using Fucus vesiculosus as an indicator - Springer Link

Marine Biology (1999) 134: 139±145

Ó Springer-Verlag 1999

A. Ruuskanen á S. BaÈck á T. Reitalu

A comparison of two cartographic exposure methods using Fucus vesiculosus as an indicator

Received: 10 August 1998 / Accepted: 11 January 1999

Abstract Morphological variation and vertical distribution of Fucus vesiculosus were quanti®ed at several sites in the Finnish archipelago (Baltic Sea). F. vesiculosus samples were obtained from skerries at geographical distances of 1 km or more (large scale) and at intervals of ca 100 m around a single island (small scale). The results were examined in relation to wave exposure, calculated by Baardseth and e€ective fetch cartographic methods. Despite the fact that the exposure indices were calculated di€erently they correlated strongly. Vegetative morphological characteristics of F. vesiculosus illustrate the morphological di€erences both within and between exposure gradients. The tallest and widest F. vesiculosus plants were found at the sheltered end of the large-scale exposure gradient. Those from equally sheltered sites of the island were smaller in all respects. Thus, the trend from small narrow plants to large wide sheltered plants was expressed di€erently over the di€erent geographical scales. Consequently localities with similar exposure indices may have morphologically di€erent F. vesiculosus populations. Shores with similar cartographic exposure indices can be di€erent in nature. Underwater topography and shore locations, either close to the mainland or at the outermost sites of the archipelago, a€ect the exposure. Although a sheltered shore is indicated, the sublittoral zone may be quite exposed to the movements of water. In contrast, in an

Communicated by L. Hagerman, Helsingùr A. Ruuskanen (&) Department of Ecology and Systematics, Division of Hydrobiology, P.O. Box 17, University of Helsinki, FIN-00014 Helsinki, Finland S. BaÈck Finnish Environment Institute, P.O. Box 140, FIN-00251 Helsinki, Finland T. Reitalu Institute of Botany and Ecology, University of Tartu, Estonia

open shore environment underwater rocks, boulders and shallow water areas can provide sheltered habitats. The depth range of the F. vesiculosus belt exhibited two distinctive patterns. At sheltered sites, around islands in the outermost reaches of the archipelago F. vesiculosus can grow to a maximum depth of 5 m. In exposed habitats the belt becomes narrower, reaching a maximum depth of 3 m. Closer to the mainland F. vesiculosus is found at exposed sites to a maximum depth of 5 m; the depth range at sheltered sites is narrower, only reaching depths of 2 m or less. In conclusion, the changes in plant morphology and in the vertical belt distribution are similar to each other along both gradients at the exposed ends of the wave action spectrum; however, the two gradients diverge at the sheltered ends of the spectrum.

Introduction Wave exposure greatly in¯uences the life of marine littoral and sublittoral organisms and the structure of the whole benthic community ± e.g. the vertical distribution of species and the morphology of algae (Cheshire and Hallam 1988; Gibbons 1988; Gutierrez and FernaÂndez 1992). Kjellman (1890) and Waern (1952) described many forms of Fucus vesiculosus in the Baltic Sea, at a time when e€ect of exposure was ignored. In recent morphological and ecological studies exposure estimations have been considered, but they have not always been arrived at by objective methods. Two commonly used cartographic methods to quantify exposure are the Baardseth index (Baardseth 1970) and the ``e€ective fetch'' (HaÊkanson 1981). On a global scale, the Baardseth index has been applied in morphological studies on algae by Russell (1978), Rice and Chapman (1985), Rice et al. (1985) and Rice and Kenchington (1990a, b). In the Baltic Sea, the Baardseth index has been used by BaÈck (1993), and the e€ective fetch with correlated depths by Kautsky and Kautsky

