A major change in the phytoplankton of a Swedish sill fjord—A consequence of engineering work?

Estuarine, Coastal and Shelf Science 63 (2005) 551e560 www.elsevier.com/locate/ECSS

A major change in the phytoplankton of a Swedish sill fjorddA consequence of engineering work? H.L. Filipssona,*, G. Bjo¨rka, R. Harlandb, M.R. McQuoidc, K. Nordberga a

Department of Earth Sciences, Oceanography, Go¨teborg University, Box 460, SE-405 30 Go¨teborg, Sweden b DinoData Services, 50 Long Acre, Bingham, Nottingham NG13 8AH, UK c Department of Marine Ecology, Go¨teborg University, Box 461, SE-405 30 Go¨teborg, Sweden Received 3 November 2003; accepted 7 January 2005

Abstract A major phytoplankton change occurred during the late 1930s and early 1940s in Koljo¨ Fjord, a sill fjord on the Swedish west coast. Dinoflagellate cyst concentrations increased tenfold over a short period of time, from hundreds of cysts per gram sediment to thousands; and the species composition of both dinoflagellate cysts and diatoms changed markedly. These changes took place during a period of extensive engineering work at the entrance to the fjord from the Skagerrak. At this time, the entire passage was straightened, a new channel was built in a previously shallow area, and the old connection was closed. This study investigates whether this engineering work could have sufficiently altered the surface-water circulation to bring about the change in the phytoplankton composition. Several mechanisms are explored by which the construction could have influenced the phytoplankton in the fjord. The primary mechanism is probably increased efficiency of tidal-generated surface-water exchange in the fjord, resulting in a larger transport of surface water from the Skagerrak and consequently a changed surface-water environment. This study highlights how engineering work can have a substantial impact on the local and regional marine environment, a factor that must not be overlooked in environmental planning. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: diatoms; dinoflagellate cysts; environmental change; engineering work; fjord; Sweden

1. Introduction Koljo¨ Fjord is a sill fjord on the Swedish west coast that is part of an open-ended fjord system (Fig. 1). Studies on the temporal record of diatoms (McQuoid and Nordberg, 2003) and dinoflagellate cysts from Koljo¨ Fjord (Harland et al., 2004a) have demonstrated that phytoplankton floras underwent a dramatic change around 1940. At this time the dinoflagellate cyst concentration increased tenfold, and new diatom and * Corresponding author. University of Bremen, Department of Geosciences, MARUM, P.O. Box 330 440, DE-28334 Bremen, Germany. E-mail address: fi[email protected] (H.L. Filipsson). 0272-7714/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2005.01.001

dinoflagellate species became dominant. This change in phytoplankton composition occurred over some five years. Concurrent with these changes in the phytoplankton was the initiation of major engineering work at the western entrance to the fjord. This construction was undertaken to provide shipping with a faster and safer sailing route to Uddevalla, an important harbour and ship-building town at the time. This study investigates if the phytoplankton changes were a result of that construction work. Distributions of diatoms and dinoflagellate cysts are increasingly used in marine environmental studies including the assessment of pre-industrial conditions within surface waters and the various effects of marine pollution (e.g. Dale, 2000; Kennington, 2002). In order

552

H.L. Filipsson et al. / Estuarine, Coastal and Shelf Science 63 (2005) 551e560

to use diatoms and dinoflagellate cysts as proxies in environmental studies, especially on short time scales, it is necessary to understand in detail how these proxies react to changes in the marine environment. Here, dominant species of diatoms and dinoflagellate cysts in Koljo¨ Fjord are examined from previously published records (McQuoid and Nordberg, 2003; Harland et al., 2004a). In order to circumscribe the environmental changes in Koljo¨ Fjord to the engineering work during the 1940s, a new high-resolution dinoflagellate cyst record from the adjoining basin of Havstens Fjord is presented. Havstens Fjord provides a control site where no construction occurred.

2. Study area 2.1. Topography The region around Koljo¨ Fjord is characterized by crystalline bedrock and clay-filled valleys and the land is dominated by mixed forest. The amount of cultivated land is small, but was higher at the beginning of the 20th century. There are no large towns in the vicinity of Koljo¨ Fjord, only small villages and summer houses. The nearest town in the area is Uddevalla, approximately 25 km from the fjord, which has a population of 30 500; in 1930s and 1940s the population was between 15 000 and 20 000 (Hansson, 1979). The Orust-Tjo¨rn fjord system consists of four main basins, restricted by sills and narrows. Koljo¨ Fjord is situated in the northern part of the system and enclosed by sill areas (Fig. 1). To the west is Malo¨stro¨mmar passage, which includes two channels, Bjo¨rnsund Channel and Malo¨sund (Fig. 1). Bjo¨rnsund Channel is the constructed passage, with a maximum depth of 9 m and a width of 40 m. Malo¨sund is linked to the Bjo¨rnsund Channel via a small sub-basin and has approximately the same dimensions. In the northwest is Nordstro¨mmarna, a shallow (!5 m), natural channel which is connected to Gullmar Fjord (Fig. 1). To the east at No¨tesund, there is a 10-m-deep sill which connects Koljo¨ Fjord with Havstens Fjord. Havstens Fjord is restricted to the south by a sill at 20 m. The maximum depths in Koljo¨ Fjord and Havstens Fjord are 56 and 46 m, respectively. 2.2. Hydrography

Fig. 1. Overview map over the fjord system with the different sill areas, a detailed map of the Malo¨stro¨mmar passage is inserted and at the top a high-resolution map over the Bjo¨rnsund Channel and the old connection. AeB indicate the cross section seen in Fig. 2.

