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DISCUSSION
The effects of weed mats on the sediments and invertebrate fauna of sheltered bays
and sandflats have been studied by both descriptive surveys and controlled
experimental manipulations. The objectives and underlying assumptions behind these
two methods are very different and consequently often produce contrasting results.
Therefore, this discussion will be limited to comparisons with, and conclusions
relative to, other controlled algal-manipulation experiments. This will avoid this
discussion becoming a hybrid between two fundamentally different approaches and,
therefore, minimise inaccuracies. A comparative survey of the ecological effects of
naturally occurring macroalgal mats on Drum Sands is given in Chapter 5.
This study aimed at specifically investigating the ecological effects of macroalgal
mats on intertidal sandflats and whether mat formation had a potential role in P.
elegans patch formation. These will be discussed separately.
The ecological effects of weed mats on intertidal sandflats.
The E. prolifera implanted within the weed plots in this study successfully established
from the time of implantation until October, a period of 20 weeks. After an initial
reduction the weed remained at a fairly constant biomass. Six weeks after
implantation the weed caused marked changes in the macrobenthos, together with
changes in all the measured sediment variables. While the numbers of P. elegans
were significantly reduced, those of C. capitata, oligochaetes and gammarids
increased under weed mats. Water, organic matter and silt/clay contents, medium phi
and sorting coefficients significantly increased in the sediments under weed mats,
these sediments also became significantly more reducing between 1-8cm depth. After
20 weeks, during weed decay, the effects of the weed on the fauna were similar to
those after 6 weeks resulting in a community numerically dominated by C. capitata,
while the sediment variables were no longer significantly different from controls.
Weed implantation experiments carried out previously have been conducted on more
sheltered sandflats and the methods of weed attachment, wire hoops for example, have
been less obtrusive (Reise, 1983a; Hull, 1987). These published studies, in contrast to
this study, did not incorporate a control treatment to determine whether there were any
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