Stykkisholmur Patreksfjordur Bolungarvik Isafjordur Ogur Holmavik Blonduos Akureyri Husavik Kopasker Thorshofn Vopnafjordur Reydarfjordur Breiddalsvik ...
/. Mar. Biol. Ass. U.K. (1998), 78, 985-1001 Printed in Great Britain
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MORPHOLOGICAL VARIATION IN FUCUS VESICULOSUS POPULATIONS ALONG TEMPERATURE AND SALINITY GRADIENTS IN ICELAND. A. KALVAS AND L. KAUTSKY Department of Botany, Stockholm University, S-106 91 Stockholm, Sweden
Geographical morphological variations in Fucus vesiculosus populations were studied along the coast of Iceland. Principal component analysis (PCA) based on 11 morphological characters clustered the 26 sampling sites into four different morphological types as follows: (1) a morphological type found along the S-SW-W coast; (2) an intermediate form comprising only two populations in the NW; (3) a type found along the N-NE-E-SE coast; and (4) a type found independent of geographical area, in estuaries and at sites influenced by large freshwater outflows. Populations influenced by low salinity had significantly (P^ 0-001) shorter thalli, a shorter distance from the holdfast to the oldest dichotomy, smaller fronds, narrower stipes and midrib width compared to the morphology of all other more saline populations. No significant difference in frond width was found between the S-SW-W and the N-NE-E-SE populations. However, significant (P< 0-001) morphological differences between them were observed, the former having shorter thalli, a greater distance from the holdfast to the oldest dichotomy, narrower stipes and smaller midrib width compared to the latter. These differences between populations along the S-SW-W and the N-NE-E-SE coasts are suggested to be due to combined effects of rare, long distance dispersal of reproductive fronds by the warm-temperate Irminger Current and the cold, Arctic East Icelandic Current respectively and the short dispersal range of Fucus gametes. Thus, over the years, the East Icelandic Current may have transported reproductive fronds from the northernmost F. vesiculosus populations on the Greenland coast and the Barents Sea to the eastern coast of Iceland while the Irminger Current has carried thalli from the more southerly distribution of Fucus on the North American east coast to the southwestern part of Iceland. Excluding the populations affected by reduced salinity, the comparatively similar morphology within these two areas, may be explained by the restricted dispersal of the short lived F. vesiculosus gametes. INTRODUCTION Macroalgal species composition, phenology and growth rates are influenced by several different environmental factors, e.g. currents, temperature, salinity, wave exposure, desiccation, frost and light regime. Such factors may affect both the species composition and the morphology along a coastal stretch in different ways. Macroalgae are also affected by hydrographical conditions, which due to currents, e.g. the Gulf Stream, change over large geographical areas. The Icelandic macroalgal vegetation is influenced by two currents, the warm-temperate North Atlantic Drift and the cold East Greenland Current. According to Liming (1990) the algal vegetation around
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A. KALVAS AND L. KAUTSKY
Iceland is mainly cold-temperate, while the species composition in the eastern fjords are considered to be subarctic (Hansen & Ingolfsson, 1993) suggesting the influence of the East Greenland Current. The bladder wrack, Fucus vesiculosus L. is a well studied seaweed in many areas (Chapman, 1995; and lit. cit. therein). However, with the exception of some taxonomyrelated, morphological descriptions (Svedelius, 1901; Levring, 1940; Waern, 1952; Gessner & Schramm, 1971; Fletcher, 1987) very few comparative morphological studies have been carried out on this species (Kalvas & Kautsky, 1993; Back, 1993a). Many seaweeds have also been found to be very plastic (Back, 1993a,b; Sideman & Mathieson, 1985) which has been suggested to be due to genetic as well as environmental factors (Mathieson et al., 1981). Among these, the genus Fucus including F. vesiculosus is known to be extremely polymorphic, as exemplified in Knight & Parke (1950), Burrows & Lodge (1951), Waern (1952) and Luther (1981). Fucus vesiculosus has a wide temperature range and is also the most euryhaline fucoid species, penetrating far into estuaries with highly