The outflow of low-salinity surface water from the Baltic Sea, together with the inflow of high-salinity North Sea water, has a major influence on the hydrography of the Skagerrak and Kattegat, causing a strong salinity stratification. Surface-water salinity in the Kattegat ranges between 15 and 25, whereas the deep water is more saline (32e35) (Svansson, 1975). The Skagerrak and Kattegat are separated by a front, which varies in position, but usually runs from Skagen, Denmark, towards the northeast, to the Swedish west coast, ending outside the southern entrance of the fjord system (Gustafsson, 1999). The surface-water salinity in the open Skagerrak is considerably higher than in the Kattegat. Salinity is about 33 in the open Skagerrak and it increases to about 35 at around 100 m (Bjo¨rk and Nordberg, 2003). In the Skagerrak, low salinity water from the Baltic is mainly confined near the coast, where a baroclinic coastal current runs in a cyclonic direction. The Baltic Current runs along the Swedish west coast, joins the Norwegian coastal current, and transports water out of the Skagerrak, although some water might recirculate back. The surface circulation in Skagerrak

H.L. Filipsson et al. / Estuarine, Coastal and Shelf Science 63 (2005) 551e560

is strongly affected by winds (Gustafsson, 1999), and during northerly and north-easterly winds the Baltic Current is forced away from the Swedish west coast and deep water is upwelled (Bjo¨rk and Nordberg, 2003). Within the fjord system, the sills and narrows have a major impact on the hydrography. The halocline depth (10e15 m) is less variable than the depth of the coastal halocline. Salinity is about 22e25 in the surface water (0e10 m) and between 28 and 32 in the deep water (20 m) with the lowest salinities in Koljo¨ Fjord (Bjo¨rk et al., 2000). The salinities inside and outside the fjord system co vary, but there is also a gradual damping of the variations with distance from the southern entrance of the fjord system (Bjo¨rk et al., 2000). The salinity difference between Koljo¨ Fjord and the coastal waters is generally larger over the northern entrance compared to the southern (and much longer) entrance via the Havstens Fjord. This indicates that the exchange of waters through the northern passage is more restricted than through the southern connection with the sea (Bjo¨rk et al., 2000). Horizontal differences in the salinity of the upper layer may cause a sea surface slope, the socalled steric effect. The salinity differences between the two ends of the fjord system generate a mean steric height of 2.8 cm. Thus, sea level is generally higher at the southern end where salinity is lower; this difference in height forces a net water flow of about 70 m3 s
1 from the southern entrance to the northern opening of the fjord system (Bjo¨rk et al., 2000).

2.3. Phytoplankton production The Swedish west coast has a rich and diverse phytoplankton community (McQuoid and Godhe, 2004). Phytoplankton production is high, especially in the frontal region between the Skagerrak and Kattegat and within some of the fjords (Heilmann et al., 1994; Lindahl et al., 2003). The spring bloom is generally characterized by diatoms, especially species of Chaetoceros, Skeletonema, and Thalassiosira, but some dinoflagellates are also common (e.g. Pentapharsodinium dalei). In late summer and autumn, a different suite of diatoms and dinoflagellates dominates, and species such as Lingulodinium polyedrum can be abundant at this time (Harland et al., 2004b). Following a bloom, many phytoplankton species produce cysts or spores as part of their natural life cycle, which sink to the sediments. Dominant taxa identified from surface sediments along the Swedish west coast include the dinoflagellates, L. polyedrum, Protoceratium reticulatum, P. dalei and Scrippsiella trochoidea, and the diatoms, Detonula confervacea, Chaetoceros spp., Skeletonema costatum and Thalassiosira spp. (Persson et al., 2000; McQuoid, 2002; Godhe and McQuoid, 2003). However, the dinoflagellate species S. trochoidea produces a calcareous

553

cyst that is not preserved following acid digestion processing techniques. The species, D. confervacea and L. polyedrum, are particularly abundant within the Orust-Tjo¨rn fjord system compared with coastal stations north and south of our study area (Persson et al., 2000; McQuoid, 2002).

