| dc.description | This thesis examines the temporal and spatial dynamics of phytoplankton assemblages, and of biotic (e.g. macroalgae, macrofauna, planktonic and benthic micrograzers) and abiotic (e.g. nutrients, temperature, salinity, pH) factors that may regulate these assemblages in tidepools, on a rocky shore, near Halifax, Nova Scotia, Canada. The abundance of phytoplankton changed over the period of tidal isolation of the pools, but these changes varied among phytoplankton groups, and within groups between a time of low phytoplankton abundance and during an autumn bloom in the surrounding seawater. The grazer field and variability in chemical and physical environment over the period of tidal isolation did not adequately explain the few recorded changes in phytoplankton abundance. Over a period of 16 months, there was little indication of vertical zonation of the phytoplankton assemblages along the intertidal gradient, and differences among zones rarely explained more than 30% of the spatial variability in phytoplankton abundance. However, the abundance of all groups of phytoplankton varied significantly among pools within intertidal zones on most sampling dates, and differences among pools explained up to 96% of the variability in phytoplankton abundance. Furthermore, there was significant variability among pools within zones for all biotic and abiotic characteristics of the pools on most sampling dates. In separate studies, I showed that there also was large variability among pools within intertidal zones in the structure of the macrobenthic and hyperbenthic assemblages. In factorial field manipulations, I examined bottom-up (nutrient availability) and top-down (grazing) effects on the composition of phytoplankton assemblages in tidepools. There were no significant bottom-up or top-down effects on any phytoplankton group in experiments conducted in November 1992 or June 1993. Although there was some variability among pools, both a reduction in grazer density and nutrient enrichment had a positive effect on some groups of phytoplankton but a negative effect on others in experiments conducted in July and August 1993. The strength of top-down effects was greater than that of bottom-up effects for all groups of phytoplankton in July 1993, but only for two groups in August 1993. The results of this thesis suggest that the factors that regulate the temporal and spatial dynamics of of phytoplankton assemblages in tidepools probably operate at the scale of the individual pool rather than the intertidal zone. The mechanisms of community regulation in tidepools differ from those on emergent substrata of rocky shores, probably due to differences in the tidal influence on the two habitats. | en_US |
