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consequence of the detrimental effects of algae on the health, amenity, economy and
general ecology of affected areas. However, understanding the causes of macroalgal
blooms is complicated as they are the result of the interaction of many factors. An
increase in the availability of nutrients such as nitrogen or phosphorus or changes in
the hydrography of the area are thought to be the most influential factors (Raffaelli et
al., 1999). Even when these suitable conditions are present, macroalgal mat formation
on more exposed intertidal sandflats has been shown to be facilitated by the
invertebrate species present. Reise (1983a, 1985), for example, has shown that the
thalli of green algae can become anchored by the feeding funnels of the lugworm
Arenicola marina. Once a feeding funnel is clogged by algae, the lugworm builds a
new funnel which again may serve to anchor more algae. He suggested that weed
mats would not have been able to become established on the sandflats of the island of
Sylt, North Sea, if it were not for the presence of the lugworm. Similarly, the
importance of tube-building polychaetes and shell fragments in the establishment of
macroalgal mats have been noted by Woodin (1977), Everett (1991) and Schories and
Reise (1993).
Generalisations concerning the effects of weed mats upon soft-sediment invertebrate
assemblages are very difficult and their effects still remain unclear (Everett, 1994).
Differences in the nature of the environment, the genera of the algal species, whether
the algae is attached or floating and differences in the weed biomass all have different
effects upon the fauna (Raffaelli et al., 1999). This makes direct comparisons
between studies very difficult. Previous studies investigating the effects of weed mats
on the sediments and associated fauna have been either descriptive surveys (e.g.,
Perkins and Abbott, 1972; Fahy et al., 1975) or controlled, manipulated experiments
(e.g., Woodin, 1977; Reise, 1983a; Hull, 1987, 1988; Everett, 1994; Cha, in prep.).
Surveys have involved comparing the invertebrate communities of areas where algal
mats are present to those where they are absent, usually in another part of the same
estuary or sandflat (e.g., Nicholls et al., 1981). However, Lowthian et al. (1985)
suggested that the presence of algal mats may be influenced by factors other than
nutrient availability and that these factors may affect the infaunal communities
independent to the effects of the weed. Consequently, because of their large-scale
separation, weed-affected and weed-free areas which have been used in observational
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