3. Materials and methods 3.1. Local history The newspaper, Bohusla¨ningen (microfilm, Go¨teborg University library), and Hansson (1979) provided detailed information on the construction of the Malo¨stro¨mmar passage. In addition, depth soundings were taken to determine water depths in the old connection, and old nautical charts were studied to determine the water depths in the area of the passage before the new channel was built. 3.2. Sediment analyses Sediment cores were collected in Koljo¨ Fjord (58  13#62N, 11  34#25E; 43 m water depth) and Havstens Fjord (58  18#65N, 11  52#22E; 47 m water depth) in September 1998 and June 1999, using Multiple and Gemini cores. Both sampling techniques give virtually undisturbed sediment surfaces. The cores were X-rayed on board using an Andrex BV 155 (140 kV/10 mA) portable X-ray machine. The sediments from Koljo¨ Fjord smelled of H2S, unlike the samples from Havstens Fjord. Cores from both locations were divided into 1cm-thick slices. During sectioning of core KG1 from Koljo¨ Fjord, it became evident that the core extruder caused some compaction. To determine the actual sample depths the presence of larger sand particles, gravel, and shells in the sectioned sediment was compared with observations from the X-radiographs. As a result, the sectioned sample depths were adjusted for compaction by the addition of 1 mm to each centimeter. 3.3. Micropaleontological and palynological analysis Each section of core KG1 was also divided into two halves, one half, KG1A, was used for dinoflagellate cysts analyses and the other, KG1B, to study the diatom flora. Core H3A from Havstens Fjord was used only for dinoflagellate cyst analysis. The dinoflagellate cyst samples were processed at the Palynology Research Facility, Department of Animal and Plant Sciences, University of Sheffield, using standard palynological processing techniques (see Wood

554

H.L. Filipsson et al. / Estuarine, Coastal and Shelf Science 63 (2005) 551e560

et al., 1996) including a wash with !10% nitric acid to rid the residues of unwanted amorphous organic material (AOM). The organic residues were separated from the post-acid material using zinc chloride solution with a specific gravity of 1.96 g cm
3. The resulting organic residue was mounted using Petrapoxy 154 with a refractive index of 1.54. Slides were counted for their dinoflagellate cyst content using an !40 objective on a Zeiss Axiolab microscope. To enable the calculation of the numbers of cysts per gram of sediment, the original dry weight of sediment was noted and aliquots of the organic residues were mounted and counted (Harland, 1989). The organic residues were counted on a single slide either representing 1 g of sediment or a fraction thereof. The raw counts are available from the authors and further details are presented in Harland et al. (2004a). The samples for diatom analysis were dried, acidcleaned (McQuoid and Hobson, 1997) and mounted in Naphrax. At least 350 diatom valves were counted in each sample. Details of counting and taxonomic identifications are described in McQuoid and Nordberg (2003).

3.4. Radiometric dating Additional cores K6A and H4A, from Koljo¨ Fjord and Havstens Fjord, respectively, were collected in 1998, using a Multiple corer, and used to establish a chronology. The cores were dated using 210Pb methodology and the constant rate of supply (CRS) model (Appleby and Oldfield, 1978). Detailed processing techniques concerning 210Pb dating are presented in Nordberg et al. (2001). The age of each sediment layer refers to the bottom part of the respective layers. Due to increasing errors when the 210Pb excess approaches zero, the older sediment layers were dated assuming the same sedimentation rates as in younger layersdabove 25 cm (w0.4 cm yr
1) for Koljo¨ Fjord and above 20 cm (w0.3 cm yr
1) for Havstens Fjorddbut corrected for compaction. The analyses were performed at Department of Radiation Physics, University of Lund, Sweden. The sediments in both Havstens and Koljo¨ Fjords are laminated in some parts, but lamination is more extensive in Koljo¨ Fjord. By comparing the positions of shell horizons and the distinctive light layers in the Koljo¨ Fjord core, KG1, with the dated core, K6A, an age model could be established. Clearly visible laminations in the X-ray images of the Havstens Fjord cores showed that core H3A was almost identical to the dated core, H4A. Accordingly, the accumulation rate determined for H4A could be used for H3A without any modifications. In the cores from both Koljo¨ Fjord and Havstens Fjord, each centimeter represents about 2e3 years of accumulation.

4. Results 4.1. Topographical changes of the Malo¨stro¨mmar passage In 1936, the first preparations to change the Malo¨stro¨mmar passage were made, and between the years 1936 and 1946 major engineering work took place. The entire passage was straightened, several shoals were blasted away, and the Bjo¨rnsund Channel was constructed. Between 1943 and 1946, the new channel was open together with the previous natural connection, however, this caused strong cross currents, which led to a number of shipping accidents. The old passage was, therefore, closed in 1946 and all shipping used the new Bjo¨rnsund Channel. The main topographical changes are summarised in Table 1, and illustrated in Figs. 1 and 2. The cross sectional area at the sill within the old connection was about 360 m2 (calculated using the sounding data), and the cross section of the new channel is about the same, 325 m2 (Bjo¨rk et al., 2000). The shape of the two passages, however, is different (Fig. 2). The total surface area of the old connection was about 3.6 ! 105 m2 with about 50% being shallower than 6 m, whereas the Bjo¨rnsund Channel has a surface area of 1.8 ! 104 m2. The volume of Bjo¨rnsund Channel is about 1.1 ! 105 m3, whereas the volume of the old connection was considerable larger, 1.9 ! 106 m3.

