiferal species,
which exceed 10% in relative abundance, in the core S65142B, have been investigated in this
study. The less abundant species (<10%) in this study are grouped as others.
The list below is a more detailed ecological description (mainly referring to the work of
Murray, 2006) of each species investigated in this study (Fig. 3).
 Hyalinea spp. in general are epifaunal and free moving. They are found in cold shelf
areas in muddy or silty substrate (Murray, 2006). Hyalinea baltica is considered an
opportunistic species, which responses positively to high food availability (Rosenthal
et al., 2011). The contexture of its test is hyaline.
 Nonionellina labradorica is infaunal and prefers a salinity >34, 5 psu and cold (<1 °C)
waters. It is common in the high fjords of Svalbard, Greenland and Scandinavia
(Murray, 2006). The contexture of its test is hyaline.
 Bulimina marginata is infaunal down to at least 4 cm and prefers a muddy substrate.
It is an opportunist in its response to high food availability (Murray, 2006) and is
able to withstand hypoxic conditions (Polovodova et al., 2011). The contexture of its
test is hyaline.
 Cassidulina laevigata is infaunal and has a positive correlation to content of coarse
sand. In favorable conditions (oxygen- and food-rich sediments) C. laevigata is a
superior competitor. It is also resistant to disturbance of macrofaunal predators
(Murray, 2006), but suggested to be sensitive to severe hypoxia (Polovodova et al.,
2011). The contexture of its test is hyaline.
9
 Textularia earlandi is infaunal in shallow surface sediment. It tolerates hypoxia
(Murray, 2006) and has an agglutinated test contexture.
 Liebusella goësi is yet to be investigated when it comes to its ecological and biological
preferences (Davidsson et al., 2013 and references therein). The contexture of its test
is agglutinated.
 Elphidium excavatum is infaunal and tolerates a wide range of salinity (15-31 psu)
(Murray, 2006). The contexture of its test is hyaline.
 Adecotryma glomerata is epifaunal to shallow infaunal and is often associated whit cold
(<4 °C) waters. It tolerates a range of salinities between 28-34 psu. In the Gullmar
fjord, the A. glomerata is a common species and, despite its agglutinated contexture of
its test, is well preserved in the fossil record (Polovodova et al., 2011).
 Nonionella turgida is infaunal species (Murray, 2006), which shows positive
correlation to a high chlorophyll a content and after spring blooms is characterized
by a green color of cytoplasm indicating herbivore feeding strategy (Gustafsson &
Nordberg, 2001). The contexture of its test is hyaline.
 Stainforthia fusiformis is an opportunist and thrives in anaerobe conditions. It is able
to increase its population up to 7 times / month. It is known to be the most abundant
species during hypoxic and periodically anoxic conditions in silled fjord basins on the
Swedish west coast (Polovodova et al., 2011). It also shows a positive correlation to
the surface water chlorophyll a content (Gustavsson & Nordberg, 2000). The
contexture of its test is hyaline.
10
Figure 3. Scanning electron microscope images showing the different species
involved in this study. (1) Stainforthia fusiformis. (2) Liebusella goësi. (3)
Textularia earlandi. (4) Nonionella turgida. (5) Hyalinea baltica. (6)
Cassidulina laevigata. (7) Bulimina marginata. (8) Adecotryma glomerata. (9)
Elphidium excavatum. (10) Noninonellina labradorica. Modified pictures
from Gustafsson & Nordberg (1999); Nordberg et al. (2000); Bergstrand
(2012).
4. Materials and Methods
In this study a 42-cm long sediment core S65142B was analyzed. It was collected in the
Gullmar fjord at the station S65 (58° 23.59’ N; 11° 37.37’ E), at a depth of 65 m in February
2014 aboard the R/V Skagerak (Fig. 1). In order to obtain an undisturbed sediment surface,
a Gemini corer was used for sampling. The core was sliced at a 1-cm interval from the top
down to the depth of 20 cm. From 20 cm and down core the sampling interval was increased
to 2 cm and only every second sediment slice was kept for analysis. Simultaneously a
protocol for a visual core description was filled.