140

(1989), Kalvas and Kautsky (1993) and, with di€erent modi®cations, by Kiirikki (1996). BaÈck (1993) and Kalvas and Kautsky (1993) found that salinity and wave force a€ect the morphology of Baltic Fucus vesiculosus. In studies on plants collected along salinity or exposure gradients, problems arose when the plants to be analysed were collected from shores with di€erent exposure values. Both wave action and salinity simultaneously a€ect plant morphology. However, there are only a few reports of shore exposure studies in which plants were collected from shores with equal exposure indices (Kalvas and Kautsky 1998; Ruuskanen and BaÈck 1999). A numerical exposure value based on the shoreline probably does not represent the true in situ conditions on the bottom. A shore may be classi®ed as sheltered if it is located close to the mainland in an archipelago, and is protected by neighbouring islands, even if the shore faces the main wind direction. A shore may also be classi®ed as sheltered on the partially exposed side of an island in the outer archipelago. These two shore types may have the same calculated exposure index; however, the water motion, turbidity and sedimentation can be totally di€erent on the bottom and have a di€erent e€ect on Fucus vesiculosus morphology and belt formation. In the Baltic Sea the upper and lower limits of the Fucus vesiculosus belt are determined by abiotic factors (Kautsky et al. 1986; RoÈnnberg et al. 1992; Kiirikki 1996; Kiirikki and Ruuskanen 1996). The lower limit is generally determined by light (Waern 1952) and the upper limit by pack ice (Kiirikki 1996). The depth range of F. vesiculosus on open shores is much wider than on sheltered shores (Kautsky et al. 1992; Kiirikki 1996). The aims of the present study were to compare two cartographic methods, on two geomorphological scales (small and large), using Fucus vesiculosus as a biological indicator of wave action, and to study the vertical depth ranges of the F. vesiculosus belt along these gradients on the coast of southern Finland, northern Baltic proper.

Materials and methods

the measured distance in kilometres and ci the angle between the measurement and the central axis. Study area and sampling The small-scale study was carried out in July 1994 at SegelskaÈr, located in the outer archipelago, about 10 km south of the Hanko peninsula; the large-scale study was carried out in the TvaÈrminne archipelago in August 1997 (Fig. 1). At SegelskaÈr, ten sampling points with Baardseth indices 0, 1, 3, 7, 8, 10, 15, 17, 18 and 19 were ®rst identi®ed from the nautical chart, scale 1:50 000. Then, their e€ective fetch values were calculated. The structure of the island and the surrounding water area did not permit sampling points with indices between 9 and 14. Later, during sampling, the points with indices 7 and 10 were rejected because Fucus vesiculosus was missing at these sites. Finally eight sampling points with di€erent degrees of exposure varying from sheltered to extremely exposed were located around the island (Fig. 1). The TvaÈrminne archipelago extends about 5 km to the east of the Hanko peninsula and includes shores with di€erent types of exposures. In the archipelago the sampling points with Baardseth indices 0, 1, 3, 8, 15 and 17 were chosen according to the same method used at SegelskaÈr. Because of the more sheltered conditions in the archipelago, we could not identify a shore with Baardseth index 1, and thus only e€ective fetch was used. At sampling point J e€ective fetch value was as similar as possible to those at SegelskaÈr. Moreover, because of sheltered structure of the archipelago, we could not identify shores with Baardseth indices 18 and 19 (Table 1). At every sampling point the upper limit, the optimal depth and the lower limit of the continuous Fucus vesiculosus belt were measured by the SCUBA diver with a dive computer (Table 2). The optimal depth was considered the depth at which F. vesiculosus had Table 1 Exposure values from SegelskaÈr (small-scale gradient) and the archipelago (large-scale gradient). Comparison of Baardseth index and e€ective fetch SegelskaÈr Island

Archipelago

Sampling Baardseth E€ective point index fetch

Sampling Baardseth E€ective point index fetch

A B C D E F G H

I J K L M N

0 1 3 8 15 17 18 19

0 1.6 10.3 117.8 155.3 180.6 180.6 180.6

0 0 3 8 15 17

0.3 1.4 13.1 106.6 159.4 189.8

Exposure measurements The wave exposure value at the sampling points was measured by the Baardseth index (Baardseth 1970) using a nautical chart with a 1:50 000 scale. To calculate the index, the centre of a transparent circular disc with a radius of 7.5 km was placed on the study site on the chart. The disc was divided into 40 sectors. The angle of each sector was 9°. A sector was ignored if there were skerries, islands or parts of the mainland shore located in that sector. The Baardseth index is the sum of the free sectors. A value of 0 indicates extreme shelter, whereas 40 represents ultimate exposure. The e€ective fetch value (HaÊkanson 1981) is based on 15 distance measurements from the study site to the opposite shore. The central radius is put in the main wind direction, or in the direction that gives the highest value. The distance (vi in km) from the sample site to a shore or to an island is measured for every deviation angle ci, where ci equals ‹6°, ‹12°, ‹18°, ‹24°, ‹30°, ‹36° and ‹42°. The e€ective fetch (Lf) may subsequently be calculated from the formula Lf = (Svi cos ci)/(S cos ci), where vi is