variable salinities affiliated with tidal dynamics. In the brackish Baltic Sea, where the salinity at any given site is comparatively stable over a long time period, it occurs down to salinities of about 4psu. Reduced size in seaweeds as a response to salinity stress is a general pattern in many marine areas (Gibb, 1957; Liming, 1990). A reduction in size of F. vesiculosus due to reduced or variable salinity (Gessner & Schramm, 1971) has been observed over limited areas in estuaries, as well as in the northern part of the Baltic Sea (Waern, 1952; Muller-Stoll & Kiintzenbach, 1956; Luther, 1981) and has been suggested to be caused by the low salinities down to 4-^4-5 psu (Kautsky et al., 1992 and lit. cit. therein; Serrao et al., 1996). Wave exposure is another factor that has been shown to account for many of the morphological differences in F. vesiculosus both over limited distances (Kalvas & Kautsky, 1993; Back, 1993a,b) as well as between populations over larger geographical areas (Liming, 1990). In all instances plasticity can be attributed to genetic differences and /or environmental growth conditions giving rise to various morphological forms. It has also been suggested that hybridization is the cause of increased morphological variation and Scott & Hardy (1994) have presented data showing the existence of hybrids between F. spiralis L. and F. vesiculosus.
The present investigation studies the morphological variation found in F. vesiculosus populations along the volcanic rocky Icelandic coastline and attempts to couple the observed differences to environmental factors, i.e. salinity, temperature, wave exposure and currents. Fucus vesiculosus grows in the mid-intertidal zone on the Icelandic coast, between the zones of F. spiralis and F. evanescens C. Agardh and/or Ascophyllum nodosum (L.) Le Jolis.
We hypothesized that morphological differences between populations occur along the Icelandic coast due to the existing temperature gradients and currents. We also hypothesized that salinity differences produce morphologically different thalli and that smaller ecotypes will be found at sites influenced by varying salinities. Effects of geographical dispersal barriers and long and short dispersal distances are discussed in relation to observed morphological patterns.
MORPHOLOGICAL VARIATION IN FUCUS VESICULOSUS
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STUDY AREA The currents around the coast of Iceland run mainly clockwise. The warmtemperate North Atlantic Drift divides into the Irminger Current south of Iceland passing along the western to north-western coast where it meets the polar water of the East Greenland Current. This cold Arctic water continues along the east coast of Iceland as the East Icelandic Current (Malmberg & Kristmannsson, 1992) (Figure 1). On the south coast, extending approximately from longitude 15-21°W, lies a 'sandur', a large sandy area, lacking rocky shores (Figure 1), which may create a dispersal barrier between the western and eastern Fucus vesiculosus populations. The larger tidal range along the south-western coast of Iceland is reflected in wider macroalgal zones compared to the north-eastern coast of Iceland (Anon., 1993, 1994). Mean water temperatures in February and August, over 17 y (Stefansson, 1969) and mean spring and neap tides (m) (Anon., 1993,1994) are given in Table 1. Mean spring and neap tides are greatest on the west coast (l-5^m) and considerably less (0-5-l-5m) along the eastern coast (Table 1). Water temperature commonly ranges between 4-ll°C along the south-western coast and between l-8-8°C on the north-east coast which is affected by the cold East Icelandic Current, thereby creating a temperature gradient along the coast (Figure 1) (Malmberg & Kristmannsson, 1992).
25°
20°
15"
10°
Figure 1. Map of Iceland showing the main ocean currents and their direction.
MATERIALS AND METHODS The study was carried out during the four summer months of 1993 and 1994 respectively (Table 1). Samples were collected at 26 sites along the Icelandic coast (Figure 2, Table 1) which covered marine open coast habitats, fjords, estuaries and a few sites in some way influenced by freshwater. Of these, one was affected by freshwater from a river flowing under the lava (Straumsvik), one was a semi-enclosed lagoon (Horn) and one, Bolungarvik, was situated in the vicinity of an estuary. The
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.
