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The possible role of algal-mat establishment in P. elegans patch formation on
Drum Sands.
Once established, the V. subsimplex mats rapidly increased both the numbers of
species present and the numbers of individuals of those species already present on
Drum Sands, including large increases in the numbers of P. elegans. This effect was
not short-lived and became even more pronounced during January, 20 weeks after the
weed mats had established. Unfortunately, during February the weed mats, together
with their accumulated sediments and associated faunal assemblages, disappeared,
presumably due to erosion by winter storms. Therefore, although the V. subsimplex
mats significantly increased the densities of P. elegans, they did not lead to P. elegans
patch formation during the year this study was carried out.
The possibility that algal mats may have created P. elegans patches during other years
cannot be ruled out. It is possible that the increased water movement due to storms
was unusually strong during the winter of 1997-1998. Indeed, sediment erosion was
probably the reason why the numbers of P. elegans had decreased in the control plots
between October and January (from 52.8 ± 8.1 to 18.2 ± 5.9). However, the majority
of the pre-existing P. elegans patches appeared unaffected by erosion during this
winter. It is perhaps more likely that the timing of weed establishment during 1997
was influential in the erosion of weed plots. During 1997, the weed mats did not
appear until September which is much later than weed mats usually establish on
sandflats in Britain and much later than it occurred on Drum Sands during 1996 (late
May/early June). Since juvenile P. elegans numbers in particular increased in
sediments covered by V. subsimplex, the late development of these mats may not have
increased P. elegans densities as much as in other years since the large recruitment
phase in May/June was missed. Since the numbers of P. elegans were possibly lower
under weed mats compared to those of previous years, and the period over which
sediment accumulation and stabilisation could occur was shorter, the sediments where
weed mats developed may have been more prone to erosion during the high-energy
wave movement during winter compared to previous years. Furthermore, the unusual
presence of the weed during the winter period may also have contributed to sediment
erosion.
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