4.2. Dinoflagellate cysts in Koljo¨ Fjord and Havstens Fjord During much of the last 200 years, the concentration and diversity of dinoflagellate cysts in Koljo¨ Fjord was very low; the concentration was about 150 cysts g
1 sediment (dry weight) in the early part of the 20th century and on average nine species were present (Fig. 3). During this time, the species Lingulodinium polyedrum was found only rarely in the record with the assemblage characterized by Spiniferites spp., Protoperidinium oblongum, and round, brown Protoperidinium cysts (Harland et al., 2004a). In the later part of the 1930s and early 1940s the total cyst abundance increased abruptly to 1500e2000 cyst g
1 sediment, and L. polyedrum becomes Table 1 Geometrical differences between the old and new passages Variable

Old, natural passage

Bjo¨rnsund Channel

Average depth (m) Area of the cross section (m2) Surface area of the passage (m2) Length (m) Volume (m3)

8 360 3.6 ! 105 ca. 1000 1.9 ! 106

9 325 1.8 ! 104 300 1.1 ! 105

555

H.L. Filipsson et al. / Estuarine, Coastal and Shelf Science 63 (2005) 551e560

Cross section of the old connection

water depth (m)

A

B

0

0

4

4

8

8

12

The Björnsunds Channel

12 0

20

40

60

80

100

120

140

160

0

180

20

40

distance (m)

distance (m)

Fig. 2. Schematic drawing of the shape of the cross sections in the old natural passage and in Bjo¨rnsund Channel. A and B indicate the location seen in Fig. 1.

about 40 000 cysts g
1 sediment between the years 1930 and 1950, and at present the cyst concentration is approximately 15 times higher in Havstens Fjord compared with Koljo¨ Fjord. Lingulodinium polyedrum has been the dominant species in Havstens Fjord for at least the last 120 years (Fig. 3), a longer presence than in Koljo¨ Fjord. Other common species in Havstens Fjord are Protoceratium reticulatum, Spiniferites bentorii, and Pentapharsodinium dalei (Fig. 3). In Havstens Fjord, P. dalei increases in the beginning of the 1930s (Fig. 3), slightly earlier than its increase in Koljo¨ Fjord. Comparison of the two records reveals that in addition to the climatic or regional environmental changes, a

a prominent member (30e50%) of the cyst assemblage. Other species which also increase at the same time are Protoceratium reticulatum, Spiniferites bentorii, and Pentapharsodinium dalei (Fig. 3). The dinoflagellate cyst record from Havstens Fjord, a less restricted basin, is somewhat different from that in Koljo¨ Fjord, even though the same species are present in both records (Fig. 3). The accumulation rates of sediment are similar in the two fjords, which enables direct comparisons of cyst concentrations between the two locations. The record from Havstens Fjord approximately represents the years 1885e1998. Cyst concentrations are much higher in Havstens Fjord; there were

Core H3A Cysts/g 0

30000

60000 0

5000 10000 0

6000 12000

0

3000 6000 0

30000

60000

2000

1980

1960

Engineering work

Year (AD)

1940

KG1A H3A

1920

1900

1880

1860

1840

1820 0

1500

3000

L. polyedrum/g

0

500

1000

P. reticulatum/g

0

150

300

S. bentorii/g

0

200

400

P. dalei/g

0

3000

6000

Total no. of cysts/g

Core KG1A Fig. 3. A selection of the most important dinoflagellate cysts species in Koljo¨ and Havstens Fjords, presented as number of specimens per gram. Note the different scales. The lower scale is for the Koljo¨ Fjord record and the upper scale is for the Havstens Fjord record.

556

H.L. Filipsson et al. / Estuarine, Coastal and Shelf Science 63 (2005) 551e560

unique change occurred in Koljo¨ Fjord. Research is ongoing to document fully the other changes occurring within this regional marine environment. 4.3. Diatoms In Koljo¨ Fjord, the diatom flora is greatly altered in the late 1930s and early 1940s. Remarkably, this change occurs at exactly the same time as the change in the dinoflagellate cyst assemblage (Fig. 4). The diatom and dinoflagellate cyst samples in Koljo¨ Fjord originate from the same core and the changes occur in precisely the same sample, 23 cm depth. At this time, there is an increase in planktonic diatoms, primarily spore-forming taxa such as Chaetoceros spp., Detonula confervacea, and Thalassiosira spp. (Fig. 4). One spore-forming diatom, Bacterosira bathyomphala, disappears at this level. Detonula confervacea increases noticeably from a relative abundance of just a few percent to well over 10%. The relative abundance of this cold-water species continues to increase until the beginning of the 1990s. At this time, there is no diatom sediment record available from Havstens Fjord so we cannot compare between the two locations as we did for the dinoflagellate cyst records. The dinoflagellate cyst and diatom record found in the sediments are only one part of the total phytoplankton record because little information is available for non-cysts/spore forming species during this time. Previous studies, however, have compared phytoplankton from the water mass and sedimented assemblages in this

area (McQuoid, 2002; Godhe and McQuoid, 2003) and show that the record of spore and cyst forming species is relatively good. Together, the dinoflagellate cyst and diatom records suggest that the phytoplankton community was greatly altered in a short period of time, over a couple of years. Because the changes occurred in species produced in late winter to spring and species that dominate in late summer (Harland et al., 2004b), the causative factor(s) must have extended over more than one season.

