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. Kraufvelin J. Exp. Mar. Biol. Ecol. 240 1999 229 –258
the causes behind the observed patterns in macrofauna community structure and to demonstrate some often overlooked problems when working with living communities.
This will be accomplished by examination of bladder-wrack macrofauna communities both in the mesocosms and in the field mother system and by pin-pointing the species
and processes basically responsible for observed differences between studied groups. This is possible thanks to the low number of species present in the mesocosms and in the
Baltic bladder-wrack zone in general, at least compared to fully marine environments Haage, 1975; Wallentinus, 1991; Kautsky et al., 1992. Finally I discuss the conse-
quences of these findings for an effective use of large-scale mesocosms in ecological and ecotoxicological research.
2. Materials and methods
Since a thorough description of the mesocosms 1989–1992, the field sampling, transplantation to mesocosms, mesocosm sampling, and analytical methods already has
been given in Kraufvelin 1998, only information of immediate relevance for this paper will be provided below. The BHB-mesocosms were situated at a field station belonging
to the Finnish Environmental Research Group in Nagu, Archipelago Sea, SW Finland 608 139 N, 228 069 E. The mesocosms consisted of 11–15 depending on year circular
3
outdoor basins volume 8 m equipped with a flow-through system of brackish water salinity 6‰, which was pumped unfiltered to the mesocosms from a nearby bay 8 m
21
of depth. The mean flow rate into each mesocosm was 168 l h . Most mesocosms
received different kinds of or dilution of pulp mill effluents, just one mesocosm for each treatment, but two to three replicated mesocosms each year served as controls. These
latter unpolluted systems are the ones to be examined closer in this paper and they will be labelled A and B 1989; C, D and E 1990; F and G 1991 and H and I 1992.
Bladder-wrack communities, consisting of brown alga 8 l per mesocosm with associated macroinvertebrates and periphyton, were usually transplanted to the meso-
cosms in June 1990: early July, 1992: late May and the mesocosms were run for | 5 months until November. At the end of the experiments five 1990–1992 or six 1989
subsamples, each consisting of one bladder-wrack specimen with associated algae and macrofauna communities, were taken from each mesocosm, in order to estimate the
amount of organisms associated with the alga. Three to six samples were also taken from the field each year. These field samples were taken both in connection with transplanta-
tion to mesocosms to get a rough measure of the initial community structure in the mesocosms and in connection with termination of experiments. Two semi-sheltered
¨ field locals, Havero 1989, 1991–1992 and Utterholm 1991, were used. In 1991,
monthly samples were taken from Utterholm for examination of seasonal differences and ¨
in 1996 a large number of field samples 64 were taken from Havero to be used for simulated transplantations to the mesocosms. The field samples from November are
labelled with the letter N and the year when they were gathered 89, 90, 91 and 92, whereas field samples from early summer are labelled with the letter S followed by the
year 90, 91 and 92. Note that no start samples are available from 1989. The monthly
P . Kraufvelin J. Exp. Mar. Biol. Ecol. 240 1999 229 –258
233
field samples from 1991 are labelled in the following way: JN samples from June, JL July, A August, S September, O October and N samples from November.
At the sampling for transplantation a plastic bag was carefully folded over each Fucus specimen including the stone to which the alga was attached at the sampling for
laboratory analyses the stone was removed. By using this method most of the mobile and agile animals associated with the bladder-wrack could not escape or loosen. At the
laboratory the volumes of algae were determined by the water displacement method. After termination of experiments, animal abundances and biomasses wet weight were
related to the dry weight of bladder-wrack after 24 h at 608C for comparisons among samples. Just eight species or taxa of macrofauna associated with the bladder-wrack are
included in the analyses, i.e. groups for which both abundance and biomass data were registered all 4 experimental years. These species taxa are: Gammarus spp. i.e. G
. ˚
oceanicus Segerstrale and G . zaddachi Sexton; Idotea spp., i.e. I. baltica Pallas and I.
chelipes Pallas; Palaemon adspersus Rathke; Theodoxus fluviatilis L.; Lymnaea spp., ¨
i.e. L . peregra Moller and L. stagnalis L.; Mytilus edulis L.; Cerastoderma glaucum
Poiret and Gasterosteus aculeatus L. The real species lists were, however, much longer. The following species or taxonomic groups were also found at least once in the
mesocosm bladder-wrack 1989–1992: Turbellaria, Acarina, Chironomidae, Tricoptera, Zygoptera, Gerris sp., Corixa sp., Notonecta sp., Gyrinus sp., Dytiscus sp., Balanus
improvisus Darwin, Jaera albifrons Leach, Praunus inernis Rathke, Electra crus-
¨ tulenta Pallas, Hydrobia spp., Macoma balthica L., Nereis diversicolor Muller,
Oligochaeta, Cottus gobio L., Pungitius pungitius L., Alburnus alburnus L., Phoxinus phoxinus L., Myoxocephalus quadricornis L. and Zoarches viviparus L..
Multivariate statistical analyses of community structure were done using the
PRIMER
program Plymouth Routines In Multivariate Ecological Research. The methods of this program have been described in detail by Clarke and Warwick 1994 and Carr 1996.
Non-parametric multivariate techniques were used as recommended for biological data by Clarke 1993, 1999. Abundance data were square-root transformed and biomass data
fourth-root transformed in order to maintain roughly the same importance of less dominant species for both types of data. The transformed data were put into triangular
matrices based on Bray–Curtis similarities. Ordination of samples was performed by NMDS Kruskal and Wish, 1978; Clarke and Green, 1988, a robust method that deals
with non-linearities by using ranks. Significance tests for differences among parallel mesocosms replicability were carried out using one-way ANOSIM permutation tests,
whereas differences among years repeatability were examined with a two-way nested ANOSIM and differences between mesocosms and the mother system realism by a
two-way crossed ANOSIM. The species contributing to Bray–Curtis dissimilarities between parallel mesocosms, among years as well as between mesocosms and the field
sites were investigated using the similarity percentage breakdown procedure, SIMPER Clarke, 1993.
The simulated transplantation of field bladder-wrack from 1996 to the mesocosms was repeated 10 times. At each simulation the 64 bladder-wrack samples were distributed to
four mesocosms in such a way that each mesocosm received 16 samples standardised to 100 g DWT. Separate coefficients of variation CVs were calculated for all 10
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. Kraufvelin J. Exp. Mar. Biol. Ecol. 240 1999 229 –258
occasions and averaged to get a measure of the overall initial variability. The results from the simulated transplantations were analysed using SPSS.
3. Results
