Introduction

Sargassum muticum (Japweed) is a brown algae (Fucales) which originates from Asia. For the past 25 years it has been known for its invasive colonisation of European waters. In Denmark, the first specimen of S. muticum was found in Nissum Bredning, Limfjorden (St. 1 fig. 1) in 1984. Five years later, in 1989, large parts of the central fjord (St. 2-6, 10-12, 14-16 fig. 1) had been colonised and by 1993 it had reached Kattegat through Langerak (St. 17-23 fig. 1), Thus giving an overall colonisation rate of approximately 10 km^-1 year. Today S. muticum is by far the most dominant macroalgae in Limfjorden. Although it has been included in the comprehensive National Monitoring Programme of the Marine Benthic Flora (NMP), only few attempts have been made to quantify its ecological impact and significance.
In the present study we:

I. Evaluate the impact of the invasion of S. muticum on the native macroalgal community structure.
II. Quantify the areal primary production of S. muticum at one location.

Figure 1: Location of Limfjorden in Denmark
Numbers refer to stations included in the NMP. The red circle indicate the site of more detailed study, Dråby Vig.

I. Invasion of Sargassum muticum and the impact on macroalgal communities in Limfjorden

Figure 2: Spatial and temporal changes in dominance of Sargassum muticum at 8 stations in Limfjorden

The dominance of S. muticum was calculated by ranking macroalgal species at each station, according to their abundance. Abundance was measured as percent cover integrated over 6 depth intervals (0-10m). Vegetation cover was registred by SCUBA divers during midsummer in connection to the NMP (cf. fig. 1).
S. muticum becomes increasingly dominant during the period 1990-95, and the progressive change in dominance at eastern stations show the invasion from west to east.

Figure 3: Temporal changes in relative abundance of macroalgal species at St. 10 in Limfjorden

The relative abundance (R_A(x)) of a species (x) at a station, is calculated as the sum of its percent cover at 6 depth intervals (0-10m) divided by the sum of the theoretical maximum cover at those same depth intervals. Plotting R_A(x) as a function of the species rank gives the species abundance curves (fig. 3).

Figure 4: Changes in complexity of the macroalgal community at St. 10 in Limfjorden (1990-95) following the invasion of Sargassum muticum

Dominance of S. muticum is calculated as described in figure 2. The relative complexity (C) is calculated as the slope of the species abundance curves (fig. 3).

This figure shows how the increasing dominance of S. muticum at station 10, affects the macroalgal community by decreasing C.

II. Primary production of Sargassum muticum in Dråby Vig, St. 10

Figure 5: Percent cover-biomass correlations of Sargassum muticum in Dråby Vig

Percent cover-biomass correlations of S. muticum were determined in ca. 15 samples in May, June and July. 3 SCUBA divers independently estimated S. muticum cover within a 0.27 m² ring, whereafter the biomass was removed for WW determination. Data were fittet to a hyperbolic equation.
100% cover of S. muticum yields 1.0, 2.0 and 2.7 kg WW m^-2 in May, June and July respectively. Differences between months are the result of individual algal growth, primarily in height. The saturation effect is probably caused by selfshading.

Figure 6 a-c: The spatial distribution of Sargassum muticum in Dråby Vig
SCUBA divers estimated the percent cover of S. muticm around 68 fix-points i a 100x100m square. Maps were produced by standard surface interpolation in IDRISI^® (a raster-based GIS) and converted to biomass by using the relationships from figure 5. The scale is divided into 15 equal intervals, each of approximately 125 g WW m^-2. Areal biomass data calculated in IDRISI^® are shown i table 1.
The 3 maps illustrate the change in distribution and biomass of S. muticum in one hectare from May to June to July 1997. A Large increase in S. muticum biomass can be seen during the period. This increase is primarily associated with the shallower areas. The range in biomass within the square indicates large spatial heterogeneity.

Figure 7: In situ growth rates of Sargassum muticum in Dråby Vig
Net growth rates of S. muticum were measured in April, May, June, and July by placing apical fronds (2-9 g WW)in transparent PVC cages (n=4) at 2.5 m depth. The algae were grown for 7 days and growth rates were calculated from the increase in biomass assuming exponential growth. Hence, growth rates represent net growth without grazing and physical disturbance.
Growth increased continously from 0.007 d^-1 in April to 0.038 d^-1 in July, thus following seasonal variation in insolation and water temperature.

Primary production of Sargassum muticum as estimated from growth rates and biomass
Assuming that only annual primary laterals were productive, total biomass of S. muticum in May was converted to productive biomass by multiplying with the ration given in table 2. A

The difference between predicted and observed values can be explained by loss- and limiting processes, that reduce the actual net increase in biomass (e.g. physical disturbance, grazing and competition). The difference is a maximum estimate of reduction caused by these processes.

Summary

• In Denmark Sargassum muticum was observed for the first time in 1984 in western parts of Limfjorden. Since then it has invaded the entire inlet and is now the most dominant macroalgae.
• The invasion of S. muticum has changed the macroalgal community structure to a less complex system with fewer species and less species of intermediate dominance.
• Mapping showed a marked spatial heterogeneity in the distribution of S. muticum. The distrubution is correlated to water depth.
• The in situ net growth rate of S. muticum is between typical slow growing perennials (e.g. Fucus vesiculosus) and fast growing annuals (e.g. Ulva lactuca). Seasonal changes in net growth rate seems to follow seasonal changes in insolation and water temperature.
• Estimating productivity on the basis of changes in biomass and net growth rates, gives a marked difference in observed and predicted values. This result stresses the importance of further investigations to assess the processes controlling net increase in biomass.

Acknowledgements

The present poster is part of our Master Thesis in Environmental Biology at Roskilde University. It was made possible through financial support from 'The staff-Student Committee og Biology' (Studienævnet for Biologi).
We wish to thank Morten Foldager Pedersen (RUC) for superb supervision and constructive criticism and Dorte Krause-Jensen (DMU, Silkeborg) for making NMP data available.

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Phenology of Sargassum muticum (Phaeophyta, Fucales) in Limfjorden, Denmark
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