The samples were wet-weighted, freeze-dried and then weighted dry, a procedure through
which the information on sample water content could be obtained. For foraminiferal analysis
approximately 10 g of the sediment from each sample was washed through a 1-mm and a 63µm sieve. Sodiumdiphosphate (Na2P2O7) was added during washing in order to disintegrate
sediment aggregates when necessary. The remaining material in the sieves (63-1000 µm)
was collected on a filter paper and dried at 50°C over night and weighted to obtain the
estimated sand-sized fraction content. Approximately 200 foraminifera from each sample
11
were randomly picked, sorted and identified to species level. For a more detailed description
on foraminiferal analyses and preparation techniques, see Murray (2006).
This study focuses on the dominant foraminiferal species which contributed to the
assemblage by >10% in relative abundance (A. glomerata, E. excavatum, N. turgida and S
fusiformis) along with those included to the Skagerrak-Kattegat fauna (H.baltica,
N.labradorica, B.marginata, C.laevigata, T.earlandi and L.goësi).
Data of the physical parameters (salinity, dissolved oxygen and temperature) along with
data on amount of chlorophyll a were downloaded from the database of the Swedish
Meteorological and Hydrological Institute (SMHI, 2014) and the data on the NAO index
was downloaded from the National Weather Service (NWS, 2014).
A significant shift in abundances of dominating species can be observed at the depth of
17,5cm in S65142B. According to Filipsson & Nordberg (2004), this shift occurred in 1980.
By using this chronological marker along with the age of core top (2014), an estimated
timescale was calculated (Fig. 5).
5. Results
5.1. Lithological description
The visual observation of the core S65142B shows quite dense clay of darker grey, from the
bottom up until about 10 cm, where the redox-cline was present. The redox-cline is the
transition where oxygen-consuming decomposition of organic matter ceases
(Nationalencyklopedin.se, 2014). The top of the core, 0–10 cm, was composed of welloxygenated, brownish sediment, with signs of bioturbation.
Figure 4 shows the water content and the sand content in percent throughout the depth of
the core S65142B. The amount of sand is rather constant through the core and lies at about
0,5%, although a decreasing trend is recognized towards the top of the core.
The water content is gently increasing towards the top. At the depth of 18,5 cm there is a
distinct notch (probably due to an error in measurements) where the amount of water
swiftly drops by ca. 10% before returning to its previous trend.
5.2. Benthic foraminiferal fauna in core S65142B
The first and second graph from the left in Figure 5 shows the results from the investigated
Skagerrak-Kattegat fauna and the Stainforthia fusiformis in percent. A significant shift in
abundances of dominating species can be observed at the depth of 17,5cm in S65142B. The
foraminiferal shift between abundances of S. fusiformis and the S-K fauna lasted from 1980 to
1995. In 1995 S. fusiformis finally decreased and the S-K fauna resumed its position as the
dominant species.
The absolute abundance in individuals per gram (ind/g) of calcareous and agglutinated
foraminifera is shown at Figure 5. Both groups indicate similar receding and progressing
trends over the years, although the agglutinated species appear to be far less abundant as
ind./g. However, they are increasing in absolute abundances towards the core top (see
Appendix).
The total amount of foraminifera in absolute abundance (ind/g) shows an increasing trend
towards the top of the core (Fig. 5 & 7). At 7,5 cm the total foraminifera show low absolute
abundance, down to 66 ind./g, after which they increase to 188 ind./g at 6,5 cm core depth.
Figure 6 display the relative and absolute abundance of the different species of foraminifera,
throughout the core. A similar signal can be observed in the deeper sediments for the species
12
H. baltica, C. laevigata, B. marginata and A. glomerata. They appear to increase in abundance
and reach their highest abundances at about 29 cm depth. Except for S. fusiformis and N.
turgida, a notable peak can be recognized for many of foraminiferal species at 18,5 cm (Fig.