5. Discussion Our results demonstrate that in Koljo¨ Fjord, changes in phytoplankton community structure occurred at the same time as engineering work modified the western entrance of the fjord; and that these changes are not associated with other changes in the regional marine environment, such as those recorded in Havstens Fjord. Several different mechanisms might have affected the surface-water circulation through the fjord and thereby the phytoplankton flora. These different mechanisms are outlined below. 5.1. Increased transport from the east Salinity differences between the southern and northern entrances of the fjord system drive a subtidal net flow from south to north through the fjord system (Bjo¨rk et al., 2000). Since Lingulodinium polyedrum and

2000

1980

1960

Engineering work

Year (AD)

1940

1920

1900

1880

1860

1840

1820 0

3

D. confervacea

6

0

1

2

0

0.2

Thalassiosira spp. B. bathyomphala

0

15

30

0

15

30

Diatoms (-spores) Chaetoceros spores

Core KG1A Cells (106 /g sed.) Fig. 4. A selection of the dominant planktonic diatoms in Koljo¨ Fjord presented as 106 cells per gram. Note the different scales.

H.L. Filipsson et al. / Estuarine, Coastal and Shelf Science 63 (2005) 551e560

the other dinoflagellate species occur in high numbers in Havstens Fjord, it is reasonable to expect that the increases documented in Koljo¨ Fjord are caused by possible increased transport from the east, from Havstens Fjord. The total subtidal transport, Q, for a given sea level difference, h, over the entire fjord system can be estimated from the relation used in the model study by Bjo¨rk et al. (2000):
 AM AN pffiffiffiffiffiffiffiffi QZkm pffiffiffiffiffiffiffiCpffiffiffiffiffiffiffi 2gh ð1Þ CM CN where AM and AN are the cross sections of the Bjo¨rnsund Channel and Nordstro¨mmarna, respectively, g is gravitational acceleration, and km is the tidal reduction factor. CM and CN are constants that describe the frictional resistance in the Malo¨stro¨mmar passage (including Bjo¨rnsund Channel) and Nordstro¨mmarna, respectively. The values of these coefficients (CM Z 2.7 and CN Z 5.5) were determined from simultaneous sea level and current measurements (Bjo¨rk et al., 2000). The larger value of C for Nordstro¨mmarna is due to the longer length and much more complicated geometry of this passage, which includes several shallow narrows. It is reasonable to assume that frictional resistance (C ) in the old Malo¨stro¨mmar passage was somewhere in between 2.7 and 5.5, but it was likely closer to 2.7 since the geometry of the old passage was longer and more complex than at present and was much shorter and less restricted than Nordstro¨mmarna. A 20% larger C value for the old passage seems realistic. However, the old connection probably had a slightly larger cross section (360 m2 compared to 325 m2) and the net change of the subtidal flow according to our Eq. (1) then becomes very small. Consequently, there is no reason to believe that the subtidal flow was significantly different before the construction of the new passage, provided that the typical water level difference was the same over the fjord system. Therefore, it is reasonable to conclude that the phytoplankton changes in Koljo¨ Fjord are not the result of increased transport from the east. 5.2. Increased water exchange generated by tides Tides are generally small on the Swedish west coast, 0.15e0.30 m. In the narrow and relatively shallow Malo¨stro¨mmar passage, however, local tidal currents can be of crucial importance. One result of shortening and straightening the passage could be increased tidal transport of surface water from the west, and this could explain the change in phytoplankton composition and abundance. An estimate of the tidal exchange can be found by comparing the total volume that passes the Bjo¨rnsund Channel with the volume of all the sub-basins of the

557

entire passage. Velocity measurements from the Bjo¨rnsund Channel (Bjo¨rk et al., 2000) have an amplitude of about 60 cm s
1 and the cross section of the Bjo¨rnsund Channel is 325 m2. This results in a water transport of 2.8 ! 106 m3 during half a tidal period (based on the semidiurnal component M2, which is the dominating tidal constituent). The volume of Malo¨stro¨mmar (including the old passage) was about 6.4 ! 106 m3 while the contemporary, and shorter, passage of Malo¨stro¨mmar (including Bjo¨rnsund Channel) has a volume of about 4.6 ! 106 m3. This gives a flushing rate of 44% for the old passage and 61% for the new passage. A flushing rate below 100% does not necessarily mean that no water is able to pass the entire passage during a tidal period. If the flow is not uniform over the cross section but includes a core with higher velocities, it is still possible for water parcels to traverse the passage from west to east. At the present time, such a core is easily seen, by eye, over several hundred meters west of the Bjo¨rnsund Channel. Since the new passage is shorter (3000 m compared to 3600 m), of smaller volume, and generally straighter, it is likely that the tidal exchange was larger after the construction. Although difficult to prove, it is also possible that the net westward subtidal circulation was often strong enough to completely block the tidal water exchange before the new passage was built. It can be concluded that the straightening and shortening of the new passage probably resulted in a more efficient tidal exchange between Koljo¨ Fjord and the Skagerrak. In addition to altering the surface-water environment, the increased tidal exchange around 1940 probably supplied Koljo¨ Fjord with additional sporeand cyst-forming diatoms and dinoflagellates from the Skagerrak. This inoculum of cells may have contributed to the floral changes documented in the sediment record. The exact properties that have changed in the fjord are difficult to determine. The few salinity measurements collected in the fjord in the early part of the 20th century do not suggest any salinity differences. The extra supply of transported plankton and their spores and cysts from the Skagerrak may have been sufficient to introduce an inoculum to Koljo¨ Fjord and to provide ongoing surface waters with all the required abiotic factors to ensure a continuing rich phytoplankton flora. Nevertheless, it is important to consider that even though the dinoflagellate cyst concentration increased tenfold in Koljo¨ Fjord, and that conditions became somewhat more suitable for dinoflagellates following the opening of the Bjo¨rnsund Channel, Koljo¨ Fjord is still a less favourable environment for dinoflagellates than neighbouring areas. As a comparison, up to 11 000 cysts g
1 sediment is recorded from the neighbouring Gullmar Fjord (Persson et al., 2000). It is also important to note that this surface water change had no significant influence on the benthic environment, since no changes in sediment textures, laminations, or benthic life are

