Successional patterns and biotic interactions in intertidal sediments
The main objective of this thesis was to investigate the role of biotic interactions, biogenic structures and successional patterns in several invertebrate soft bottom assemblages in the Ythan estuary, Aberdeenshire, Scotland. The seasonal dynamics of the fauna found within a dense tube-field of Lanice conchilega were investigated. The abundance and composition of this fauna differed from that in the adjacent sediments and from that in the nearby sandflat at Red Inches. No consistent differences in abundance could be detected for any species throughout the tube-field, and the mussel (Mytilus edulis) was the only species significantly positively correlated with Lanice tube density. Pygospio elegans was found in significantly reduced numbers in tube-field areas affected by weed. Accumulation of weed and subsequent settlement of mussels in summer and periodical intense sediment movement during winter may have partly accounted for the large seasonal fluctuations seen in the abundance and composition of the tube-field fauna. The size of distributed patches is thought to influence the rate of colonization, but there have been few experimental tests of this proposition. The hypothesis was therefore tested by a field experiment where the rates and modes of colonization of azoic patches of different size (1600cm2 vs. 55cm2) were compared. The results of this experiment were also considered when designing subsequent experiments (see below). None of the invertebrates recorded differed significantly in abundance between patch size on any sampling occasion. The total number of species and individuals rapidly attained control (natural sediment) values. However, the patterns of colonization of individual species could be categorised into three types. The abundances of Type I species mirrored closely those in control sediments. Type II species were opportunists, such as the polychaete Capitella spp., which exceeded the control density rapidly but then showed a decline. Type III species, such as the polychaete Pygospio elegans, showed a very slow increase in azoic areas and never attained control densities, although size distributions were markedly different between the two patch sizes. This experiment showed that over the range studied, size of patch was not important in determining overall colonization patterns on sandflats.