6). This coincides with the notch in the water content of the core (Fig. 4) and thus may
represent an artifact of a weight measurement error. For N. turgida and N. labradorica there
is a clear increasing trend towards the core top. At the same time their abundances are low
deeper down the core. Species T. earlandi and C. laevigata also increase in absolute
abundances from ca. 11 cm towards the core top, although the increase in T. earlandi is much
higher. Species S. fusiformis begins to increase at the depth of 19 cm and reaches its
maximum of 58 ind./g at the depth of 7 cm.
5.3. Combined results from the transect studies
Figure 7 presents the total abundance of foraminifera (ind./g) in all four cores, that make up
the transect of the Gullmar fjord, AE73142B (Davidsson, 2014), G113142B (course material,
2014), B88141B (Lindström, 2014) and S65142B (present study). The core B88141B has two
alternative time scales since the chronological marker, used in the other studies, was not
applicable at this station. For further explanation, see Lindström (2014). The dashed line
indicates the year 1980.
Figure 8 shows the relative abundance of the S-K fauna and the species S.fusiformis,
throughout the same cores as above. Also here the core B88141B has two alternative time
scales. For further explanation, see Lindström (2014). The dashed line indicates the year of
1980.
Figure 4. Showing the results of the sand(left) and water content (right) in percent,
throughout the depth of the core S65142B.
13
Relative abundance
(%)
0
Total forams
S-K fauna
100
1933
S.fusiformis
41
50
1943
0
36
75
1953
S-K Fauna
31
50
1963
0
26
S.fusiformis
1973
25
21
Absolute abundance
(ind./g)
Figure 5. Graphs showing the general results of the foraminiferal analysis in
core S65142B. First and second graph from the left shows the shifts in
dominating foraminiferal fauna between the S-K fauna and Stainforthia
fusiformis in both relative (first graph) and absolute (second graph)
abundance. Third graph shows the absolute abundance of calcareous and
agglutinated foraminifera and forth graph shows the total abundance of
foraminifera. The dashed line indicates the year 1980. Note the different units
for the x-axis.
14
200
1983
100
16
0
1993
Calcareous forams
11
100
2003
Agglutinated forams
6
50
2013
Year
Depth (cm)
1
15
10
5
75
0
30
40
50
25
15
20
10
0
20
20
C.laevigata
20
10
45
0
T.earlandi
B.marginata
20
1933
H.balthica
41
20
1943
10
36
0
1953
15
31
10
1963
0
26
N.labradorica
1973
5
21
20
1983
0
16
0
1993
10
11
10
2003
0
6
0
2013
Year
1
Absolute abundance (ind/g)
30
20
10
0
60
30
0
20
10
20
20
0
10
10
10
10
5
5
15
0
0
0
Relative abundance (%)
2013
2003
1993
1983
Year
1973
Others
N.turgida
1943
A.glomerata
1953
S.fusiformis
1963
E.excavatum
Absolute abundance (ind/g)
Figure 6. Results of the distribution of benthic foraminifera in the core S65142B. Each
species presented in this study is shown in a separate graph in both absolute and relative
abundance. The dashed line indicates the year of 1980. Note that the scale on the x-axis
varies between the different species.
15
40
30
20
10
45
0
30
15
0
20
10
0
1933
15
0
Depth (cm)
0
10
Relative abundance (%)
1980
Depth (cm)
0
5
10
15
20
25
30
35
40
45
50
0
AE73142B
300
600
Depth (cm)
0
5
10
15
20
25
30
35
40
45
50
55
60
0
G113142B
200
400
600
Depth (cm)
0
5
10
15
20
25
30
35
40
0
400
600
Total forams (ind/g)
B88141B Alt. 1
200
Depth (cm)
0
5
10
15
20
25
30
35
40
0
B88141B Alt. 2
200
400
600
Depth (cm)
0
5
10
15
20
25
30
35
40
41
0
S65142B
200
400
600
1980
Figure 7. Combined results from the cores used in the transect showing the
total abundance of foraminifera. Dashed line indicates the year 1980. The
cores are placed in the same order as they were located along the transect,
with AE73142B to the left, located closest to the sill and S65142B to the
right, located in the innermost part of the fjord.
