Botanica Marina Vol. 39, 1996, pp. 129-132 © 1996 by Walter de Gruyter · Berlin · New York
Apical Growth-measurements of Fucus vesiculosus L.: Limited Value in Monitoring E. Bonsdorff*'* and W. G. Nelsonb a
Huso Biological Station, Department of Biology, Abo Akademi University, FIN-22220 Emkarby, Finland Oceanography Program, Division of Marine and Environmental Systems, Florida Institute of Technology, Melbourne FL 32901, U.S.A. * Corresponding author b
In monitoring the marine ecosystem, the search to find simple and universal methods has led to selected species being commonly used as indicator organisms. In the Baltic Sea, the bladder wrack (Fucus vesiculosus L., Phaeophyceae), the only broadly distributed perennial macroalga, is commonly used in monitoring the state of the coastal ecosystem. One parameter used is growth measured as apical tip length. The possible utility of apical growth of E vesiculosus as an indicator of environmental stress in relation to within- and between-plant variation, within- and between-site variation, and between habitat (wave exposed vs. sheltered) variation was tested in the northern Baltic Sea. The analysis showed that variation within plants, among plants within a single site, and among sites of similar wave exposure was significant. Although the difference between exposures also was important, the apical growth of R vesiculosus should not, without great caution, be used as a measure of environmental condition.
Introduction
Materials and Methods
In monitoring the ecological condition of coastal sea The present study was carried out in the Aland archiareas, the use of indicator species has long been a pelago, northern Baltic Sea (60°25'N, 19°49'E), a standard procedure (Levine 1984, Wilson 1994), for vast archipelago forming a mosaic of habitats, rangdescription of pollution effects and natural environ- ing from sheltered bays to the open exposed sea zone mental gradients, such as salinity or wave exposure. (Bonsdorff and Blomqvist 1993). The area is low-salPerennial macroalgae have been commonly used as inity (5—7%o), non-tidal, and characterized by annual biological indicators (Jordan and Vadas 1972, Levine ice-cover in the winter. Fucus vesiculosus has wide 1984). In the Baltic Sea, the major perennial alga is horizontal and vertical distribution in the rocky litthe brown alga Fucus vesiculosus L., which occurs toral (Kautsky et al 1992). The entire Archipelago from the Danish Sounds in the south to the Bothan- Sea is also influenced by eutrophication, with largeian Bay in the north and the inner reaches of the scale negative effects on the distribution of F. vesicGulf of Finland in the east. The biology of Baltic F ulosus (Kangas et al. 1982, Kautsky 1991). Field sampling of Fucus vesiculosus was carried out vesiculosus is well known (Kautsky et al 1992). It has been regarded as suitable for monitoring purposes, in the middle (sheltered) and outer (exposed) archiboth through direct field measurements and through pelago of NW Aland, which receives minimal influexperimental manipulation (Kangas et al 1982, ence of local point source pollution or disturbance Rönnberg et al 1985, Lindblad et al 1986, Vogt and . other than eutrophication (Bonsdorff et al 1996). Schramm 1991). One parameter commonly used to Entire plants (all fronds originating from the same measure growth is apical tip length (Strömgren 1977, holdfast) of F. vesiculosus were collected at random 1994, Rönnberg et al 1991, 1992, Back et al in the field according to the following three-level 1992 a, b), which has been proposed as a suitable me- hierarchic sampling strategy designed to evaluate the thod for detection and evaluation of changes or dif- sources of variation in F vesiculosus apical growth. ferences in environmental conditions resulting from Ten sample sites were selected (five representing sheltered and five representing relatively exposed localithuman activities (Rönnberg et al 1991, 1992). The aim of this paper is to analyse critically the ies according to Rönnberg and Ruokolahti 1986) on validity of the use of apical growth in comparing the five different islands. At. each sampling site, five effect-parameter on the environment. The sources of plants of F vesiculosus were collected at random by variation of this parameter are identified, and their hand through snorkeling at about l m depth, and five importance evaluated using a three-level hierarchic randomly selected bifurcated apical tips were measured on each plant. Measurements were carried out field sampling design. Brought to you by | De Gruyter / TCS Authenticated |
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E. BonsdorfT and W. G. Nelson
by teams of two individuals, each visiting two sites on the same day in August, 1991. Each measurement was confirmed by both individuals before recording. Thus, the sampling design evaluated variation in frond length at the following levels using 250 measurements:
Table I. Results of a two-way analysis of variance of apical tip length measurements for 10 F. vesiculosus fronds measured by 8 independent persons. Source of variation
df
Mean square
F
P
(a) (b) (c) (d)
Among individuals Among fronds Remainder
7 9 63
30.86 123.93 29.97
1.03 4.14
n. s. P < 0.001
between exposed vs sheltered localities (n = 2) among sites within an exposure type (n = 5) among plants with the same sampling site (n = 5) among fronds within a single plant (n = 5)
The basic measurement methodology follows Back et αϊ (1992 a, b), and R nnberg et al (1991, 1992). The growing apical tips of F. vesiculosus are bifurcated, and the length along the midrib of both forks were measured for the uppermost fork, with the average of the two tips constituting a single measurement. As a preliminary study, the error variation associated with using different persons to carry out the measurements was evaluated to eliminate possible systematic errors in the material. Eight advanced level marine ecology students were presented 10 numbered, bifurcated F. vesiculosus apical tips connected with a pair of bladders, under standard conditions in the laboratory. All apical bifurcations of each sample were measured using a mm-scale, by each student separately. Because size varied among tips, data were analyzed using a two-way ANOVA without replication to separate variation in measurements due to persons from that of the fronds. As all fronds came from a single plant, significant variation within a plant could also be demonstrated. The 1991 field data were analyzed using a three level nested Analysis of Variance. The 1991 data were compared to monitoring data from 1994 on apical growth measurements (550 tips) and total plant length (60 plants) from 11 sites representing sheltered and exposed localities sampled at random, and analyzed using one-way ANOVA (Haldin 1994).
Results Measurements of the same numbered apical tips by 8 different persons showed no differences for measurements among persons (Table I), while there was a highly significant difference between fronds within a plant. This allowed us to carry out the field survey using several individuals doing the measurements. The field measurements of F vesiculosus gave highly significant differences for apical tip length measurements from different plants of F. vesiculosus within a single site. Apical tip length also varied significantly for measurements made at different localities judged to be of similar wave exposure. The comparison between exposure types was not significant, with 0.05 < p < 0.10 (Fig. 1, Table II). Because this comparison (exposed vs. sheltered) had only one degree of freedom, we take this as an indication of a high degree of variation at this level as well (Table II). Corn-
70-
e
60-
+ +
τ
g, 50jj 40·§· 1 30·§"
2010-
T
i
s
-Η
+
nExposed
Sheltered
Fig. 1. The mean apical tip length ± 95% confidence limits ofFucus vesiculosus from exposed and sheltered localities in the Aland archipelago, northern Baltic Sea, August 1991. + = range between minimum and maximum measurements. putation of the percent contribution of each level to total variation of apical tip length (total variation: 56.4 mm; mean length: 44.1 mm) indicated that the greatest contribution was between sites within exposure (44%). This was followed by between exposure types (27%), between apical tips within a plant (15.4%), and between plants within a site (12.7%). Data on F. vesiculosus from the field monitoring programme in 1994 showed that variation in apical growth of F. vesiculosus was significant between areas (sheltered/exposed; ρ < 0.05), but that similar or higher values (0.01
