The first reports of attached Sargassum muticum occurring in British waters coincided with
a warning that this invasive alga may replace the seagrass Zostera marina. While many
subsequent studies have investigated the biology of the alga and the effects of the
introduction of the indigenous algal and the associated faunal community, few have
responded to the original warning. Presented here are three studies into the interactions
between the two species and any effects on the associated fauna of Z. marina. I show that
contrary to previous thoughts, S. muticum can inhabitat soft substratum and successfully
attach and grow within a Z. marina bed, although the size of the plant is reduced.
Moreover, it may be the presence of the Zostera itself which enhances S. muticum
settlement. Growth and photosynthetic responses to inter and intra-specific competition in
each species were also recorded. Although the growth measures were shown to be
unaffected by the presence of the other species, a reduction in photosynthetic efficiency
was demonstrated in both species when involved in inter-specific competition. The mobile
epifauna and mobile epibenthos occurring in a range of vegetation types containing both Z.
marina and S. muticum was sampled using FBA and seine nets over a four month period, to
assess the effect of S. muticum on faunal assemblage structure within two Zostera beds in
South-West England. S. muticum stands and mixed areas of Z. marina and S. muticum were
found to contain a richer, more abundant, and faunally distinct community compared to
Zostera beds. The implications of the S. muticum invasion on Z. marina and the associated
fauna are discussed with relation to the original warning that S. muticum may replace Z.
marina beds.Faculty of Scienc