16
1980
Depth (cm)
0
5
10
15
20
25
30
35
40
45
Year
0
5
10
15
20
25
30
35
40
45
50
55
60
2010
2000
1990
1980
1970
1960
1950
1940
1938
G113142B
Realtive Abundance
(%)
0
S. fusiform is
50
S-K fauna
100
Depth (cm)
0
5
10
15
20
25
30
35
40
2010
2000
1990
1980
1970
1960
1950
1940
1930
1920
1910
1900
1890
1880
1870
1860
1850
1844
Realtive Abundance
(%)
B88141B Alt. 1
Year
0
S. fusifor mis
50
S-K fauna
100
Depth (cm)
0
5
10
15
20
25
30
35
40
Year
B88141B Alt. 2
2010
2004
1998
1992
1986
1980
1974
1968
1962
1956
1950
1944
1938
1932
1926
1924
Realtive Abundance
(%)
0
50
Opportunists
100
H. balthica
C. laevigata
N. labrodor ica
Depth (cm)
0
5
10
15
20
25
30
35
40
45
Year
S65142B
2006
S. fusiformis
1996
1986
1976
1966
1956
1946
1936
1926
Realtive Abundance
(%)
0
50
S-K fauna
100
1980
Figure 8. Combined results from the cores used in the transect showing the S-K
fauna and S.fusiformis. The dashed line indicates the year 1980. The cores are
placed in the same order as they were located along the transect, with AE73142B
to the left, located closest to the sill and S65142B to the right, located in the
innermost part of the fjord.
17
AE73142B
Depth (cm)
2014
S. fusiformis
100
2010
S-K fauna
50
2006
2002
1998
1994
1990
1986
1982
1978
1974
1970
Realtive Abundance
(%)
0
6. Discussion
6.1. North Atlantic Oscillation and its influence on fjord’s hydrography
Several studies noted a link between the different phases of the NAO index and the
temperature, salinity and concentration of dissolved oxygen in the Gullmar Fjord.
According to Filipsson & Nordberg (2004), a positive phase of the NAO index brings
predominately westerly winds from the Atlantic Ocean, which results in more mild and
humid winters. This theory is confirmed by the results of salinity and temperature
measurements shown at Figure 2. The bottom water salinity in the fjord is lower during a
positive NAO phase, which could be due to the increased access of fresh water, along with
the inhibition of deep-water exchange. The bottom-water temperatures during the positive
NAO index appear to be generally higher (Fig. 2). During a phase of negative NAO index
there is a reversed relationship.
From 1964 the NAO index was generally increasing, which caused a gradual decline of
bottom water oxygen in the fjord and through the positive phase of the NAO index, the deep
water of the Gullmar fjord endured more frequent episodes of hypoxia (Fig. 2). Nordberg et
al. (2000) and Filipsson and Nordberg (2004) both speculate about the possible connection
between a positive NAO phase and a declining trend in concentration of dissolved oxygen.
The results of the current study support their hypothesis. The link between the positive
NAO index and hypoxia is probably due to the inhibition of the deep-water exchange in the
fjord, which is caused by the predominately westerly winds. From the early 2000 and
onwards, the NAO index decreases towards zero and an increasing trend of concentration in
dissolved oxygen is observed (Fig. 2).
6.2. Estimated time scale
The change in the foraminiferal fauna, where the Skagerrak-Kattegat fauna decreases and
the Stainforthia fusiformis progresses, takes place at a depth of 17,5 cm in core S65142B (Fig.
