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DISCUSSION
The two main aims of this study were;
1 - to investigate the ecological effects of macroalgal mats on an intertidal sandflat;
2 - to investigate whether macroalgal mat establishment could promote patch
formation of P. elegans on Drum Sands.
These were addressed by a descriptive survey by sampling from weed-affected areas
and weed-free areas.
The ecological effects of weed mats on intertidal sandflats.
Vaucheria subsimplex mat establishment, which occurred during September 1997, had
a marked effect on both the fauna and the sediments of Drum Sands. Four weeks after
establishment the mean number of individuals and diversity significantly increased in
weed-affected areas compared to non-weed plots. The mean abundances of 8 of the
10 most abundant species significantly increased under weed mats as did the values of
% sediment water, organic and silt/clay contents, medium phi and sorting coefficient,
while the sediments between 1-4cm depth became significantly more reduced. In
January, approximately 20 weeks after V. subsimplex mat establishment, the faunal
differences between weed plots and non-weed plots were still very marked, P. elegans
and C. capitata numerically dominating the communities of weed-affected sediments.
All the sediment variables were still significantly increased in weed plots after 20
weeks although the sediments were no longer significantly more reduced.
The physical and physico-chemical effects of V. subsimplex on the underlying
sediments during this study were essentially similar to those reported for other
macroalgal mat species (see Chapter 4). These include a decreased sediment redox
potential, increased silt/clay and organic contents and an increased sorting coefficient
due to a reduction in water current velocity. V. subsimplex is a green filamentous alga
which does not possess the long filaments of other green macroalgal mat forming
genera and, therefore, has a very close association with the sediment surface.
Consequently, the alga formed a velvet-like carpet over the sediments (pers. obs.).
Raffaelli et al. (1999) proposed that there was little evidence that the observed
differences of macroalgal mats on infaunal species abundances were due to
differences in algal morphologies, although Everett (1994) stated that this was the
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