558

H.L. Filipsson et al. / Estuarine, Coastal and Shelf Science 63 (2005) 551e560

recorded in the sediment record from this time period (cf. Nordberg et al., 2001; Filipsson and Nordberg, 2004). This suggests that the influence of the engineering work on the marine environment in Koljo¨ Fjord is mainly restricted to the surface water. 5.3. A general increased nutrient supply It is possible that the altered phytoplankton composition and increased dinoflagellate cyst production in Koljo¨ Fjord was the result of increased nutrients within the surface water. One potential source of nutrients is sewage discharge from the town Uddevalla and surrounding areas; however, a gradual increase of nutrients from these sources would likely result in a corresponding gradual increase of phytoplankton abundance. The changes documented herein for Koljo¨ Fjord are marked and abrupt, which suggests a sudden rather than gradual change. Additionally, we would have seen corresponding and synchronous changes in the dinoflagellate cyst record from Havstens Fjord, which is situated closer to Uddevalla. Interestingly, in Havstens Fjord there is a gradual increase of dinoflagellate cysts since the 1910s, and this indeed might reflect increased nutrient supply from Uddevalla. Consequently, it is not likely that a change in nutrient levels within the surface waters caused the marked change in the phytoplankton in Koljo¨ Fjord.

velocities as in the Malo¨ Stro¨mmar, it is possible to estimate the turbulent turnover time. The turbulent velocity scale was 0.02 m s
1 in O¨resund and using 8 m as average depth of the passage gives a turbulent turnover time of about 400 s. This is much smaller than the advective time scale for the water to pass the passage, which is about 7500 s based on a total length of 3000 m and a 0.4 m s
1 velocity. The new channel considerably shortened the distance that water has to pass to reach Koljo¨ Fjord, and the total surface area of the Malo¨ stro¨mmar connection was reduced by about 30% and it is, therefore, realistic to assume that the population of filter feeders has decreased to the corresponding degree. The old passage has a number of shallow rock and sandy bottom areas, which are suitable habitats for mussels and other filter feeders (Fig. 1) that are excluded in the new passage. If fewer filter feeders were present after the construction, it is likely that more phytoplankton cells and different species (cf. Figs. 3 and 4) would reach the fjord. However, there are no measurements of mussel density in the Malo¨ Stro¨mmar area before and after the channel construction, so it is not possible to verify if the filtering capacity has changed, but changes in filter feeders may have contributed to the sharp increase in phytoplankton numbers following the engineering work. 5.5. Environmental factors not related to the engineering work

5.4. Reduced filtering by mussels Another process that significantly may have influenced phytoplankton abundances and species composition and reinforced the mechanism of increased transport of phytoplankton from the Skagerrak, is a reduction in the filtering capacity of mussels and other filter feeders in the fjord entrance. The effect of mussels on phytoplankton is well known (e.g. Møhlenberg, 1995; Nore´n, et al., 1999; Haamer and Rodhe, 2000). Both phytoplankton composition and concentration can change when water passes an area containing dense mussel populations. For instance Nore´n et al. (1999) showed that the phytoplankton biomass was reduced by 74% and the proportion of small cells (2e 12 mm) increased after passing a mussel bed in O¨resund strait (Fig. 1). The new channel has mussels (Mytilus edulis) in the deeper parts and mussels (M. edulis) were also abundant in the old passage. Unfortunately there is no information on how dense the mussel population may have been in early part of the 20th century. A prerequisite for the mussels to be effective is that the turbulence is intensive enough such that the major part of the water column in the passage is available for filtering by mussels. Using the results from an investigation of mussel banks in O¨resund strait (Haamer and Rodhe, 2000) with similar depth range and current

The changes to the phytoplankton community occurred rapidly and the timing with the engineering work is striking; however, other factors can also influence the marine environment and affect the phytoplankton composition. The only dinoflagellate species which responds at about the same time in both Havstens Fjord and Koljo¨ Fjord, is Pentapharsodinium dalei. Although little is known about its ecology, P. dalei is common in the spring bloom (Dale, 1977; Harland et al., 2004a) and in polar and subpolar areas (Dale, 1996; Rochon et al., 1999). Our results suggest that this species could be a part of larger regional signal, not related to changes discussed in this study, since it increases in several other fjords along the Swedish west coast at around the same time (Harland and Nordberg, unpublished data). The diatom assemblage changes from a community dominated by Bacterosira bathyomphala in the 1800s to a community dominated by Chaetoceros spp., Detonula confervacea, and Thalassiosira spp. in the late 1930s to early 1940s. Since D. confervacea is a spore-forming diatom, it may have had an advantage over other species and could become a dominant species; however, the species which it replaced, B. bathyomphala, is also sporeforming, so it is probable that other factors also contributed to its increase. Both B. bathyomphala and