5 & 8). According to Filipsson & Nordberg (2004) this shift occurred in 1980, as a
consequence of a severe low-oxygen event, which in turn was influenced by the only partial
deep-water exchange in the 1979/80. This low-oxygen event was seemingly beneficial and
probably eased the progression of the S. fusiformis, since this species is very tolerant of lower
oxygen concentrations (Polovodova et at., 2011). This explanation seems most likely and
therefore the depth of 17, 5 cm was correlated to the year of 1980 in S65142B. Through this
reference point the accumulation rate of 0,5 cm/year was calculated and the estimated
timescale on the y-axes (Figs 4, 5 & 6) could be modeled. However, there is an impending
probability that the sediment core suffered some level of compaction during the sampling.
Since this source of error has not been compensated for, the credibility of the modeled
timescale decreases with depth.
6.3. The distribution of benthic foraminifera
6.3.1. Hypoxic events and predating macro fauna
The Stainforthia fusiformis increases its numbers by ca. 66% at the onset of the low-oxygen
event of 1979/80 (Fig. 6). This opportunistic species shows a positive correlation to lower
oxygen-levels in the fjord bottom waters, or rather to the lack of competition from other
species which does not tolerate these environments. This is supported by the fact that
during an episode of higher concentrations of dissolved oxygen (1998 to 2001) S. fusiformis
decreases its numbers by roughly 54%. However, there might be an alternative or additional
18
explanation to this decline in abundance. By comparing the absolute abundance of other
foraminiferal species (Fig 6) or by simply viewing the graph for total forams (Fig. 5), a
similar decreasing trend is recognized between the years 1998 and 2001, regardless the
species preferences of oxygen concentrations. Polovodova Asteman and Nordberg (2013)
observed a similar trend at their station G113-119A, where the years 2001-2007, were also
characterized by low absolute abundance of foraminifera, despite the fact that the ecosystem
should be recovering from low-oxygen events. The authors speculated that the ecosystem in
fact was recovering quite well and that the foraminiferal decrease in abundance was due to
heavier macrofaunal predation. Even though Polovodova Asteman and Nordberg (2013)
describe a later event than what was discovered in the core S65142B, their explanation to
this phenomenon applicable for the results of present study as well. Particularly since the
total abundance of foraminifera rapidly increases after the year 2001, as the concentrations
of dissolved oxygen in the fjord again sinks below 2 ml/L, which is a critical limitation
below which many macrofaunal species die or escape the area (Polovodova Asteman &
Nordberg, 2013). The species that has the highest increase in abundance (from 6 ind./g to
39 ind./g) after the year 2001 is to no surprise S. fusiformis, once again supporting the theory
that this species is the superior competitor in low-oxygen conditions.
A similar, yet less profound decrease in abundance of the species B. marginata, T. earlandi,
E. excavatum, A. glomerata, N. turgida, S. fusiformis and Others is observed at the year 2010,
even though the oxygen-level between the years 2009 to 2011 was quite high (Fig. 2).
During the same time C. laevigata increased its numbers (Fig. 6). Keeping in mind that this
species correlates positively to higher oxygen-levels and is more resistant to predation
(Murray, 2006), one can assume that the theory of increasing abundance of predating
macrofauna, is applicable for this event as well.
6.3.2. Foraminiferal fauna in respond to food availability
In the lower part of the core corresponding to ca. 1956, the population of C. laevigata
reached its highest peak. At that time, no measurements of the oxygen, salinity or
temperature were carried out at a regular basis in the Gullmar fjord, though the NAO index
had already been measured for several decades (Hurrell, 1995). The NAO during that time
had a generally negative trend. A negative NAO index would indicate that the deep-water
environment in the fjord was well oxygenated at the year 1956, which agrees whit the
higher abundance of C. laevigata since this species in known to correlate positively to higher
oxygen concentrations (Murray, 2006). Although, it should be noted that the inner part of
the fjord at this time, was heavily exposed to oxygen-consuming pollutants (Nordberg et al.,
2000), which contradicts to the previous assumption of well-oxygenated bottom waters.