Table 2 Upper, optimal and lower limits of a continuous Fucus vesiculosus belt related to Baardseth exposure index at SegelskaÈr (small-scale gradient) and in the archipelago (large-scale gradient) +, belt ends in sandy bottom; ±, data missing) Location

SegelskaÈr Island Upper limit Optimum limit Lower limit Archipelago Upper limit Optimum limit Lower limit

Baardseth index 0

1

3

8

15

17

18

19

0.5 1.8 5

0.5 1.6 4.8

0.5 1 +

0.5 1.8 +

1 3 5.2

1.5 3.5 4.6

1.6 2.4 3.4

1.6 3 3.8

0.5 0.7 1.0

0.5 1.0 1.5

0.7 1.5 3.5

± 1.5 ±

2 2.7 5

2 2.5 5

141 Fig. 1 Study area and sampling points at SegelskaÈr (small-scale gradient) (A±H) and in the TvaÈrminne archipelago (large-scale gradient) (I±N). Baardseth index values in parentheses

I(0) J(0) Hanko peninsula

K(3)

Tvärminne Zoological Station

Tvärminne Zoological Station

L(8) N(17)

ON

C RE

D

1

2

3

4 km

DI

IN

N

AI

M

W

A(0) N B(1)

F(17) Finland 59 50' Sweden

Baltic Sea

1 km

TI

Gulf of Finland 0

M(15)

C(3)

H(19)

G(18)

D(8) E(15)

100 m

23 15'

maximum coverage and density. From homogeneous substrata, at the optimal depth, 13 to 15 F. vesiculosus plants were randomly collected (Kalvas and Kautsky 1993). From holdfasts with several fronds only one was randomly chosen for measurements (BaÈck et al. 1991). In order to avoid ontogenetic di€erences only mature branches bearing receptacles were collected (Sideman and Mathieson 1983). Linear regression analysis and two-way analysis of variance (ANOVA) were used to analyse changes in plant morphology along the exposure gradients. From each plant seven vegetative characteristics, describing the morphological variation, were measured (Rice and Chapman 1985; BaÈck 1993) (Fig. 2). Measured vegetative morphological characteristics: 1. Frond length (mm) from the base of the holdfast to the tip of the most distal apex. 2. Stipe length (mm) from the base to the oldest dichotomy. 3. Number of dichotomies from the longest part of the frond. 4. Distance of dichotomies (mm, average of all) from the base to the top of the longest part of the frond. 5. Frond width (mm, average of 5) at a point midway between the youngest and the next youngest dichotomy. 6. Stipe width (mm) at the middle of the stipe. 7. Midrib width (mm, average of 5) at a point midway between the youngest and the next youngest dichotomy.

Results The results showed that at Baardseth values >15 the e€ective fetch levelled o€. Despite the fact that the ex-

Fig. 2 Fucus vesiculosus. Morphological characteristics measured from each plant (1 frond length; 2 stipe length; 3 number of dichotomies; 4 distance of dichotomies; 5 frond width; 6 stipe width; 7 midrib width)

142

Fig. 3 A scatter plot of Baardseth index versus e€ective fetch with linear regression line shows equal exposure values, r2 ˆ 0:96, p < 0:01

posure indices were calculated di€erently they correlated strongly (Fig. 3). Although the e€ective fetch levels o€ at the exposed end of the scale, it probably more reliably describes the degree of exposure. This is the case especially in the sheltered archipelago. Linear regression analysis was carried out using the mean values of morphological characteristics derived from eight di€erent sampling sites at SegelskaÈr and from six in the archipelago. In Fig. 4 the changes of measured characteristics are plotted against the Baardseth index values for small- and large-scale gradients. The results show that along the small-scale gradient (SegelskaÈr Island) frond width, stipe length and frond length decrease signi®cantly with increasing wave action. Other measured characteristics, though demonstrating the same trend, were not statistically signi®cant. Along the largescale gradient (archipelago), the frond width, midrib width and the distance of dichotomies decrease signi®cantly with increasing wave action; in contrast, the number of dichotomies increases with increasing wave action. Frond width is the only characteristic which changes signi®cantly along both small- and large-scale gradients. A general trend is decreasing frond width with increasing wave exposure. However, at the sheltered end of the large-scale gradient, plants are considerably wider than along the small-scale gradient. At the exposed ends of both gradients, frond width is almost the same. The di€erence becomes clear below a value of about 3 on the Baardseth scale. Another remarkable di€erence at the sheltered end of the gradients may be observed in the relationship between the number and distance of dichotomies. This makes plant habitus di€erent, the plants seeming more sparse (i.e. longer frond with fewer branches) on the sheltered shore along the large-scale gradient as opposed to the small-scale (Fig. 4). Two-way ANOVA was performed for frond width (Table 3). The result shows a highly signi®cant (p < 0.000) di€erence in frond width between smalland large-scale exposure gradients. There are highly