H.L. Filipsson et al. / Estuarine, Coastal and Shelf Science 63 (2005) 551e560

D. confervacea are Arctic-Boreal species (Hasle, 1973; Matishov et al., 2000). Since both species are common in cold environments, it is curious that D. confervacea increased its population so abruptly once B. bathyomphala declined, and also that it reaches considerably higher concentrations compared with B. bathyomphala. Air temperature records (3-year running mean) from Vinga Island, a station near Koljo¨ Fjord, show that from the mid-1800s to 1930 winter temperatures varied only slightly (ca.
2 to C2.5  C), whereas after 1930 winter temperatures become more variable (ca.
4 to C4  C) with especially cold winters around 1940 (Nordberg et al., 2001). These changes in temperature extremes may have contributed to the sudden predominance of D. confervacea if this species is more adapted to temperature fluctuations.

6. Conclusions In the late 1930s to early 1940s the phytoplankton community of dinoflagellate cysts and diatoms changed markedly in Koljo¨ Fjord. This occurred concurrently with major engineering work at the western entrance to the fjord. It is possible and probable that these two events are related. Construction of the Bjo¨rnsund Channel and the general straightening of the route likely increased the efficiency of tidal-generated surfacewater exchange in the fjord, resulting in a larger transport of surface water and plankton from the west (i.e. the Skagerrak). This increased flow to the fjord and the subsequent environmental change is considered here to be the primary cause of the documented phytoplankton changes. Another mechanism, which might have influenced the flora but cannot be quantified, is the decrease in filtering capacity from filter feeders in the new sill area, a feature that also may reinforce the effects from the increased inflow of Skagerrak water. These results from Koljo¨ Fjord show how relatively small changes in topography and hydrography result in alterations to the pelagic environment that cause substantial changes of phytoplankton abundance and composition. Because some dinoflagellates and diatoms can form harmful algal blooms (HABs) that can have negative impacts on mariculture, it is essential that environmental impact studies for marine engineering work consider effects on the pelagic environment.

Acknowledgements The authors would like to thank the crews of R/V Arne Tiselius and R/V Skagerrak for their assistance in collecting the cores. We thank Dr. Olof Liungman for sharing his volume calculations for Malo¨stro¨mmar and Bohusla¨ns Museum for information concerning the

559

engineering work in the fjord. R. Harland acknowledges the assistance of Steve Ellin, Rob Cook and Ahwad B. Ibrahim in providing all the palynological processing at the Palynology Research Facility, University of Sheffield. We also thank the three anonymous reviewers for helpful and constructive comments. We are grateful for financial support from the Swedish Natural Science Research council (NFR, grants no G 650-1998/576/ 2001), Futura Foundation, Carl Tryggers Foundation, Oscar and Lili Lamm Foundation, Magnus Bergvall’s Foundation, Wa˚hlstro¨m’s Foundation, and Go¨teborg University Marine Research Centre (GMF). Finally H.L. Filipsson acknowledges support from EUROPROX (European Graduate CollegeeProxies in Earth History).

References Appleby, P.G., Oldfield, F., 1978. The calculation of lead-210 dates assuming a constant rate of supply of unsupported 210Pb to the sediment. Catena 5, 1e8. Bjo¨rk, G., Liungman, O., Rydberg, L., 2000. Net circulation and salinity variations in an open-ended Swedish fjord system. Estuaries 23, 367e380. Bjo¨rk, G., Nordberg, K., 2003. Upwelling along the Swedish west coast during the 20th century. Continental Shelf Research 23, 1143e1159. Dale, B., 1977. New observations on Peridinium faeroense Paulsen (1905), and classification of small orthoperidinioid dinoflagellates. British Phycological Journal 12, 241e253. Dale, B., 1996. Dinoflagellate cysts ecology: modeling and geological applications. In: Jansonius, J., McGregor, D.C. (Eds.), Palynology: principles and applications, vol. 3. American Association of Stratigraphic Palynologists Foundation, Salt Lake City, pp. 1249e1275. Dale, B., 2000. Dinoflagellate cysts as indicators of cultural eutrophication and industrial pollution in coastal sediments. In: Martin, R.E. (Ed.), Environmental micropaleontology: the application to environmental geology. Topics in geobiology, vol. 15. Kluwer Academic/Plenum Publishers, New York, pp. 305e321. Filipsson, H.L., Nordberg, K., 2004. A 200-year environmental record of a low oxygen fjord, Sweden, elucidated by benthic foraminifera, sediment characteristics and hydrographic data. Journal of Foraminiferal Research 34, 277e293. Godhe, A., McQuoid, M.R., 2003. Influence of benthic and pelagic environmental factors on the distribution of dinoflagellate cysts in surface sediments along the Swedish west coast. Aquatic Microbial Ecology 32, 185e201. Gustafsson, B., 1999. High frequency variability of the surface layers in the Skagerrak during SKAGEX. Continental Shelf Research 19, 1021e1047. Haamer, J., Rodhe, J., 2000. Mussel Mytilus edulis (L.) filtering of the Baltic Sea outflow through the O¨resund e an example of a natural, large-scale ecosystem restoration. Journal of Shellfish Research 19, 413e421. Hansson, W., 1979. Bohusla¨n genom hundra a˚r. Zindermans, Go¨teborg, 318 pp. Harland, R., 1989. A dinoflagellate cyst record for the last 0.7 Ma from the Rockall Plateau, northeast Atlantic Ocean. Journal of the Geological Society 146, 945e951. Harland, R., Nordberg, K., Filipsson, H.L., 2004a. A high-resolution dinoflagellate cyst record from Koljo¨ Fjord, Sweden and its

