Download (20MB) - Repository@Napier

eason why some invertebrates showed mixed responses between studies. However,
this controversy mainly revolves around the results of studies investigating the effects
of macroalgal genera with very different morphologies from V. subsimplex (e.g.,
Enteromorpha, Ulva and Cladophora) and therefore the effect of this species on
macrofaunal abundances could not have been predicted with any certainty without a
comprehensive survey. Furthermore, it is possible that since the growth of Vaucheria
spp. on intertidal sandflats is not a common occurrence, this is possibly the first
detailed survey to assess the effects of this macroalgal genera on the invertebrate
abundances on intertidal sandflats within this area of Britain.
Many studies have documented the effects of natural macroalgal mat cover on
intertidal invertebrates using community parameters such as changes in the numbers
of species, individuals and changes in diversity (Nicholls et al., 1981; Soulsby et al.,
1982). These studies have found that although the total numbers of individuals may
increase due to opportunistic species such as C. capitata and oligochaetes, and species
such as the gastropod H. ulvae, the number of species and diversity decrease. The
establishment of V. subsimplex mats on Drum Sands, however, appeared to have had
an 'enrichment' effect on the faunal assemblages present. Although the number of
species were not significantly higher in the weed plots until 20 weeks after
establishment, the mean numbers of individuals and diversity showed more rapid
responses being significantly higher than weed-free areas after 4 weeks (although
diversity was significantly lower after 20 weeks due to the numerical dominance of P.
elegans). Throughout the period of algal cover, the change in actual species
composition was only small - the species present in the two plot types remained
similar, i.e., the increase in total abundances and diversity were mainly due to an
increased abundance of those species already present. The amphipod C. volutator and
the nudibranch Doto spp. were the only taxa which were found exclusively in the
weed plots while no taxa were only present in the control plots.
These results, given the low biomass of V. subsimplex compared with biomasses
reported in other studies, are fitting with the suggestion that weed has similar effects
as those of observed during organic enrichment. While higher densities of weed
create a more hostile environment resulting in an impoverished faunal community
125