Keeping this in mind, one could argue that the increase in abundance of C. laevigata could
possibly be due to sewage-derived pollution resulting in high food availability. Also B.
marginata, H. baltica, S. fusiformis and A. glomerata have high abundances at 1956. This
supports previous statement since B. marginata, H. baltica, S. fusiformis and C. laevigata all
have a positive respond to high food availability.
The graph showing the amount chlorophyll a (Fig. 2) present a distinct peak at the year
2012. This sudden increase in food availability potentially may benefit most of the species in
this study. Indeed, species N. labradorica, B. marginata, T. earlandi, E. excavatum, A. glomerata,
N. turgida, S. fusiformis and Others all show an increase in abundances in response to this
event (Fig. 6). The total foraminifera/g also demonstrates the maximum absolute abundance
at this very year.
19
6.3.3. Possible effects of taphonomic processes
Although, the generally increasing numbers of foraminifera towards the top (Fig. 5 & 7) is
probably a result of taphonomic processes such as time averaging (Murray, 2006), rather
than a scarcer food availability, in the older sediments.
Before the mid 1960s, the inner parts of the fjord was exposed to pollution from the
Munkedal industries (Nordberg, 2014, personal com.). This correlates to the extremely low
numbers of foraminifera for the same time. Possibly pollutants caused dissolution of the
tests, because the species preserved from this period are such of more robust structure.
The abundance of agglutinated species is increasing towards the top, but is far less than
the abundance of the calcareous species (Fig. 5). This is probably due to the fact that
agglutinated tests are more susceptible to taphonomic processes. It is not surprising that the
agglutinated species found in present study are T.earlandi and A.glomerata. According to
Schröder (1988) such species, due to their morphology, have the highest fossilization
potential in the group of agglutinated species.
6.3.4. Correlations between present, previous and parallel studies
The general trend observed in present study is that the Skagerrak-Kattegat fauna (in
particular H. baltica, B. marginata and C. laevigata) are dominating at station S65 in Gullmar
Fjord during the years before the severe hypoxic event in 1979/80 (Fig. 5 & 8). After 1980
the S. fusiformis increased in abundances and a minor shift in regime of dominating species
takes place. Between the years 1980 and 1994, several minor shifts between the S-K fauna
and S. fusiformis occur, where these species continuously out-compete each other. From
1994, the S-K fauna starts to dominate again, but this time has T. earlandi as the most
dominant species, reaching nearly 40% of the total assemblage. This dominance of T.
earlandi in the newly re-established S-K fauna also occurs in the study of Polovodova
Asteman & Nordberg (2013) and in the core of Lindström (2014) and the timing of T.
earlandi dominance is approximately the same (1986) as in core S65142B from present study.
An overall similar trend concerning the development of the S-K fauna and S. fusiformis
throughout the depth, is observed in the investigations done by Davidsson (2014); course
material (2014); Polovodova Asteman and Nordberg (2013); Davidsson et al. (2013) and
Filipsson and Nordberg (2004). Although, the depth of which the shift in regime of
dominating species occurs, differs between the cores, indicating that the accumulation rate
varies throughout the fjord.
A comparison has been performed between core S65142B (present study) and three other
cores; AE73142B (Davidsson, 2014), B88141B (Lindström, 2014) and G113142B (Course
material, 2014), which together make up a transect through the Gullmar fjord (fig.1). The
general trend of an increase in the abundance of T. earlandi was noted in all cores and in
both S65142B and in B88141B, this increase occurs at the same year of 1986. Also the
disappearance of the H. baltica towards the core top was observed between all stations. The
same goes for the increase of Nonionella turgida, which reaches its peak at about 2012 in all
cores.