signi®cant (p < 0.000) di€erences between sampling points within their own exposure gradients, and in the frond width at each sampling point along the wave exposure gradients (p < 0.000). At SegelskaÈr Fucus vesiculosus forms a continuous vertical belt from the exposed to the sheltered shores around the island. In the archipelago, a clear vertical F. vesiculosus belt can be found only in the outer areas. On moderately exposed shores the F. vesiculosus belt is not continuous and has an irregular structure. In the sheltered part of the archipelago the plants grow separately and a belt is missing. Around SegelskaÈr F. vesiculosus grew deeper on sheltered shores than it did on open shores. The lower limit in sheltered locations reached a depth of 5 m (Table 2). Along the large-scale gradient on exposed shores, the F. vesiculosus zone lies between 0.7 and 5 m (Fig. 5; Table 2). Along both small- and large-scale gradients, on shores up to about 8 on the Baardseth scale, the upper limit of the Fucus vesiculosus belt lies at a depth of ca 0.5 m (Fig. 5; Table 2). There is a clear di€erence in the lower limit of the F. vesiculosus belt between 0 and 8 Baardseth index units. Along the small-scale gradient, around a very exposed island in the outer archipelago, the lower limit of the belt was deeper than on the sheltered shores along the large-scale gradient (Fig. 5; Table 2). Between 8 and 17 on the Baardseth scale the vertical belt formation is similar along both small- and largescale gradients. However, along the small-scale gradient the lower limit of the belt rises at values >17 on the Baardseth index (Fig. 5; Table 2). These sites are located on the most exposed shore of the island. The stronger the wave action is the deeper the optimal growth depth of the continuous belt. At SegelskaÈr a clear drop was observed when the Baardseth index was higher than 8. In the archipelago this sudden change did not occur, but the optimal growth depth gradually became deeper (Table 2).

Discussion Fucus vesiculosus undergoes morphological changes both within and between small- and large-scale gradients. At the sheltered end of the gradients the F. vesiculosus morphology is di€erent even if the numerical exposure values are the same. Along a small-scale gradient around SegelskaÈr Island, the plants are considerably narrower on the sheltered shore than on sheltered shores of the archipelago. This means that wave action is also strong on the ``sheltered'' shore at SegelskaÈr. At the exposed end of both gradients plant morphology is quite similar. At SegelskaÈr, the exposure changes from 0 to 19 on the Baardseth scale within a few hundred metres. SegelskaÈr is such a small island that waves can go around it. Sharp waves do not a€ect the sheltered side, but the ¯ow around the island causes continuous water move-

143

Fig. 4 Fucus vesiculosus. A to G Scatter plot of measured morphological characteristics versus Baardseth index (mean ‹ SEM) with a linear regression line along exposure gradients (small scale, n = 8, ®lled dots; large scale, n = 6, open dots)

144 Table 3 Two-way ANOVA for frond width

Source

df

SS

Exposure gradient (small scale vs large scale) Baardseth index (0±17 along exposure gradients) Exposure gradient + Baardseth index Residual Total

1 5 5 165 176

7325.26 7325.26 72503.5 14500.7 28765.6 5753.12 36847.0 223.315 1.454E+05

Depth increases

Upper limit of the belt

Large scale gradient= Along the archipelago zone

Small scale gradient= Around an island Lower limit of the belt

Exposure increases

Fig. 5 Fucus vesiculosus. A schematic illustration with a 3-D visual e€ect of the vertical distribution of a continuous F. vesiculosus belt (upper and lower limits) related to exposure along large- and smallscale gradients based on Table 2

ments. This means that a sheltered site may not actually represent a sheltered shore. Near the mainland, several islands block the waves, and continuous water motion does not exist as at SegelskaÈr. It is possible that the e€ect of circulating waves on Fucus vesiculosus morphology may di€er from the e€ect of small and sharp waves on the truly sheltered shore near the mainland. The slope of the bottom and the topographical conditions, e.g. shallow areas (