560

H.L. Filipsson et al. / Estuarine, Coastal and Shelf Science 63 (2005) 551e560

implications for palaeoenvironmental analysis. Review of Palaeobotany and Palynology 128, 119e141. Harland, R., Nordberg, K., Filipsson, H.L., 2004b. The seasonal succession of dinoflagellate cysts in Koljo¨ Fjord, west coast of Sweden. Review of Palaeobotany and Palynology 128, 107e117. Hasle, G.R., 1973. Some marine plankton genera of the diatom family Thalassiosiraceae. Nova Hedwigia Beihefte 45, 1e49. Heilmann, J.P., Richardson, K., Ærtebjerg, G., 1994. Annual distribution and activity of phytoplankton in the Skagerrak/ Kattegat frontal region. Marine Ecology Progress Series 112, 213e223. Kennington, K., 2002. The environmental applications of diatoms. In: Haslett, S.K. (Ed.), Quaternary Environmental Micropalaeontology. Arnold, London, pp. 166e184. Lindahl, O., Belgrano, A., Malmgren, B.A., 2003. Increased phytoplankton production in the Gullmar Fjord, Sweden, 1985e1999. ICES Marine Science Symposia 219, 387e389. Matishov, G.G., Makarevich, P., Timofeev, S., Kuznetsov, N., Druzhkov, N., Larionov, V., Golubev, V., Zuev, A., Adrov, N., Denisov, V., Iliyn, G., Kuznetsov, A., Denisenko, S., Savinov, V., Shavikyn, A., Smolyar, I., Levitus, S., O’Brien, T., Baranov, O., 2000. Biological atlas of the Arctic Seas 2000: plankton of the Barents and Kara Seas. NOAA Atlas NESDIS 39. U.S. Gov. Printing Office. McQuoid, M.R., 2002. Pelagic and benthic environmental controls on the spatial distribution of a viable diatom propagule bank on the Swedish west coast. Journal of Phycology 38, 881e893. McQuoid, M.R., Godhe, A., 2004. Recruitment of coastal planktonic diatoms from benthic versus pelagic cells: variations in bloom development and species composition. Limnology and Oceanography 49, 1123e1133.

McQuoid, M.R., Hobson, L.A., 1997. A 91-year record of seasonal and interannual variability of diatoms from laminated sediments in Saanich Inlet, British Columbia. Journal of Plankton Research 19, 173e194. McQuoid, M.R., Nordberg, K., 2003. Environmental influence on the diatom and silicoflagellate assemblages in Koljo¨ Fjord (Sweden) over the last two centuries. Estuaries 26, 927e937. Møhlenberg, F., 1995. Regulating mechanisms of phytoplankton growth and biomass in a shallow estuary. Ophelia 42, 239e256. Nordberg, K., Filipsson, H.L., Gustafsson, M., Harland, R., Roos, P., 2001. Climate, hydrographic variations and marine benthic hypoxia in Koljo¨ Fjord, Sweden. Journal of Sea Research 46, 187e200. Nore´n, F., Haamer, J., Lindahl, O., 1999. Changes in the plankton community passing a Mytilus edulis mussel bed. Marine Ecology Progress Series 191, 187e194. Persson, A., Godhe, A., Karlson, B., 2000. Dinoflagellate cysts in recent sediments from the west coast of Sweden. Botanica Marina 43, 69e79. Rochon, A., de Vernal, A., Turon, J.L., Matthiessen, J., Head, M.J., 1999. Distribution of recent dinoflagellate cysts in surface sediments from the North Atlantic Ocean and adjacent seas in relation to sea-surface parameters. American Association of Stratigraphic Palynologists Foundation, Contribution Series 35, 1e146. Svansson, A., 1975. Physical and chemical oceanography of the Skagerrak and the Kattegat I Open sea conditions. Fishery Board Sweden, Institute of Marine Research, 72 pp. Wood, G.D., Gabriel, A.M., Lawson, J.C., 1996. Palynological techniques e processing and microscopy. In: Jansonius, J., McGregor, D.C. (Eds.), Palynology: principles and applications, vol. 1. American Association of Stratigraphic Palynologists Foundation, pp. 29e50.