An intriguing discovery is that there seems to be a general increase in total abundance of
foraminifera outwards in the fjord (Fig. 6). The highest numbers of foraminifera are fewest
at station S65142B (200 ind./g), followed by B88141B (335 ind./g) and G113142B (456
ind./g). Highest numbers of foraminifera are found at station AE73142B (582 ind./g), which
is located close to the sill. The cause to this correlation between total abundance of
foraminifera and distance to the sea could be due to transportation. The juvenile
20
foraminifera along with fossilized tests, which gets brought during upwelling and water
exchange, probably accumulates closer to the sill.
It is difficult to make more specific conclusions about the transect, based on only four
samples. Therefore future, more thorough studies throughout the Gullmar fjord could be of
some interest.
7. Conclusions
The record from present study indicates that the same general dominating foraminiferal
faunas that has been reported in previous studies from Alsbäck is also characteristic at
station S65. The Skagerrak-Kattegat fauna were dominating until about 1980, when the
index of the NAO begins to shift from a negative, to a positive phase. The years of 1980 to
1994 are characterized of the continuing positive NAO and dominating was the
opportunistic species Stainforthia fusiformis. Since 1994 the S-K fauna is once again most
prominent, but with the Textularia earlandi as dominating species. A positive NAO phase no
longer dominates since the year 2010 and since then more species from the S-K fauna,
Cassidulina laevigata and Nonionellina labradorica, have been reestablishing the seafloor at
S65142B.
The species C.laevigata shows a close correlation with the variations of the NAO by
decreasing in abundance during a positive phase and increasing during a negative phase.
In the comparison with parallel studies a clear correlation between numbers of foraminifera
in individuals per gram and distance to the sill was discovered. This is probably caused by
an accumulation of transported foraminifera more closely to the sill.
The disappearance of the species Hyalinea baltica as well as the rapid increase of T.earlandi
towards the top of the sediment was observed at all stations.
21
8. Acknowledgements
I would like to thank my supervisors Dr. Irina Polovodova Asteman and Professor Kjell
Nordberg for the opportunity to do this project and for their provided guidance and
assistance in the laboratory. I would also like to thank the crew of the R/V Skagerrak for
their assistance during sampling and Dr. Lennart Bornmalm for helpful comments.
Finally I would like to thank Christoffer Bergstrand and my fellow students and partners
in crime; Sandra Davidsson and Camilla Lindström for valuable comments and interesting
scientific discussion, and for encouraging and supporting me throughout the whole process.
Thanks!
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material from 1927. Department of Earth Sciences, University of Gothenburg. pp. 1-22.
Björk, G., & Nordberg, K. (2003). Upwelling along the Swedish west coast during the 20th
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Culver, S., in Lipps, J.H., (1993). Fossil Prokaryotes and Protists. Blackwell Scientific
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Davidsson, S., Faxén, A., Guðmundsdóttir, A., Lindström, C.,
Nilsson, E. & Carl Regnéll (2013). The distribution of benthic foraminiferal fauna in the
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Filipsson, H. L., & Nordberg, K. (2004). Climate variations, an overlooked factor influencing
the recent marine environment. An example from Gullmar Fjord, Sweden, illustrated by
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22
Murray, J. W. (2006). Ecology and applications of benthic foraminifera. Cambridge
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Gullmar Fjord: The influence of seasonal hypoxia and North Atlantic Oscillation. Journal of
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Polovodova, I., Nordberg, K., & Filipsson, H. L. (2011). The benthic foraminiferal record of
the Medieval Warm Period and the recent warming in the Gullmar Fjord, Swedish west
coast. Marine Micropaleontology, 81(3), pp. 95-106.
Rosenthal, Y., Morley, A., Barras, C., Katz, M. E., Jorissen, F., Reichart, G. J., Linsley, B. K.
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23
10. Appendix
24
10. Appendix continued
25