Invasion of Sargassum muticum (Phaeophyta, Fucales) in Limfjorden, Denmark
    P. A. Stæhr, T. Wernberg-Møller & M. S. Thomsen
    Roskilde University, Dept. of Life Sciences and Chemistry, Po. box 260, 4000 Roskilde, Denmark
    (This poster was presented at the 10. Marine Research Conference in Hirtshals (Denmark) january 21.-23. 1998) 

    Introduction

    Sargassum muticum (Japweed) is a brown algae which originates from Asia. For the past 25 years it has been known for its invasive colonization of European waters. In Denmark, the first specimen of S. muticum was found in Nissum Bredning (St. 1, figure 1) in 1984. Since then S. muticum has successfully colonized the fjord, becomming a dominant macroalgae. S. muticum has been included in the comprehensive Danish National Monitoring Program of the Marine Benthic Flora (NMP) since 1989. Still only few attempts have been made to investigate its ecological impact, and it remains unknown whether S. muticum has reached its potential distribution in Danish waters.

    In the present study we:

    I. Describe the temporal and spatial pattern of invasion in Limfjorden.
    II. Evaluate the impact of the invasion of S. muticum on the native macroalgal community structure in Limfjorden.

    Fig1
    Figure 1. Location of Limfjorden in Denmark. Black numbers refer to stations included in the NMP. The red circle indicate first site of observation. Yellow bars separate reference areas. 

    I. Temporal and spatial pattern of invasion

    Successive colonization of Sargassum muticum since 1984 has produced high abundances, most conspicuously at 2-4 meters depth (fig. 2).

    Fig2
    Figure 2. Percent cover of Sargassum muticum at different depth intervals in the central part of Limfjorden (reference area 4, cf. fig. 1).

    Colonization of Limfjorden (measured as percent cover) saturates around 1995 at all depth intervals. The overall drop in abundance from 1993-94 in the depth interval 0.5-4m has been explained by a prolonged period of anoxia decreasing biomass of S. muticum by 74% (Limfjordssamarbejdet, 1994). Maximum depth distribution is constantly at 6-8m.

    The successful colonization of depth 0.5-6m has occurred progressively from west to east. Figure 3 examplifies the colonization process at depth interval 2-4m.

    Fig3
    Figure 3. Temporal and spatial development in abundance of S. muticum in Limfjorden, at depth interval 2 - 4m.

    The overall rate of colonization in Limfjorden has been approximately 10 km year-1. The high colonization rate indicates that absence of S. muticum in central and far regions is determined by other barriers than colonization time (e.g. physico-chemical or biological factors).

    II. Impact of invasion on the native macroalgal community complexity and species composition

    A. Impact on community complexity

    Temporal and spatial changes in structures of biological communities can be adequately evaluated on the basis of species abundance curves (Tokeshi, 1993). Changes in community structure caused by an increase in abundance of single species and/or decrease in species of intermediate dominance produce steeper and straighter abundance curves (Tokeshi, 1993). Figure 4 illustrates classic LogAbundance-species curves of macroalgae in Limfjorden 1989-97.

    Fig4
    Figure 4. Temporal changes in relative abundance of macroalgal species in Limfjorden. Analyses include all depth intervals.

    The relative abundance (RA(x)) of a species (x) at a station, is calculated as the sum of its cover (%) at i depth intervals, divided by the total macroalgal cover at the station at those same depth intervals. Plotting (RA(x)) as a function of the species rank gives the species abundance curves. In agreement with Tokeshi (1993) we use curve steepness, calculated as its slope, as a measure of community complexity (C). An increasing slope indicates a decline in C. See figure 5A and B.

    Fig5
    Figure 5A+B. Changes in complexity of the macroalgal community in Limfjorden following the invasion of S. muticum. Figure 5A includes all depth intervals. Figure 5B only include depth interval 2-4m where S. muticum is most prominent (cf. fig.2).

    According to figure 5A+B, the increasing abundance of S. muticum results in a decreasing macroalgal community complexity in Limfjorden. The effect of S. muticum is most marked at depth interval 2-4m, possibly caused by a higher abundance of S. muticum. The positive correlation from 1989-91 (fig.5B) may indicate that S. muticum actually have a positive effect on C during its initial phase of colonization.

    B. Impact on species composition and abundance

    The next step in our analysis is to address the question: "Can we detect effects on the macroalgal community structure, in presence and abundance of specific groups of native macroalgal species? " First we look on effects at higher levels of organization (fig. 6).

