Download (20MB) - Repository@Napier

(e.g., Nicholls et al., 1981; Soulsby et al., 1982), areas with a low weed biomass tend
to support higher numbers of individuals due to presumably an increased food supply
(this study). Furthermore, it seems that the physical presence of V. subsimplex had no
detrimental effect on the species present.
Unfortunately, previous surveys of this type have not recorded the redox potentials of
the sediments under the weed mats. Although the V. subsimplex at Drum Sands
rapidly caused the sediments below to become more reduced, it appears this was not
severe enough to have had a significant deleterious effect on the species present. Hull
(1987, 1988) reported much lower redox potentials in the sediments under his
experimentally-implanted Enteromorpha spp. than those under the V. subsimplex mats
in the present study. Although Perkins and Abbott (1972) did not measure redox
potentials, they noted that C. edule were forced to migrate to the sediment surface by
the reduced conditions just below the sediment surface in weed-covered areas; this
was never observed at Drum Sands where their numbers actually increased.
Some of the species which increased in numbers with weed cover in this study, e.g.,
C. cap itata and oligochaetes (mainly tubificids), have been found to increase under
both experimentally implanted and naturally occurring weed mats (Wharfe, 1977;
Nicholls et al., 1981; Wiltse et al., 1981). These opportunists increase in numbers due
to the increased food supply and their relative tolerance of reduced sediments.
However, many species which have been shown to decrease in abundance under weed
cover; spionids, bivalves and the amphipod C. volutator (Perkins and Abbott, 1972;
Nicholls et al., 1981; Wiltse et al., 1981; Reise, 1983a) increased in numbers in the
weed plots in this study. The increased abundances of these species in this study
presumably resulted from the increased food supply and the more stable sediments
afforded by the weed-covered areas, together with the fact that V. subsimplex, at least
at the density observed, seemed to have no deleterious effect on the feeding
mechanisms of most deposit-feeding invertebrates. The nudibranch Doto spp. was
found only in the weed plots and in moderate abundance. Prescott (1970) suggested
that Vaucheria spp. harbours its own faunal invertebrate species in marine intertidal
benthic environments and proposed that nudibranchs utilise Vaucheria spp. as a food
source.
126