Vestmannaeyjar Stokkseyri Sandgerdi Straumsvik Seltjarnarnes Reykjavik;Ellidaar Alfsnes Laxa Tradir Arnarstapi Stykkisholmur Patreksfjordur Bolungarvik Isafjordur Ogur Holmavik Blonduos Akureyri Hiisavik Kopasker Thorshofn Vopnafjordur Reydarfjordur Breiddalsvik Djiipivogur Hofn
Sampling sites:
63"27' 63°50' 64"03' 64°01' 64°10' 64"07' 64°11' 64°20' 64°34' 64°46' 65°05' 65°35' 66°09' 66°04' 66°02' 65°42' 64"40' 65°41' 66°04' 66°18' 66°12' 65°45' 65°02' 64°47' 64°40' 64°15'
Latitude °N
20° 13' 21"06' 22°43' 22°02' 22"01' 21°52' 21°46' 21°38' 22°22' 23°27' 22°42' 24'W 23°13' 23°08' 22°43' 21°43' 20"18' 18 TO' 1T2V 16°28' 15 20' 14C49' 14°13' 14°00' 14'15' 15"12'
Longitude °W
open coast open coast open coast estuary open coast estuary open coast estuary open coast open coast open coast fjord marine bay fjord estuary fjord open coast fjord open coast open coast harbour fjord fjord fjord fjord marine lagoon
Habitat
June June June June August August June July
August September September September September August August
June June June
September August
July June June August June July
Sampling month
3 2 3 1 3 1 3 1 3 3 3 2 2 2 1 2 3 1 3 3 3 2 2 2 3 2
Salinity relative scale
3 3 3 1 2 1 3 1 3 2 2 1 2 1 1 1 2 1 2 3 1 1 2 1 1 1
Exposure relative scale
17 18 17 1 14 0 5 1 5 5 2 3 18 5 6 2 16 3 12 16 0 1 1 1 2 1
Baardseth's exposure index
5-9 5-9 4-3 2-5 2-5 2-5 2-5 2-5 5-0 4-2 4-2 1-8 1-6 1-6 1-6 1-8 1-8 1-6 2-0 20 1-9 20 1-9 1-9 1-9 3-5
11-2 11-2 10-5 10-5 10-5 10-5 10-5 10-5 10-9 10-2 10-2 9-9 8-9 8-9 8-9 81 81 8-2 8-8 8-2 8-1 8-2 6-7 6-7 6-6 8-4
Water temp (°C) mean 'values for: February August
2-5 2-8 3-5 3-8 3-8 3-8 3-8 3-8 3-8 3-7 4-1 3-0 2-1 21 2-1 1-4 1-3 1-2 1-2 11 1-3 1-3 1-6 1-7 1-9 10
Mean tidal range (m) at spring tides
Table 1. List of sampling sites with environmental variables; salinity (relative scale: 1, low, variable; 2, intermediate; 3, high, stable) and relative exposure {1, sheltered; 2, intermediate; 3, exposed), mean water temperature (°C) in February and August measured over 17 years, 1949-1966, in the vicinity of the sampling site and mean tidal range (m).
o
H cn
C
a
2
cn
00 00
MORPHOLOGICAL VARIATION IN FUCUS VESICULOSUS I
989 I
Bolungarvfk Isafjor&l
-66°N
rfjVopnafjor&ur Patreksfj6r&
Rey&arfjdr&ur
StykkisholmurQ
Brei&dalsv Djupivo
Arnarstapi
_ g^o
Horn
Sandger6i< Stokkseyri ^
Vestmannaeyjar .® 25° I
j
i
i
100 km
20° j
Figure 2. Map of the sampling sites of Fucus vesiculosus around Iceland.
site at Stokkseyri, although situated on the open coast, is located between two of Iceland's biggest rivers and has reduced salinity occasionally. Finally, at Breiddalsvik the material was collected close to a sewage outlet. Since the salinity may vary greatly with time within a site, a three graded relative scale, based on momentary salinity measurements was used and the distance from freshwater influence was noted. The sites were grouped into the following classes: (1) sites with low and /or very variable salinity (i.e. estuaries and sites influenced by large freshwater outflow), momentary salinity measurement (S