    Fig6
    Figure 6. Temporal development of S. muticum and functional groups in Limfjorden. Calculated as mean cover (%) of depth interval 2-4m.

    Again we have focused on depth interval 2-4m where S. muticum is present in highest abundance (cf. fig. 2).

    The most important trends of figure 6 are :

    • a significant decrease in perennial brown algae (rs = 0,70; p < 0,05) and ephemeral green algae (rs =0,70; p < 0,05) abundance.
    • an almost significant increase of S.muticum (rs = 0,63; p = 0,058) abundance.
    • no significant effect of S. muticum on decrease in perennial brown algae or ephemeral green algae.
    Functionally S. muticum is more similar to the related perennial brown algae than to ephemeral green algae. We therefore assume that a decrease in perennial brown algae is likely to be explained by competition from S. muticum, whereas the decrease in ephemeral green algae is more likely to be related to other regulating factors e.g. nutrients. Assuming that the fall in ephemeral green algae is related to a drop in nutrient input, would support the idea that S. muticum, and not nutrients, plays an important role in lowering the abundance of perennial brown algae, although not statistically significant.

    In figure 7 we focus on temporal development of S. muticum and perennial brown algae. Here we have focused on the central region of Limfjorden (reference area 4, cf. fig. 1), because it shows the largest temporal change in abundance of S. muticum, corresponding to a pre- and post Sargassum era. We therefore assume that an effect on species level will be most obvious when limiting the geographical area used in the analysis.

    Fig7
    Figure 7. Temporal development of S. muticum and selected perennial brown algae. The analysis is limited to the central region of Limfjorden (cf. fig. 1, reference area 4). and depth interval 2-4m.

    The most important trends of figure 7:

    • a significant increase in S. muticum (rs = 0,728; p < 0,05) and a significant decrease in abundance of Fucus vesiculosus (rs = -0,89; p < 0,001)
    • an almost significant decrease in Halidrys siliquosa (rs = -0,58; p = 0,087)
    • a significant effect of S. muticum in decrease of Halidrys siliquosa (r = -0,73; p < 0,05)
    • an almost significant effect in decrease of Fucus vesiculosus (r = -0,70; p = 0,053).
    Additional observations based on all stations at 2-4m depth interval :
    • a significant increase in S. muticum, decrease in Halidrys siliquosa and Laminaria sp. (p < 0,05).
    • an almost significant decrease in Fucus serratus (rs = -0,62; p = 0,067).
    • no significant effect of S. muticum in decrease of any of the species.

    Summary

    I. Temporal and spatial pattern of invasion in Limfjorden:

    • Abundance of S. muticum has increased significantly with time, saturation around 1995, with maximum abundance at 2-4m depth interval.
    • Maximum depth distribution is constant at 6-8m.
    • Colonization has occurred progressively from west to east.
    • Present /absence of S. muticum in central and eastern parts of Limfjorden is determined by physic-chemical and/or biological barriers - not time.
    II. Impact of the invasion on the native community structure in Limfjorden:
    • S. muticum is today the far most dominant macroalgae in Limfjorden.
    • The increasing abundance of S. muticum results in a decreasing macroalgal community complexity, most distinct at 2-4m depth.
    • A significant decrease in abundance of perennial brown algae and ephemeral green algae is evident, since the appearence of S. muticum, although no significant correlation with increase in abundance of S. muticum was found.
    • An increase in abundance of S. muticum significantly explains only the decrease in abundance of H. siliquosa at 2-4m depth.
    • Based on our statistical analysis, there is no straight forward effect of S. muticum on the changes in macroalgal community structure. We suggest that this supports the hypothesis that the success of S. muticum in Limfjorden, is partly determined by invasion of a previously unused niche.
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    Acknowledgements

    The present poster is part of our Master Thesis in Environmental Biology af Roskilde University. It was made possible through financial support from ‘The staff-Student Committee of Biology’ (Studienævnet for Biologi). We wish to thank Morten Foldager Pedersen (RUC) for superb supervision and constructive criticism and Dorte Krause-Jensen (DMU, Silkeborg) as well as Martha Laursen (Ringkøbing county) for making NMP data available. Also we wish to thank Dorte Krause-Jensen for her constructive criticism.

    References

    Limfjordssamarbejdet (1994): Vegetationsundersøgelser i Limfjorden 1994. Bio/consult.

    Tokeshi (1993): Tokeshi M.; Species abundance patterns and community structure. Advances in ecological research. 24:111-179.

    Comments are very welcome! Please send us an email:
    Thomas Wernberg-Møller, Peter A. Stæhr & Mads S. Thomsen
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