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emoved from the sediment surface before these cores were inserted to ensure that
differences in organic content were not due to above surface weed. Water and organic
contents and particle size distributions were analysed in the same way as described in
Chapter 2.
Redox potential values were taken to characterise the physico-chemical conditions
within the sediments. The redox measurements were taken at 5 depths; 1, 2, 4, 6 and
8cm. The redox potentials were taken after 60 seconds of the probe being inserted or
before if the reading had stabilised (Pearson and Stanley, 1979). The redox meter,
supplied by Russel Ltd., model RL100, had previously been calibrated with Zobell's
solution to 228mV (Zobell, 1946). One redox profile was taken within each plot, the
redox probe being washed with distilled water between each plot.
Hull (1987) found that the effects of macroalgal mats on benthic species was biomass
dependent. Since no information was available on the algal biomass of the weed mats
establishing on Drum Sands (i.e., no weed samples were taken for biomass
determination during 1996), the 'medium' weed density used by Hull was considered
appropriate, i.e., lkg FW/m2. Three kilograms fresh weight of algae were added to
each weed plot and covered with plastic mesh (5cm mesh size). The weed biomass
initially used was much more than lkg/FWm 2 since preliminary experiments had
shown that a significant proportion (30-40%) is lost soon after implantation on Drum
Sands. The algae were then threaded up through the mesh so they could drift in the
water. Net plots were included in the experiment to determine whether there were any
artefacts associated with the mesh. These consisted of 1m 2 of plastic mesh without
the addition of algae. The weed plots soon accumulated 5-6 cm of sediment within
just 3 days. The weed plots were examined after 2 weeks and more weed added in
areas where the weed had not properly established.
After 6 weeks, the plots were sampled for faunal and sediment analyses and redox
measurements were taken as described above. These measurements were aimed at
investigating the short-term effects of macroalgal cover, i.e. the effects of the weed at
the established density. Redox measurements were also taken after 12 weeks. At the
start of October (after 20 weeks) the weed started to disappear and faunal and
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